Aeromonas

Ralph D. Feigin

Aeromonas species cause opportunistic infections and are identified increasingly as pathogens in healthy persons. Aeromonas organisms are found as normal flora in nonfecal sewage and can be isolated from rivers, streams, canals, and tap water. These organisms cannot be recovered from water sources in which the saline content approaches that of sea water. Aeromonas can survive readily on work surfaces and can be recovered from moistened paper towels.

Aeromonas organisms are asporogenous, gram-negative, facultatively anaerobic, motile rods that have a single polar flagellum. These organisms are oxidase and catalase-positive and produce acid or gas during carbohydrate fermentation. Aeromonas organisms grow well on blood agar, and most strains produce a large zone of beta-hemolysis on this medium. Aeromonas organisms also grow on Salmonella-Shigella, MacConkey, eosin-methylene blue, and triple sugar-iron media.

Aeromonas organisms are confused most often with Enterobacteriaceae. The oxidase tests aid in differentiation: Aeromonas species generally are oxidase-positive, whereas Entero-bacteriaceae are oxidase-negative. Aeromonas species are susceptible to ceftriaxone, cefamandole, chloramphenicol, gentamicin, fluoroquinolones, and trimethoprim-sulfamethoxazole. Aeromonas species consistently are resistant to penicillin, ampicillin, streptomycin, cephalothin, and carbenicillin.

PATHOGENESIS

A. hydrophila produces alpha and beta-hemolysins that are significant virulent factors in the pathogenesis of A. hydrophila infection. Alpha-hemolysin released from cells can produce dermonecrosis and may be cytotoxic to HeLa cells and human embryonic lung fibroblasts. Beta-hemolysin also may produce dermonecrosis and is cytotoxic to HeLa cells and to human diploid lung fibroblasts. Antibodies to either hemolysin neutralize both toxins.

Aeromonas species elaborate a cytotoxic enterotoxin that stimulates the cyclic adenosine monophosphate-mediated sequence of events in cells. This enterotoxin may cause diarrhea in humans. An association between enterotoxigenicity and multiple drug-resistant isolates has been established. Serum-resistant strains have been shown to cause more fluid accumulation in rabbit ileal loops than do drug-resistant isolates. Aeromonas species also produce endopeptidase, fibrinolysin, leukocidin, proteinase A and B, and staphylolytic enzyme.

Agglutinating, precipitating, and antihemolysin antibodies to A. hydrophila have been detected in patients with systemic Aeromonas infections but not in those with superficial infections. Antihemolysin titers as high as 1:1,280 and agglutinin titers up to 1:640 have been found. A specific opsonizing antibody in normal serum and the normal bactericidal activity of neutrophils are required to prevent invasive A. hydrophila infections.

CLINICAL MANIFESTATIONS

Septicemia caused by Aeromonas has been reported in more than 40 children, but because this infection is not a reportable disease, the total number of affected children is unknown. Although septicemia caused by Aeromonas has occurred in physiologically normal children, most patients have had a disorder known to impair the normal host response to infection. Clinical manifestations of septicemia are similar to those of other gram-negative enteric bloodstream infections. High fever and shock are common manifestations, and ecthyma gangrenosum, seen more commonly in Aeromonas infections, has been described. The reported fatality rate has been 50%, despite the introduction of antibiotic therapy. The high fatality rate may be related to the severity of the underlying disorder and does not reflect an unusual virulence of this microorganism.

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