Chapter 140. Infections Due to the HACEK Group and Miscellaneous Gram-Negative Bacteria (Part 3) pot

Chapter 140. Infections Due to the HACEK Group
and Miscellaneous Gram-Negative Bacteria
(Part 3)

Other Gram-Negative Bacteria
Achromobacter xylosoxidans
A. xylosoxidans (previously Alcaligenes xylosoxidans) is probably part of
the endogenous intestinal flora and has been isolated from water sources.
Immunocompromised hosts, including patients with cancer and post-
chemotherapy neutropenia, cirrhosis, and chronic renal failure, are at increased
risk. Nosocomial outbreaks of A. xylosoxidans infection have been attributed to
contaminated fluids, and clinical illness has been associated with isolates from
many sites, including blood (often in the setting of intravascular devices).
Community-acquired bacteremia with A. xylosoxidans usually occurs in the setting
of pneumonia. Metastatic skin lesions are present in one-fifth of cases. The
reported mortality rate is 67%—a figure similar to rates for other bacteremic
gram-negative pneumonias.
Achromobacter xylosoxidans Infections: Treatment
Treatment is based on in vitro susceptibility testing of all clinically relevant
isolates.
Aeromonas Species
More than 85% of Aeromonas infections are caused by A. hydrophila, A.
caviae, and A. veronii biovar sobria. Aeromonas proliferates in potable and fresh
water and in soil. It remains controversial whether Aeromonas is a cause of
bacterial gastroenteritis; asymptomatic colonization of the intestinal tract with
Aeromonas occurs frequently. However, rare cases of hemolytic-uremic syndrome
following bloody diarrhea have been shown to be secondary to the presence of
Aeromonas.
Aeromonas causes sepsis and bacteremia in infants with multiple medical
problems and in immunocompromised hosts, particularly those with cancer or
hepatobiliary disease. Aeromonas infection and sepsis can occur in patients with

trauma (including severe trauma with myonecrosis) and in burn patients exposed
to Aeromonas by environmental (freshwater or soil) contamination of their
wounds. Reported mortality rates range from 25% among immunocompromised
adults with sepsis to >90% among patients with myonecrosis. Aeromonas can
produce ecthyma gangrenosum (hemorrhagic vesicles surrounded by a rim of
erythema with central necrosis and ulceration) resembling the lesions seen in
Pseudomonas aeruginosa infection. Aeromonas causes nosocomial infections
related to catheters, surgical incisions, or use of leeches. Other manifestations
include meningitis, peritonitis, pneumonia, and ocular infections.

Aeromonas Infections: Treatment
Aeromonas species are generally susceptible to fluoroquinolones (e.g.,
ciprofloxacin at a dosage of 500 mg every 12 h PO or 400 mg every 12 h IV),
trimethoprim-sulfamethoxazole (TMP-SMX; trimethoprim dosage, 10 mg/kg per
day in 3 or 4 divided doses), third-generation cephalosporins, and
aminoglycosides. Because Aeromonas can produce various β-lactamases,
including carbapenemases, susceptibility testing must be used to guide therapy.

Capnocytophaga Species
This genus of fastidious, fusiform, gram-negative coccobacilli is
facultatively anaerobic and requires an atmosphere enriched in carbon dioxide for
optimal growth. C. ochracea, C. gingivalis, and C. sputigena have been associated
with sepsis in immunocompromised hosts, particularly neutropenic patients with
hematologic malignancy, and probably play a role in localized juvenile
periodontitis in the immunocompetent host. These species have been isolated from
many other sites as well, usually as part of a polymicrobial infection.
C. canimorsus and C. cynodegmi are endogenous to the canine mouth
(Chap. e14). Patients infected with these species frequently have a history of dog
bites or of exposure to dogs without scratches or bites. Asplenia, glucocorticoid
therapy, and alcohol abuse are predisposing conditions that can be associated with

fulminant infections. C. canimorsus causes a wide range of infections, including
severe sepsis with shock and disseminated intravascular coagulation, meningitis,
endocarditis, cellulitis, and septic arthritis.

Capnocytophaga Infections: Treatment
Because of increasing β-lactamase production, clindamycin (600–900 mg
every 6–8 h) or drug combinations including a penicillin derivative plus a β-
lactamase inhibitor—such as ampicillin/sulbactam (1.5–3.0 g of ampicillin every 6
h)—are currently recommended for empirical treatment of infections caused by C.
ochracea, C. gingivalis, and C. sputigena. Infections with C. canimorsus should
be treated with penicillin (12–18 million units every 4 h). This regimen or
ampicillin/sulbactam should be given prophylactically to asplenic patients
sustaining dog-bite injuries. C. canimorsus is also susceptible to clindamycin,
imipenem, fluoroquinolones, and third-generation cephalosporins.