Chapter 129. Staphylococcal Infections (Part 10) pptx

Chapter 129. Staphylococcal Infections
(Part 10)

Diagnosis
While the detection of CoNS at sites of infection or in the bloodstream is
not difficult by standard microbiologic culture methods, interpretation of these
results is frequently problematic. Since these organisms are present in large
numbers on the skin, they often contaminate cultures. It has been estimated that
only 10–25% of blood cultures positive for CoNS reflect true bacteremia. Similar
problems arise with cultures of other sites. Among the clinical findings suggestive
of true bacteremia are fever, evidence of local infection (e.g., erythema or purulent
drainage at the IV catheter site), leukocytosis, and systemic signs of sepsis.
Laboratory findings suggestive of true bacteremia include multiple isolations of
the same strain (i.e., the same species with the same antibiogram or a closely
related DNA fingerprint) from separate cultures, growth of the strain within 48 h,
and bacterial growth in both aerobic and anaerobic bottles.
Clinical Syndromes
CoNS cause diverse prosthetic device–related infections, including those
that involve prosthetic cardiac valves and joints, vascular grafts, intravascular
devices, and CNS shunts. In all of these settings, the clinical presentation is
similar. The signs of localized infection are often subtle, the rate of disease
progression is slow, and the systemic findings are often limited. Signs of infection,
such as purulent drainage, pain at the site, or loosening of prosthetic implants, are
sometimes evident. Fever is frequently but not always present, and there may be
mild leukocytosis.
Infections that are not associated with prosthetic devices are infrequent,
although native-valve endocarditis due to CoNS has accounted for ~5% of cases in
some reviews. S. lugdunensis appears to be a more aggressive pathogen in this
setting, causing greater mortality and rapid valvular destruction with abscess
formation.
Staphylococcal Infections: Treatment

General Principles of Therapy
Surgical incision and drainage of all suppurative collections constitute the
most important therapeutic intervention for staphylococcal infections. The
emergence of MRSA in the community has increased the importance of culturing
all collections in order to identify pathogens and to determine antimicrobial
susceptibility. Prosthetic-device infections are unlikely to be successfully managed
unless the device is removed. In the limited number of situations in which removal
is not possible or the infection is due to CoNS, an initial attempt at medical
therapy without device removal may be warranted. Because of the well-recognized
risk of complications associated with S. aureus bacteremia, therapy is generally
prolonged (4–8 weeks) unless the patient is identified as being one of the small
percentage of individuals who are at low risk for complications—e.g.,
immunocompetent patients and patients whose S. aureus infection is associated
with a removable focus (such as an IV catheter) and whose device is promptly
removed.
Duration of Antimicrobial Therapy
Debate continues regarding the duration of therapy for bacteremic S. aureus
infections. No carefully controlled, prospective study has addressed this question.
A meta-analysis reviewing studies relevant to this issue concluded that insufficient
information was available to determine which patients were candidates for short-
course therapy (2 weeks rather than 4–8 weeks).
Among the findings associated with an increased risk of complicated
bacteremia are persistently positive blood cultures 48–96 h after institution of
therapy, acquisition of the infection in the community, a removable focus of
infection (i.e., an intravascular catheter) that is not removed, and cutaneous or
embolic manifestations of infection. In those immunocompetent patients for whom
short-course therapy is planned, TEE to rule out endocarditis is warranted since
neither clinical nor laboratory findings are adequate to detect cardiac involvement.
In addition, an aggressive radiologic investigation to identify potential metastatic
collections is indicated. All symptomatic sites must be carefully evaluated.

Choice of Antimicrobial Agents
The choice of antimicrobial agents to treat both coagulase-positive
staphylococcal and CoNS infections has become increasingly problematic because
of the prevalence of multidrug-resistant strains. Data collected by the Centers for
Disease Control and Prevention from intensive care units in the United States
(1988–1998) show a dramatic increase in the number of isolates susceptible only
to vancomycin. This trend is especially apparent with CoNS: >80% of nosocomial
isolates are resistant to methicillin, and these MRSA strains are usually resistant to
most other antibiotics as well. Because the selection of antimicrobial agents for the
treatment of S. aureus infections is similar to that for CoNS infections, treatment
options for these pathogens are discussed together and are summarized in Table
129-3.