Achromobacter (previously Alcaligenes) xylosoxidans is probably part of the endogenous intestinal flora and has been isolated from a variety of water sources, including well water, IV fluids, and humidifiers. Immunocompromised hosts, including patients with cancer and postchemotherapy neutropenia, cirrhosis, chronic renal failure, and cystic fibrosis, are at increased risk. Nosocomial outbreaks and pseudo-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 A. xylosoxidans bacteremia usually occurs in the setting of pneumonia. Metastatic skin lesions are present in one-fifth of cases. The reported mortality rate is as high as 67%—a figure similar to rates for other bacteremic gram-negative pneumonias.
TREATMENT Achromobacter xylosoxidans Infections
(Table 183e-2) Treatment is based on in vitro susceptibility testing of all clinically relevant isolates; multidrug resistance is common. Meropenem, tigecycline, and colistin are typically the most active agents.
More than 85% of Aeromonas infections are caused by A. hydrophila, A. caviae, and A. veronii biovar sobria. Aeromonas proliferates in potable water, freshwater, and 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. A. caviae is associated with health care–related bacteremia. 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; see Fig. 25e-35) resembling the lesions seen in Pseudomonas aeruginosa infection. This organism causes nosocomial infections related to catheters, surgical incisions, or use of leeches. Other manifestations include necrotizing fasciitis, meningitis, peritonitis, pneumonia, and ocular infections.
TREATMENT Aeromonas Infections
(Table 183e-2) 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), third- and fourth-generation cephalosporins, carbapenems, and aminoglycosides, but resistance has been described to all those agents. Because Aeromonas can produce various β-lactamases, including carbapenemases, susceptibility testing must be used to guide therapy. Antibiotic prophylaxis (e.g., with ciprofloxacin) is indicated when medicinal leeches are used.
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, C. haemolytica, and C. sputigena have been associated with sepsis in immunocompromised hosts, particularly neutropenic patients with oral ulcerations. These species have been isolated from many other sites as well, usually as part of a polymicrobial infection. Most Capnocytophaga infections are contiguous with the oropharynx (e.g., periodontal disease, respiratory tract infections, cervical abscesses, and endophthalmitis).
C. canimorsus and C. cynodegmi are endogenous to the canine mouth (Chap. 167e). Patients infected with these species frequently have a history of dog bites or of canine exposure without scratches or bites. Asplenia, glucocorticoid therapy, and alcohol abuse are predisposing conditions that can be associated with severe sepsis with shock and disseminated intravascular coagulation. Patients typically have a petechial rash that can progress from purpuric lesions to gangrene. Meningitis, endocarditis, cellulitis, osteomyelitis, and septic arthritis also have been associated with these organisms.
TREATMENT Capnocytophaga Infections
(Table 183e-2) Because of increasing β-lactamase production, a penicillin derivative plus a β-lactamase inhibitor—such as ampicillin/sulbactam (1.5–3.0 g of ampicillin every 6 h)—is currently recommended for empirical treatment of infections caused by Capnocytophaga species. If the isolate is known to be susceptible, infections with C. canimorsus should be treated with penicillin (12–18 million units every 4 h). Capnocytophaga is also susceptible to clindamycin (600–900 mg every 6–8 h). This regimen or ampicillin/sulbactam should be given prophylactically to asplenic patients who have sustained dog-bite injuries.
Elizabethkingia meningoseptica (formerly Chryseobacterium meningosepticum) is an important cause of nosocomial infections, including outbreaks due to contaminated fluids (e.g., disinfectants and aerosolized antibiotics) and sporadic infections due to indwelling devices, feeding tubes, and other fluid-associated apparatuses. Nosocomial E. meningoseptica infection usually involves neonates or patients with underlying immunosuppression (e.g., related to malignancy or diabetes). E. meningoseptica has been reported to cause meningitis (primarily in neonates), pneumonia, sepsis, endocarditis, bacteremia, and soft tissue infections. Most published reports have originated from Taiwan. Chryseobacterium indologenes has caused bacteremia, sepsis, and pneumonia, typically in immunocompromised patients with indwelling devices.
TREATMENT Elizabethkingia/Chryseobacterium Infections
(Table 183e-2) These organisms are often susceptible to fluoroquinolones and trimethoprim-sulfamethoxazole. They may be susceptible to β-lactam/β-lactamase inhibitor combinations such as piperacillin/tazobactam but can possess extended-spectrum β-lactamases and metallo-β-lactamases. Susceptibility testing should be performed.
P. multocida is a bipolar-staining, gram-negative coccobacillus that colonizes the respiratory and gastrointestinal tracts of domestic animals; oropharyngeal colonization rates are 70–90% in cats and 50–65% in dogs. P. multocida can be transmitted to humans through bites or scratches, via the respiratory tract from contact with contaminated dust or infectious droplets, or via deposition of the organism on injured skin or mucosal surfaces during licking. Most human infections affect skin and soft tissue; almost two-thirds of these infections are caused by cats. Patients at the extremes of age or with serious underlying disorders (e.g., cirrhosis, diabetes) are at increased risk for systemic manifestations, including meningitis, peritonitis, osteomyelitis and septic arthritis, endocarditis, and septic shock, but cases have also occurred in healthy individuals of all ages. If inhaled, P. multocida can cause acute respiratory tract infection, particularly in patients with underlying sinus and pulmonary disease.
TREATMENT Pasteurella multocida Infections
P. multocida is susceptible to penicillin, ampicillin, ampicillin/sulbactam, second- and third-generation cephalosporins, tetracyclines, and fluoroquinolones. β-lactamase-producing strains have been reported.