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