ENCYCLOPEDIA OF ENVIRONMENTAL SCIENCE AND ENGINEERING - MICROBIOLOGY ppt

684
MICROBIOLOGY
INTRODUCTION
Microbiology is the study of organisms which are small
enough to require the aid of a microscope to be seen. In a
few cases, organisms are included in this group which can
be seen by the unaided eye because these organisms are
clearly related to the smaller ones. Microorganisms include
viruses, bacteria including rickettsia, mycoplasma, fungi
(yeast and molds), most algae, protozoa and, if one inter-
prets “micro” broadly, certain tiny multicellular plants and
animals. The study of cells and tissues from higher plants
and animals ( tissue culture ) uses techniques common to the
microbiologist and is frequently considered part of modern
microbiology.
Cells in general vary greatly in size but have many simi-
larities in internal organization. Among the most primitive type
of cells, it is impossible to clearly distinguish whether they are
distinctly “plants” or “animals” since they may have some of
the properties of each type. Viruses, on the other hand, are not
cells at all. Instead of arguing endlessly about whether a micro-
organisms is more plant-like or more animal-like and worrying
how to assign viruses, many scientists have divided organisms
in general into those which have (1) only animal characteristics,
(2) only plant characteristics and (3) the Protista (Table 1),
which have some characteristics of both plants and animals.
Some protists, viruses, may have characteristics not shared by
either plants or animals, that is, crystallizability and ability
to reproduce only by infecting some cell and using the cell’s
manufacturing machinery.
PHYSICAL CHARACTERISTICS OF

MICROORGANISMS
Protists vary greatly in size, shape and internal architecture.
Protists are subdivided into prokaryotes, and eukaryotes.
Prokaryotes do not have their genetic material (chromo-
somes) separated from the rest of the cell by a membrane
whereas eukaryotes have a true nucleus ( eu —true, karyo —
nucleus) separated from the rest of the cell by a nuclear
membrane. Viruses (virions) are usually included among the
prokaryotes. There are 9 types of prokaryotes.
Prokaryotes
1) Viruses are the smallest protists. They range in
size from about 30–300 nm. The smallest viruses
can only be visualized with an electron micro-
scope while the largest can be seen with a light
microscope. Viruses are composed of two general
molecular types (1) only one nucleic acid, either
ribonucleic acid (RNA) or deoxyribonucleic acid
(DNA), and (2) a group of proteins also called pro-
tein subunits or capsomeres, which surround the
TABLE 1
Characteristics of the Protista
Virion (virus) 30–300 nm icosahedron, hollow cylinder
icosahedral head ϩ tail
RNA, DNA requires participation of host
machinery
Mycoplasma 100–300 nm pleomorphic prokaryotes DNA fission
True bacteria 250–3000 nm spherical, rod, spiral rods,
prokaryotes
DNA fission
Higher bacteria 500–5000 nm spherical, rod, spiral rods,

filamentous, prokaryotes
DNA fission, budding
Prokaryotic algae 500–5000 nm spherical, rods in chains, spiral
rods in chains
DNA fission, internal septation,
gonidia
Eukaryotic algae 500 nm to macroscopic unicellular or multicellular,
filamentous, leafy
DNA in nucleus,
chloroplasts,
mitochondria
asexual or sexual simple fission
to complex life cycles
Protozoa 500–500,000 nm unicellular or colonial various
forms
DNA in nucleus,
mitochondria
asexual or sexual simple fission
to complex life cycles
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MICROBIOLOGY 685
nucleic acid and form a protective coat or capsid.
The smallest viruses appear spherical but magnifi-
cation in the order of 150,000–700,000 ϫ reveals
that they are icosahedrons (20 triangular faces and
12 corners) for example, wart virus. Other viruses,
for example, tobacco mosaic virus (TMV), the first
virus crystallized in 1935 by Wendell Stanley, is
grossly rodlike. Tobacco mosaic virus is composed

of a central, spirally-attached RNA to which capso-
meres are attached to form the outside of a cylinder.
The center of the RNA spiral of TMV is hollow.
Structurally, the most complicated viruses are some
which attack bacteria and blue-green algae.
These complicated viruses are composed of an icosahedral
head, containing DNA, a protenaceous tail and sometimes
accessory tail structures which are important for the attach-
ment of the virus to its host cell.
2) Mycoplasma are prokaryotes which overlap viruses
in size. They range from 100–300 nm in size. They
are highly pleomorphic: they do not have one typi-
cal shape but rather can appear coccoid, filamentous,
or highly branched. Unlike most other prokaryotes,
they do not have cell walls external to their cell
membranes. Their cell membranes usually contain
sterols, which are thought to lend strength to these
cell-limiting membranes (see also Table 2).
3) The true bacteria or Eubacteriales are prokaryotes
which are built on three general geometric forms:
spheres or cocci, rods, and spirals (including spi-
ral helices). All true bacteria have rigid cell walls.
They are either permanently immotile or move
by means of one to many flagella. They may be
aerobes or anaerobes. Some of the anaerobes are
photosynthetic. Their sizes and shapes are usually
constant except among the rods, in which rapidly-
multiplying cells may be somewhat smaller than
usual. When the cells divide, they often remain
attached to each other and form characteristic,

multicellular clusters. The shape of the cluster is
determined by the number of division planes.
When cocci divide in only one plane, they form chains which
may be as much as 20 cells long. Diplococcus pneumoniae
forms chains only two cells long while Streptococcus is
an example of the long-chain forming type. On the other
hand, cocci which divide along two planes, at right angles
to each other, form sheets of cells, and cocci which divide in
three planes form cube-shaped packets. If there is no regu-
lar pattern of the orientation of successive division planes,
a randomly-shaped cluster is formed. Staphylococcus is an
example of a coccus which forms random clusters. A typical
coccus is in the size range of 0.15–1.5 ␮ m in diameter.

Rods always divide in only one plane. They may appear
as single cells or groups of only two when they separate rap-
idly. The common intestinal bacterium Escherichia coli (size
0.5 ϫ 2.0 ␮ m) is an example of this type. Frequently rods
form long chains or streptobacilli. Bacillus megaterium (size
1.35 ϫ 3.0 ␮ m), the organism responsible for the “bloody
bread” of ancient times, is an example of a chain forming
rod. Some basically rodshaped bacteria are either curved or
helical rods. Their sizes range from almost as small as the
smallest straight rod shaped form to close to twice the length
of the largest straight rod.
True bacteria always divide by binary fi ssion after their
single circular chromosome replicates in a semiconservative
fashion.
Some true bacteria have complicated life cycles which
includes spore-formation. Spore-formers are all rods but

belong to diverse genera. They are ecologically related
in that they are found primarily in soil. Since that natural
TABLE 2
Some characteristics of prokaryotic and eukaryotic cells
Structure Prokaryote Eukaryote
Weight Chromosome 0.001–1.0 pg
one, single circular DNA double helix not
complexed with histones
10–10,000 pg
several linear DNA double helices (several
chromosomes usually complex with histones)
Nucleus No true nucleus. Chromosomes not separated from
cytoplasm by a membrane
True nucleus. Chromosomes enclosed in a nuclear
membrane
Reproduction Usually asexual, conjugation takes place rarely, no
mitosis or meiosis
Asexually by mitosis or sexually after meiosis
Membranes Only cell limiting membrane present. Usually lacks
sterols (except for mycoplasma)
Cell limiting membrane plus membrane limited
organelles present. Composition includes sterols
Organelles None Many including mitochondria, chloroplasts (plants
only), Golgi apparatus, lysosomes, etc.
Apparatus for protein synthesis Ribosomes, 70 S type usually not associated with
membranes
Ribosomes, 80 S type in cytoplasm associated with
endoplasmic reticulum. 70 S type in mitochondria
and chloroplasts not associated with membranes
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686 MICROBIOLOGY
environment is rather variable in that it can range from very
hot to very cold and from very wet to very dry, the heat- and
cold-resistant dormant spores offer the bacteria a means of
surviving adverse environmental conditions for months or
even years. Many important pathogens and commercially
important organisms are spore formers, e.g. Bacillus anthra-
cis which causes anthrax, Clostridium tetani which causes
tetanus and Clostridium acetobutylicum which can ferment
corn or potato mash into acetone, ethanol and butanol.
Corynebacteria are also rod-shaped bacteria but they
are pleomorphic and often look club-shaped. One of the
best known members of the genus is C. diphtheriae, which
causes diphtheria. Other members of the genus are commer-
cially important as producers of the vitamin folic acid.
Arthrobacter species are found widely in soil and water.
Depending upon the nutrients supplied, they can appear as
cocci or pleomorphic rods.
4) Spirochetes are NOT true bacteria though they
resemble Eubacteriales in that they are spirally
curved, unicellular and multiply by binary fission.
They differ from eubacteria by the absence of
a rigid cell wall which allows them to be quite
flexible. They are all motile by means of axial
filaments attached at the cell poles and spirally
wrapped around the cell. The smallest spiro-
chete is 0.1 ϫ 5 nm while the largest is 3.0 ϫ
120 ␮ m. One of the most important spirochetes is
Treponema pallidum, which causes syphilis.

5) Actinomycetes are NOT true bacteria. Rather,
they are naturally-branching, filamentous, spore-
forming organisms which have a mycelial struc-
ture similar to that of filamentous fungi. Many
actinomycetes, especially those from the genus
Streptomyces, are commercially important sources
of antibiotics.
6) Mycobacteria are rods which can form a rudimen-
tary mycelium which resembles actinomycetes,
but they differ in that their cell walls are particu-
larly rich in waxes, which allows them to retain
stain imparted by such dyes as basic fuchsin even
after treatment with dilute acid. This property,
called acid fastness, is characteristic of myco-
bacteria. Many species occur in soil but the best
known are the human pathogens M. tuberculosis
and M. leprae , which cause tuberculosis and lep-
rosy respectively.
7) Budding bacteria are NOT true bacteria. They
possess a complicated life cycle which includes
multiplication by budding rather than binary fis-
sion. Their type of budding can be readily dis-
tinguished from that of true fungi such as yeast.
The budding bacterium Hyphomicrobium exists
for part of its life cycle as a flagellated, slightly
curved rod. For multiplication, the flagellum is
lost, the chromosome replicates, and one chro-
mosome migrates to one end of the cell where
a hypha-like lengthening takes place. When the
hyphal extension ceases, it becomes a rounded

bud which contains the chromosome. The bud
grows in length and diameter until it reaches the
size of the mother cell, grows a new flagellum,
and separates from the hyphal extension.
8) Gliding bacteria are diverse group of prokaryotes
which are motile without having flagella. They
have very close affinities to blue-green algae
although gliding bacteria are not themselves pho-
tosynthetic. They may be unicellular rods, helical
or spiral-helical, or filamentous.
9) Blue-green algae or Cyanophyta are the only
prokaryotic algae. They are a diverse group that
include both unicellular and filamentous forms.
They have cell walls that resemble Gram-negative
bacteria but their photosynthesis more closely
resembles that of eukaryotes in that it is aerobic
rather than anaerobic (as in photosynthetic bacte-
ria). They are among the most complex prokary-
otes. Even though they lack defined organelles,
e.g. they lack chloroplasts, many species have
complex membranous or vesicular substructures
which are continuous with the cell membrane.
Some fi lamentous forms contain specialized structures such
as gas vacuoles, heterocysts, or resting spores
( akinetes ).
Gas vacuoles are frequently found in planktonic species, i.e.
those which live in open water. These vacuoles are thought
to provide the algae with a means of fl oating and sinking
to the depth most appropriate to support photosynthesis.
Heterocysts arise from vegetative cells and are thought to

function in N
2
fi xation. Some blue-green algae show gliding
motility. None are fl agellated. They are very widely distrib-
uted either in terrestrial or aquatic habitats from the arctic
to the tropics. Some forms are found in hot springs. Other
Cyanophyta are symbionts in a variety of plants and animals.
For example a species of Anabaena fi xes N
2
for its host the
water fern, Azolla. Many blue-green algae form especially
luxuriant mats of growth called blooms which clog water-
ways and limit their use for navigation, etc.
FIGURE 1 Animal viruses are often grown in embry-
onated eggs. The position of the hypodermic needles
indicates three common inoculation places.
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MICROBIOLOGY 687
The prokaryotic blue-green algae, Cyanophyta, are
usually divided into 5 groups:
Chooccocales are unicellular. They sometimes occur in
irregular packets or colonies. Cells multiply by binary fi ssion.
Chamaesiphonales are unicellular, fi lamentous, or colo-
nial epiphytes or lithophytes. Cells show distinct polarity
from apex to base. The base usually has a holdfast which
permits attachment to the substrate. Cells multiply by inter-
nal septation or by formation of spherical cells ( gonidia ) at
the ends of fi laments.
Pleurocapsales are fi lamentous with differentiation into

aerial and nonaerial elements. Cells multiply by crosswall
formation or by internal septation.
Nostocales are fi lamentous without differentiation into
aerial and nonaerial elements. They are unbranched or falsely
branched and frequently have pale, empty-looking cells called
heterocysts and resting spores ( akinetes ). Reproduction is by
liberation of a short fi lament only a few cells long, called a
hormogonium, which then elongates.
a) Nostacaceae are unbranched and produce hetero-
cysts. They frequently produce akinetes.
b) Rivulariaceae are unbranched or falsely branched.
Filaments taper from base to tip. Heterocysts are
usually present at the base. There is some akinete
formation.
c) Scytonemataceae are false branched. Heterocysts
are frequently found at branch points.
d) Stigonematalis are filamentous with aerial and
nonaerial differentiation. Hormogonia and hetero-
cysts are present. They often show true branching
and have pit connections between cells. Akinetes
are rare.
Eukaryotes
Eukaryotic microorganisms include all the algae (except
the Cyanophyta ), all the protozoa, and most fungi. All are
microscopic in size.

The eukaryotic algae are separated into nine divi-
sions based upon their pigment and carbohydrate reserves
(Table 3). They are all photosynthetic and, like higher plants,
evolve oxygen during photosynthesis. Many algae are obli-

gate phototrophs. That is, they are completely dependent
upon photosynthesis: they can not use exogenously supplied
organic compounds for growth in either the dark or light.
Some algae are facultative phototrophs; they are able to uti-
lize organic compounds for growth in the dark but fi x carbon
dioxide photosynthetically in the light.
Occasionally algae, especially unicellular forms, per-
manently lose their chloroplasts by exposure to any one
of several adverse conditions, e.g. heat or chemicals. If the
organism had been a facultative phototroph, before the loss of
the chloroplasts, it has the enzymatic machinery necessary to
survive except that now, in its chloroplastless state, it is indis-
tinguishable from certain other unicellular organisms more
commonly called protozoa. The ease with which an organ-
ism at this primitive level of evolution may be interchanged
between groups containing a preponderance of plant-like or
animal-like attributes underlines the need for the term protist
rather than plant or animal to describe them. Indeed both
botanists and zoologists claim the protists. Some algae e.g.
Euglena spp., normally only form chloroplasts when they
grow in the light while others e.g. Chlorella spp. form chlo-
roplasts regardless of the presence of absence of light. There
is great diversity in size, shape, presence or absence of life
cycles, type of multiplication, motility, cell wall chemistry,
and chloroplast structure. Although these parameters are of
great assistance in defi ning affi nities among algae, there are
still groups whose proper place is debated.
Many algae are important as sources of food, chemi-
cal intermediates of industrial and medical importance, and
research tools. Others are nuisances which clog waterways

or poison other aquatic life with their potent toxins.
Eukaryotic Algal Groups
The eight groups are:
1) Chlorophyta (green algae) are either marine or fresh-
water forms. This large and diverse group includes
forms which are either unicellular, colonial, filamen-
tous, tetrasporal (cells separated but held together
in groups of four in a mucilaginous material), coe-
nobial (cells more or less attached to each other in
an aggregate), or siphonaceous (simple, nonseptate
filaments). They frequently have life cycles which
RELATIVE SIZE OF BACTERIA
Clostridium 1x3.10m
Salmonella 0.6x2.3m
Hemophilus 0.3x0.6–1.5m
Pseudomonas 0.5x1.3m
Fusibacterium 0.75–1.5x8.80m
Neisseria 0.6x0.8m
Streptococcus 0.5–0.75
m
Staphylococcus 0.8–1m
Erythocyte 7m diameter
FIGURE 2 Relative sizes of bacteria.
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688 MICROBIOLOGY
include motile, flagellated stages. Both asexual and
sexual reproduction occurs.
2) Euglenophyta differ from the other algae by pos-
sessing a rather flexible cell wall which allows con-

siderable plasticity of form. They are either fresh
water or marine forms. They all have two flagella
but in some genera the second flagellum is often
rudimentary. Many forms are phagotrophic (can
ingest particles). Chloroplastless forms are fairly
common. Multiplication is only by asexual means.
3) Xanthophyta are mostly freshwater forms. They
may be unicellular, colonial, filamentous or
siphonaceous. Some forms have life cycles which
include both asexual and sexual reproduction.
Motile anteriorly flagellated cells are found.
4) Chrysophyta are mainly freshwater forms but
important marine forms are known. Most genera
are unicellular but there are some colonial forms.
Cell walls are often composed of siliceous or cal-
careous plates. Some form siliceous cysts. They
are mainly found in fresh water but some impor-
tant marine forms exist. Reproduction is asexual.
5) Phaeophyta (diatoms) are unicellular or colonial
forms with distinctly patterned siliceous cell walls.
Both asexual and sexual multiplication is found.
Freshwater, marine, soil and aerial forms exist.
6) Pyrrophyta are unicellular flagellates with cel-
lulose cell walls which are sometimes formed in
plates. Reproduction is asexual. Sexual reproduc-
tion is rare.
7) Cryptophyta are unicellular, usually flagellated
forms which produce asexually.
8) Rhodophyta (red algae) are unicellular, filamentous
or leafy forms with complex sexual cycles. Most

are marine but there are a few freshwater forms.
Fungi
The “true” fungi or Eumycota are eukaryotes which are
related to both protozoa and algae. They are divided between
Reserve material (cont.)
b-1,3 glucans Sugars Sugars
alcohols
Mannitol
Division Laminarin Paramylon Chrysolamainarin Floridoside Sucrose Lipid
Chlorophyta (green algae)
ϩ
Euglenophyta
Xanthophyta
ϩ ϩ
Chrysophyta
Phaeophyta (brown algae)
ϩ o
ϩϩ
Bacillariophyta (diatoms)
ϩϩ
Pyrrophyta
ϩ
Cryptophyta
Rhodophyta (red algae)
ϩ
TABLE 3
Divisions and characteristics of the eukaryotic algae
Pigments Reserve material
Chlorophyll Biliproteins Starches (a-1,4-glucans)
a b c d e Phyco-cyanin Phyco-erythrin True

starch
Floridian
starch
Chlorophyta (green algae)
ϩϩϪϪϪ ϩ
Euglenophyta
ϩϩϪϪϪ Ϫ Ϫ
Xanthophyta
ϩϪϪϪϩ Ϫ Ϫ
Chrysophyta
ϩϪ Ϫ Ϫ
Phaeophyta (brown algae)
ϩϪϩϪϪ Ϫ Ϫ
Bacillariophyta (diatoms)
ϩϪϩϪϪ Ϫ Ϫ
Pyrrophyta
ϩϪϩϪϪ Ϫ Ϫ ϩ
Cryptophyta
ϩϪϩϪϪ ϩ ϩ ϩ
Rhodaphyta (red algae)
ϩϪϪ
?
Ϫϩ ϩ ϩ
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MICROBIOLOGY 689
microscopic and macroscopic and macroscopic groups. In
general, they have rigid cell walls, lack chlorophyll, and are
usually immotile. Most fungi reproduce asexually or sexu-
ally by means of spores though important budding groups

such as yeasts are well known. Since fungi are classifi ed by
the pattern of their sexual structures, fungi whose sexual
stages are unknown are placed into a group called Fungi
Imperfecti and assigned genera on the basis of their asexual
structures. They are further subdivided into the so-called
lower and higher fungi. The lower fungi, Phycomycetes, are
also called water molds but not all are aquatic (e.g. black
bread molds). Some species multiply by means of fl agellated
gametes or fl agellated spores i.e. more like certain green
algae than other fungi; Most, but not all, Phycomycetes have
COCCI
BACILLI
VIBRIOS
SPIRILLA
SPIROCHAETES
ACTINOMYCETALES
(A) MORPHOLOGICAL CHARACTERIZATION OF BACTERIA
(B)
(C)
FIGURE 3 A. General morphological characteristics of bacteria; B. Variety of morphological types among the
cocci; C. Variety of morphological types among the bacilli (rods).
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690 MICROBIOLOGY
ELEVATION
EDGE
Flat
Raised
Low Convex
High Convex

Entire
Umbonate
Convex with
papillate surface
Erose
Crenated
Undulate
Lobate
Rhizoid
FIGURE 5 Diagrammatic representa-
tion of types of bacterial colonies. These
shapes are specific for individual types
and are therefore quite useful as a step in
the process of identification of unknown
organisms.
SPORE FORMS
FIGURE 4 Diagrammatic represen-
tation of spores (clear areas) inside
rod-shaped bacteria. Note (bottom
row) that free spores may be ball or
egg-shaped.
hyphae, microscopic cytoplasm-fi lled tube-like branches
(lacking crosswalls), which together make a felty mat called
a mycelium. Individual hyphae are microscopic but the
mycelium, equivalent to a bacterial colony, is macroscopic.
Growth takes place by extension of the hyphae. Specialized
spore-containing bodies called sporangia can form at the
ends of some hyphae. Sexual reproduction requires fusion
of hyphae from two different mycelia to form a specialized
zygospore.

It is more common now to discard the term Phycomyetes
and instead subdivide the group into 4 classes in which affi n-
ities are much clearer. However, at present, the literature is
divided in its use of the older and newer terminology. As
with bacteria, chemical analyses of structures and metabolic
pathways followed are important in defi ning the classes.
These four classes are:
1) Chytridiomycetes lack true mycelia. They are aquatic,
have posteriorly uniflagellated zoospores and cell
walls composed of chitin.
2) Hyphochytridiomycetes have true mycelia. They are
aquatic, have anteriorly uniflagellated zoospores and
cell walls composed of chitin.
3) Oomycetes have true well developed mycelia and
cell walls composed of cellulose.
a) Saprolegniales are generally aquatic and have
asexual spores on specialized mycelear structures.
Only male gametes are motile.
b) Peronosporales are generally terrestrial. Sporan-
gia either produce asexual zoospores or may
germinate directly to form hyphae. Both gametes
are nonmotile.
4) Zygomycetes are terrestrial and have large and
well developed mycelia and nonmotile spores.
Asexual spores are produced in sporangia. Cell
walls are made of chitosan or chitin.
There are two classes included in the higher fungi.
1) The Ascomycetes are the best known and largest
class of fungi. Ascomycetes have hyphae divided by
porous crosswalls. Each of these hyphal compart-

ments usually contains a separate nucleus. Asexual
spores called conidia, form singly or in chains at
the tip of a specialized hypha. The sexual structure
called ascus, is formed at the enlarged end of a spe-
cialized fruiting structure and usually contains eight
ascospores. Some important microscopic members
of this group include yeasts, mildews, the common
red bread mold and many species which produce
antibiotics. On the other hand macroscopic forms
include Morchella esculenta or morels which are
highly regarded as a delicacy by gourmets.
2) The Basidiomycetes are entirely macroscopic and are
commonly known as mushrooms and toadstools.
Slime Molds
The slime molds, Myxomycetes, are at times classifi ed with
either true fungi or protozoa or, as here, treated separately.
They produce vegetative structures which look like ameboid
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MICROBIOLOGY 691
protozoa and fruiting bodies which produce spores with cell
walls like fungi. There are two major subdivisions (a) Cellular
and (b) Acellular. They both primarily live on decaying plant
material and can ingest other microorganisms, such as bacte-
ria, phagocytically. Both have life cycles, but that of the acel-
lular slime molds is more complicated.
Cellular slime molds have vegetative forms composed of
single ameboid cells. Cyclically, ameboid cells aggregate to
form a slug-shaped pseudoplasmodium that begins to form
fruiting bodies when the slug becomes immotile. Spores are

fi nally produced by the fruiting bodies.
Acellular slime molds have vegetative forms called plas-
modia which are composed of naked masses of protoplasm
of indefi nite size and shape and which travel by ameboid
movement (protoplasmic streaming). Two kinds of nesting
structures are produced: fruiting bodies (part of the sexual
cycle) and sclerolia.
Protozoa
The last major group of microorganisms are the protozoa.
As already stated, it is very hard to distinguish plants from
animals at this primitive stage in evolution where organisms
have some attributes of each. Most workers therefore are less
interested in whether protozoa should be claimed by bota-
nists or zoologists as they are in studying the group as the
root of a phylogenetic tree which gave rise to clearly sepa-
rable plants and animals. Protozoa range in size from that
of large bacteria to just visible without a microscope. They
have a variety of shapes, multiplication methods and associ-
ations which range from single cells to specialized colonies.
They are variously found in fresh water, marine, terrestrial,
and occasionally, aerial habitats. Both freeliving and para-
sitic forms are included. Most are motile but there are also
important nonmotile forms. The protozoa are divided into
four subphyla (I–IV).
I . Sarcomastigophora include forms which have either fl a-
gella, pseudopodia or both. Usually a single-type of nucleus
(though opalinids contain multiples of this one type) is pres-
ent except in development stages of a few forms. Asexual
reproduction by binary fi ssion is common. One whole class
contains chloroplasts and are claimed by both protozoolo-

gists and algologists (they are considered here in detail with
the eukaryotic algae). Many important parasites of diverse
animal and some plant groups are found here. Sexual repro-
duction is present in a few forms.
The Sarcomastigophora are divided into three super-
classes.
A. Mastigophora ( fl agellates ) Are further sub-divided into
Phytomastigophorea or plant-like fl agellates (see eukaryotic
FIGURE 6 Bacterial motility. Motility is tested by
stabbing an inoculated needle into a tube of very vis-
cous growth medium. The motile organisms (S. typhi
and P. vulgaris) grow away from the stab mark.
4
3
5
1
2
FIGURE 7 Isolation of single bacterial colonies
on agar plates by dilution streaking. A diagrammatic
representation of method of streaking inoculated
needle across nutrient-containing plate. Stippled
area is the primary inoculation. The inoculation
needle is then flamed to sterilize and is then drawn
across the stippled areas as indicated for area 1.
The needle is then resterilized and drawn across
area 2, etc.
FIGURE 8 Isolation of single colonies by pour plate
technique.
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692 MICROBIOLOGY
algae) and Zoomastigophorea or animal-like fl agellates which
are divided into nine orders.
1) Choanoflagellida have a single anterior flagellum
surrounded posteriorly by a collar. Some forms
are attached to substrates. They are solitary or
colonial and are all free-living.
2) Bicosoecida have 2 flagella (one free, the other
attached to the posterior of the organism). They
are free-living.
3) Rhizomastigida have pseudopodia and 1–4 or
more flagella. Most species are free-living.
4) Kinetoplastida have 1–4 flagella and all have a
kinetoplast (specialized mitochondrion). Many
important pathogens (e.g. trypanosomes) and
some free-living genera are included.
5) Retortamonadida have 2–4 flagella. The cytostome
is fibril-bordered. All are parasitic.
6) Diplomonadida have 2 karyomastigonts, each with
4 flagella and sets of accessory organelles. Most
species are parasitic.
7) Oxymonadida have one or more karyomastigonts,
each with 4 flagella. All species are parasitic.
8) Trichomonadida have mastigont systems with
4–6 flagella. Some have undulated membranes.
Many important pathogens (e.g. Trichomonas )
are included.
9) Hypermastigida have mastigont systems with
numerous flagella and multiple parabasal apparatus.
All are parasitic. Some forms reproduce sexually.

B. Opalinata Are an intermediary group related to both
ciliates and fl agellates and are entirely parasitic. Opalinics
have many cilia-like organelles arranged in oblique rows over
their entire body surface. They lack cytosomes (oral open-
ings). They have multiple nuclei (ranging from 2 to many)
which divide acentrically. The whole organism divides by
binary fi ssion. Life cycles are complex.
C. Sarcodina Or ameboid organisms have Pseudopodia
which are typically present but fl agella may be present
during certain restricted developmental stages. Some forms
have external or internal tests or skeletons which vary widely
in type and chemical composition. All reproduce asexually
by fi ssion but some also reproduce asexually. Most species
are free-living (in both aquatic and terrestrial habitats) but
some are important pathogens; for example, Entameba his-
tolytica , which causes amebic dysentary. The sarcodinids are
further divided into three classes.
1) Rhizopodae, a free-living, mostly particle-eating
(phagotrophic) group which includes both naked
and shelled species. The specialized pseudopodia
are called lobopodia, filopodia, or reticulopodia.
2) Piroplasmea. These parasitic small, piriform,
round, rod-shaped or ameboid organisms do not
form spores, flagella or cilia. Locomotion is by
body-flexing or gliding. They reproduce by binary
fission or schizogony.
3) Actinopodea are free-living, spherical, typically
floating forms with typically delicate and radiose
pseudopodia. Forms may be naked or have mem-
braneous, clutenoid, or silicated tests. Both asex-

ual and sexual reproduction occurs. Gametes are
usually flagellated.
II. Sporozoa typically form spores without polar fi laments
and lack fl agella or cilia. Both asexual and sexual reproduc-
tion takes place. All species are parasitic. Some have rather
complicated life cycles.
The Sporozoa are divided into three classes:
A. Telesporea Can reproduce sexually or asexually, have
spores, move by body fl exion or gliding and generally do not
have pseudopodia.
B. Toxoplasmea Reproduce asexually, lack spores, pseudo-
podia or fl agella, and move by body fl exion or gliding.
C. Haplosporea Reproduce asexually and lack fl agella. They
have spores and may have pseudopodia.
III. Cnidospora have spores with one or more polar fi laments
and one or more sporoplasms. All species are parasitic. There
are two classes.
IV. Ciliophora have simple cilia or compound ciliary organ-
elles in at least one stage of their life cycle. They usually
have two types of nucleus. Reproduction is asexually by
fi ssion or sexually by various means. Most species are free-
living but parasitic forms are known.
ENERGY AND CARBON METABOLISM
All cells require a source of chemical energy and of carbon
for building protoplasm. Regardless of whether the cell type
is prokaryote or eukaryote or whether it is more plant-like
or more animal-like, this basic requirement is the same. The
most basic division relates to the source of carbon used to
build protoplasm. Organisms which can manufacture all their
carbon-containing compounds from originally ingested inor-

ganic carbon (CO
2
) are called autotrophs while those which
require ingestion of one or several organic compounds for use
in the manufacture of cellular carbon compounds are called
heterotrophs. Some organisms are nutritionally versatile and
may operate either as autotrophs or heterotrophs and are
therefore referred to as facultative-autotrophs or facultative-
heterotrophs (depending upon which mode of nutrition usu-
ally predominates).
Autotrophs are further divided according to the manner
in which they obtain energy. Chemoautotrophs (also called
chemotrophs or chemolithotrophs oxidize various inorganic
compounds to obtain energy while photoautotrophs (also called
phototrophs or photolithotrophs ) convert light to chemical
energy via the absorption of light energy by special pig-
ments (chlorophylls and carotenoids). In both cases, chemical
energy is stored in the form of chemical bond energy in the
compound adenosine triphosphate (ATP).
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MICROBIOLOGY 693
When bonds of ATP indicated by ~ are broken, a con-
siderable amount of energy is released. This ~ bond cleav-
age energy operates the biological engines: it is the universal
chemical power which operates in all cells, autotroph or het-
erotroph.
Chemolithotrophic nutrition is only used by certain true
bacteria. These bacteria are of ecological importance in that
they are used to convert one form of nitrogen to another (i.e.

in the nitrogen cycle) or industrially to oxidize low grade
metallic or non-metallic ores. There are six bacterial groups
which are chemolithotrophic.
1) The ammonia oxidizers such as Nitrosomonas,
Nitrosococcus, Nitrosocystis, Nitrosogloea and
Nitrosospira.
One scheme for ammonia oxidation had hydrox-
ylamine as an obligate intermediate and has been
proposed for Nitrosomonas.
2) The nitrite oxidizers such as Nitrobacter and
Nitrocystis. One proposed scheme for nitrite oxi-
dation for Nitrobacter is:
NO
cytochrome
reductase
cytochrome C
cytochrome
oxi
2
Ϫ
⎯→⎯⎯⎯⎯→⎯⎯⎯
ddase
O
ATP ADP
2



3) Hydrogen oxidizers Hydrogenomonas. One pro-
posed hydrogen oxidation scheme is:

H
2
→2H
ϩ
ϩ 2e→unknown→fl avor protein compound
→ubiquinone→O
2

cytochrome
b compex
Nicotinamide adenine→menadione
→cytochrome C→cytochrome a
→O
2
dinucleotide (NAD)
4) Ferrous compound oxidizing bacteria such as
Ferrobacillus and Thiobacillus ferroxidans.
One proposed ferrous oxidizing scheme for
F. ferrooxidans is:
4FeCO
3
ϩ O
2
ϩ 6H
2
O→4Fe(OH)
3
ϩ 4CO
2

5) Methane oxidizers such as Methanomonas methano-
oxidans and Pseudomonas methanica are common
in the upper layers of marine sediments and soil.
Methane is oxidized in the following manner:
CH
4
→CH
3
OH→HCHO→HCOOH→CO
2

6) The sulfur-compound oxidizing bacteria Thioba-
cillus.
Four pathways for oxidation of thiosulfate (S
2
O
3
ϩ2
) by
different Thiobacillus species are known. These are:
a) 6Na
2
S
2
O
3
ϩ SO
2
→4Na
2

SO
4
ϩ 2Na
2
S
4
O
6

2Na
2
S
4
O
6
ϩ 6H
2
O ϩ 7O
2
→2Na
2
SO
4
ϩ 6H
2
SO
4

b) Na
2

S
2
O
3
ϩ 2O
2
ϩ H
2
O→Na
2
SO
4
ϩ H
2
SO
4

c) 5Na
2
S
2
O
3
ϩ H
2
O ϩ 4O
2

→5Na
2

SO
2
ϩ H
2
SO
4
ϩ 4S
2S ϩ 3O
2
ϩ 2H
2
O→2H
2
SO
4

d) 2Na
2
S
2
O
3
ϩ H
2
O ϩ 1/2O
2
→Na
2
S
4

O
6
ϩ 2NaOH
Photolithotrophic nutrition is used by photosynthetic
bacteria, blue green algae and eukaryotic algae. The general
reaction in which both utilization of CO
2
(carbon dioxide
fi xation) and energy generation is summarized is:



CO H A CH O A H O
22 2
ϩϩϩ
nv
⎯→⎯ ()2
2

Where A is either oxygen for all eukaryotic algae and the
prokaryotic blue-green algae (H
2
A = H
2
O), or sulfur for green
sulfur bacteria, Chlorobacteriaceae, and purple sulfur bacte-
ria, Thiorhodaceae (H
2
A = H
2

S) or any one of several organic
compounds for nonsulfur purple bacteria, Athiorhodaceao
(H
2
A = H
2
-organic compound which is oxidizable).
Both green and purple sulfur bacteria are obligate
anaerobes whereas the non-sulfur purple bacteria are facul-
tative anaerobes (they are anaerobic when growing hetero-
trophically). In all cases, photosynthetic organisms operate
by the initial transduction of light to chemical energy. In
this transduction, chlorophyll ϩ light quanta Ch1
ϩ
(excited
chlorophyll) ϩ e
Ϫ
(electron driven off of Ch1). Many such
events take place simultaneously and electrons released
during these reactions migrate through the photosynthetic
unit to the reaction center and transfer energy to a special
reaction-center chlorophyll. At the reaction center, a charge
separation of the oxidant and reductant occurs. Electron
fl ow after this event differs in photosynthetic bacteria as
compared with algae and higher plants (Figures 9 and
10).
In addition, differences in photosynthetic ability exist
among organisms based upon the absorption maxima of
their light-transducing pigments (primarily chlorophylls).
The combination of light intensity, wavelength of available

light, wavelength of operation of principal energy trans-
ducing pigment, degree of aerobiasis, and availability of
oxidizable compound (H
2
O, H
2
S, or H
2
-organic compound)
all infl uence the effi ciency of photosynthesis. These factors
should be borne in mind when one looks for the ecological
niche occupied by these various organisms.
Ecology of Microorganisms
One should understand the physiological requirements of
microorganisms before investigating the effects of environ-
mental changes on the distribution and activity of diverse
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694 MICROBIOLOGY
microbial types, their interactions and their relationships to
higher plants and animals. It is important to note that the
“natural” balance may be undesirable. Thus studied efforts
to change these bal ances would be quite desirable. The
most important precaution to observe relates to the ancillary
consequences of these changes, i.e. do the changes produce
side effects which may be as unappetizing as the original
condition.
The development of techniques which form the bases for
studying microbial ecology comprise an important chapter in
Bacteriochlorophyll

+
light
cytochrome c
cytochrome b
Ubiquinone
Ferredoxin
Intermediate
ADP + Pi
AT P
e
–
E
'
0
(REDOX POTENTIAL)
+0.6
+0.4
+0.2
0
–
0.2
–
0.4
– 0.6
– 0.8
FIGURE 9 Electron flow in bacterial photosynthesis.
cytochrome f
cytochrome b
ADP + Pi
ADP + Pi

AT P
AT P
e
–
e
–
e
–
photosystem I
photosystem II
Plastoquinone
light 400–500 nm
light, 700 nm
chlorophyll
+
chlorophyll
+
intermediate
oxidant
intermediate
reductant
reduced ferrodoxin
NADP
NADPH
H
2
O
2H+1/2

O

2
+0.6
+0.8
+1.0
+0.4
+0.2
0
–0.2
–0.4
–0.6
–0.8
–1.0
FIGURE 10 Electron flow in algal and higher plant photo-synthesis.
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MICROBIOLOGY 695
TABLE 4
Absorption maxima of chlorophylls from various sources
Organism Chlorophyll type Principal absorption maxima in nm
Green sulfur bacteria Bacterial Chl
c
Bacterial Chl
d
660
650
Purple sulfur bacteria Bacterial Chl
a
Bacterial Chl
b
820

1025
Non sulfur purple bacteria Bacterial Chl
a
Bacterial Chl
b
820
1025
Green algae and Euglenids, higher plants Chl
a
Chl
b
683
650
Diatoms, brown algae Chl
a
683
Pyrrophyta Chl
c
620
Xanthophyta Chl
a
Chl
e
682
Cyano-, Chryso- and Rhodophyta Chl
a
683
classical microbiology. These laboratory methods, pioneered
by Winogradsky (1856–1953) and Beijerinck (1851–1931)
and refi ned by others, utilize specialized, restrictive, physi-

cal and chemical conditions to select and cause to predomi-
nate one or few types of organisms from a highly diverse
mixture. Hence this method is termed selective enrichment
culture. Once one understands how to manipulate these labo-
ratory systems, it is easier to analyze fi eld observations in
which specifi c conditions which result in microbial changes
can be recognized and, if necessary, altered. We will fi rst
explain the principles of selective enrichment techniques and
then look at the natural distribution of microorganisms and
their relationship to higher plants and animals. Highlights
of microbial characteristics which are useful taxonomically
have been described in the various sections listed under
Physical Characteristics of Microorganisms.

Selective Enrichment Methods
To determine whether a given sample of soil, water, or air
contains microorganisms capable of living under a particu-
lar set of conditions, one prepares a growth medium which
is selective for a property peculiar to those conditions. For
example, if organisms which can fi x atmospheric nitrogen
are sought, all non-atmospheric sources of nitrogen (such
as nitrites, nitrates, ammonia, amino acids) are eliminated
from the growth medium. If organisms which obligately fi x
carbon dioxide are required, all nonatmospheric (organic)
sources of carbon are eliminated from the growth medium
and, frequently, additional CO
2
is bubbled through the
medium.
On the other hand, it might be of particular interest to

determine if a certain weed-killer is biodegradable before
it is used under fi eld conditions. Many different approaches
to this important problem are possible. Thus more than one
mode of attack is described. The approach which is closest
to the general principle of revealing (selectively enriching
for) a minor population of desired microbial type among a
multitude of undesirable organisms makes use of an enrich-
ment medium in which the weed-killer is used as either
(a) the only source of organic carbon and nitrogen, (b) the only
source of organic carbon though other sources of nitrogen
are present, or (c) the only source of nitrogen though other
sources of organic carbon are present. Subsequent microbial
growth indicates biodegradability. The organisms may be
isolated and used as seed cultures for a percolation system
which is fed by the aqueous runoff from a fi eld which was
treated with the weed-killer. Thus the more public water-
ways fed by aqueous effl uents from treated fi elds would not
be polluted by potentially-toxic agricultural chemicals.
In order to prepare selective enrichment media, one
needs to provide the microorganisms with all their nutri-
tional requirements in proper proportions. Insuffi cient
quantities will not support growth and excesses are fre-
quently toxic. In addition conditions must be biased in some
way to insure that most of the undesired organisms will not
grow at all or will grow appreciably slower than the desired
organisms. It should be recognized that it is rare for any
single enrichment to select out only one microbial species.
Thus further purifi cation steps are required if one wishes to
isolate only one species uncontaminated with other living
things. An uncontaminated, single membered culture is

called a pure or axenic (a = absence of, xenos = strangers)
culture. The common nutritional requirements of microor-
ganisms, the quantities in which they must be supplied, and
the biological uses of each substance are shown in Table 5.
The selective enrichment techniques to be described are
most frequently used for the isolation of bacteria, yeast, and
certain prokaryotic algae. An outline of selecting properties
is given in Table 6.
A demonstration of factors involved during the natural
selection which takes place under fi eld conditions is shown
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696 MICROBIOLOGY
in Figure 11. These models are called Winogradsky columns.
The variety of microbial life which develops over a period
of 2–10 days is determined by (a) the degree of acidity or
alkalinity of these natural growth media, (b) the nutrients
contained in the liquid and solid phases, and, of course,
(c) the initial populations of microorganisms. In the exam-
ples shown, light is provided to ensure growth of photosyn-
thetic organisms.

If the basic principles of the Winogradsky column are
to be used to reveal the microbial population in a particular
soil or water sample, then the column and all its components
are fi rst sterilized and then inoculated with a nonsterile soil
or water sample. The microbial population of the sample
will develop in the portions of the column which provide the
proper physical and chemical conditions.
An important application of Winogradsky columns can

be made for testing various chemicals for potential ecologi-
cal changes. The chemical agent is percolated through a
soil column or is simply added to a predominantly liquid
column either when the column is started or after its micro-
bial population has developed. Signifi cant changes in
the column’s normal population (diversity or population
density) is indicative of toxicity to one or more types of
microorganisms. The profound changes in populations of
higher plants and animals due to disruption of the balance
of microbial life can be readily appreciated when the cyclic
nature of nitrogen and sulfur dissimilation is considered
(Figures 12 and 13).
These interdependences emphasize
the key role played by microorganisms in maintaining the
balance of soil nutrients.
Use of the Winogradsky Column for Testing
Biodegradability Capacity of a Natural Soil or
Water Body
In the last section, an example was given in which one of
these bodies with little capacity for biodegradation of a
weed-killer can be controlled so that the body in question
does not spread the potential pollutant to a bordering body.
We now consider a method for pretesting the biodegrada-
tion test. One would hope that this or a parallel test would
become standard before new agricultural chemicals are mar-
keted. That is, chemicals which are not decomposed before
they leave the immediate land or water body in which they
are used would not be marketed or would only be marketed
after controls against accumulation of the chemical had been
worked out.

The Winogradsky column test for ability of a potential
soil or water body to degrade a potenlially dangerous chemi-
cal such as a weed-killer consists of (a) preparing a standard
column composed of soil or water from the body in ques-
tion, and (b) after the column has been allowed to develop
its natural population (c) an isotopically labelled version of
the weed-killer can be added in the concentration (and 10 ϫ
the concentration that the weed-killer is to be used). After a
time equivalent to that in which the weed-killer is expected
to remain in the natural body (i.e. account for fl ushing time
from rain or water currents), the column is tapped by elution
with water or buffer, and the effl uent is analyzed for per cent
undergraded weed-killer as well as the nature of the deg-
radation products. The latter point is particularly important
TABLE 5
Nutritional requirements of typical heterotrophic microorganisms with limited synthetic capacity
Type compound Example Quantity in typical medium (%)
Energy source Glucose, sucrose,
glutamic acid,
succinic acid
0.1Ϫ2.0%
0.1Ϫ0.5%
Synthesis of
protein and fat
sodium acetate 0.01Ϫ0.1%
Lecithin
amino acids
0.001Ϫ0.005%
0.002Ϫ0.1%
Synthesis of

nucleic acids
purines and
pyrimidines
0.0005Ϫ0.002%
Coenzymes Vitamins 0.0001Ϫ0.005%
Major inorganic
requirements
PO
4
, Mn
ϩϩ
,
Mg
ϩϩ
, Na
ϩ
, K
ϩ
0.01Ϫ0.05%
Minor inorganic
requirements
Ca
ϩϩ
, Co
ϩϩ
, Zn
ϩϩ
,
Fe
ϩϩϩ

,Cu
ϩϩ
, Cl
Ϫ

NH
4
ϩ
or NO
3
Ϫ

0.001Ϫ0.01%
Water All the above are
prepared in aqueous
solution
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MICROBIOLOGY 697
Soil
Coarse sintered
glass
Sample to be tested
for microbial
population
Water plus pesticide
To
Reservoir
Sampling pipette
Aqueous layer

Areas of microbial
Growth
Soil sample
Aerobic: bacteria spp.,
blue-green+eukaryotic
algae, protozoa
Microae rophilic: bacteria
Anaerobic
purple sulfur bacteria
green sulfur bacteria
facultative hetero-
tropic bacteria
PONDWATER
Pond bottom=
MUD, GYPSUM
ROTTED PLANTS
AEROBIC
A
B
C
D
E
FIGURE 11 The Winogradsky column. A laboratory model for microbial population development in natural environments.
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698 MICROBIOLOGY
aerobic fixation by
bacteria (Azotobacter),
blue green algae,
fungi

bacterial autotroph
bacterial autotroph
NH
4
+
NH
4
+
NO
2
–
NO
3
–
–
Nitrosomonas
Nitrobacter
algal autotroph
Ankistromonas
organic compounds
N-containing
(atmospheric)
leghemoglobin and other N-containing
organic compounds of higher plants
Animal protoplasm
N-containing animal wastes
[NO
2
]
[N

2
O
+
]
anaerobic bacterial denitrification
by several organisms
Clostridium
Anaerobic fixation
bacterial bacterial symbiotic fixation
Rhizobium
N
2
FIGURE 12 The nitrogen cycle presented in a generalized way so that the role of various microorganisms are
indicated as well as the relationship to higher plants and animals. The precise arrangement will vary according to
several physical parameters of the natural environment.
since the degradation products may themselves be noxious.
Thus, the only degradation products which are acceptable
are those which may enter normal metabolism or are labile
enough to be further degraded to metabolizable compounds
by the physical conditions in the body in question.
Determination of Gross Populations of
Microorganisms
Quantitative sampling techniques are required for the deter-
mination of the microbial population in air, water, or soil
samples. The method used for sample collection must ensure
against (a) loss of more than a trivial number of microorgan-
isms and (b) cross-contamination from other sources during
sample transport and laboratory manipulation. Once the
sample has arrived in the laboratory, selective enrichment
techniques can be used to reveal the diversity of microorgan-

isms or mixed populations can be counted by using some
variation of the pour plate technique (see Figure 8).
A. Air sampling techniques Non-spore forming organ-
isms are rarely found in air samples because they
are too delicate to survive for long in the gener-
ally dehydrating conditions of atmospheric trans-
port. A convenient sampler consists of a sterile
membrane filter connected to a metering vacuum
pump. At the beginning of the sampling period, the
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700 MICROBIOLOGY
memb rane is exposed and the desired amount of
air passed through it. Air-borne microorganisms
are trapped on or in the membrane. Some dis-
crimination among organisms can be made during
sampling by using membranes with graded porosi-
ties. In this way organisms are segregated accord-
ing to their cross-sectional dimensions. To reveal
the organisms impinged upon the membrane, the
membrane is treated as inoculum for the various
selecting media.
B. Water sampling techniques Any container which
can be sterilized and can be opened and closed
by remote signal can be used. It should be rec-
ognized that pressure changes after retrieval may
influence viability of organisms collected at great

depths.
C. Soil sampling techniques A sterilized coring device
is usually used to ensure against contamination
with airborne organisms. The soil samples are han-
dled aseptically and weighed amounts are tested
for their microbial population.
REFERENCES
1. Environmental Microbiology, in The Natural Environment and the Bio-
ecochemical Cycles, O. Hutzinger, ed., Springer Verlag, Heidelberg,
1985.
2. Saunders, V.A. and Saunders, J.R., Microbial Genetics Applied to Bio-
technology, Croom-Helm, London, 1987.
3. Zehnder, A.J.B., Biology of Anaerobic Microorganisms, Wiley Inter-
science, New York, 1988.
4. Fleckenstein, L.J., Release of Genetically Engineered Microorganisms
to the Environment, Resource Management and Optimization, Vol. 6,
no. 4, 1989.
HELENE N. GUTTMAN
US Department of Agriculture

Elemental sulfur
colorless aerobic sulfur
bacteria:
Beggiatoa,
Thiothrix, Thiobacilli
SO
4
=
Desulfovibrio
many microorganisms

plants and animals anaerobic putrefaction
bacteria
SH of amino acids
to make protoplasm
anaerobic, photosynthetic
bacteria
H
2
S
FIGURE 13 The sulfur cycle. Microorganisms are becoming increasingly useful for processing
low-grade sulfur-containing ores. Sometimes overlooked is the natural cyclic distribution of
sulfur-containing compounds in which microorganisms play principle roles.
MINE DRAINAGE: see POLLUTION FROM MINE DRAINAGE
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