Final Diagnosis -- Aeromonas hydrophila/caviae infection


Aeromonas hydrophila/caviae infection. The remel RapidID NF plus panel and the Vitek 2 were not able to resolve between A. hydrophila/caviae.



Aeromonas is typically found in aquatic environments, including fresh water, polluted or chlorinated water, brackish water, and occasionally, marine water. The organism may also transiently colonize the gastrointestinal tract; however, it is not considered part of the normal intestinal flora. Transmission can occur by contaminated food (i.e. dairy, meat, produce) or water if disrupted skin or mucosal surfaces come into contact with contaminated areas.

Pathogenesis and Spectrum of Diseases:

Although Aeromonas can cause a clinical picture similar to Vibrio spp., their role in causing infections is not always clear. They can cause diseases in both immunocompetent and immunecompromised individuals. Aeromonas has been linked to causing gastroenteritits, wound infections, bacteremia, and other miscellaneous other infections like endocarditis, meningitis, and osteomyelolitis. Various toxins and factors have been identified, but their specific role in virulence is uncertain.

Laboratory Diagnosis:

Gram Stain:

Aeromonads are gram-negative, straight rods appearing singly or in pairs with or without short chains.

Media and Incubation:

Aeromonas spp. grow well on common laboratory media including blood agar, nutrient, Hecteon enteric agar, MacConkey's, and Luria Bertani. On blood agar, Aeromonas forms circular colonies that are 1-3 mm in diameter that start off grayish in color due to beta-hemolysis and after three days become dark green. DNase culture is positive. Maximal growth is seen when the temperature is between 37 and 44 C.

Colony morphology on:

Blood agar - large, round, raised, opaque; most colonies are beta-hemolyitc except A. caviae

MacConkey agar - typically non-lactose fermenting; however, some lactose fermenting Aeromonas spp. have been observed

Approach to Identification:

Aeromonas species are facultative anaerobes that are oxidase and catalase positive. This test must be performed on media without a fermentable sugar (ie MacConkey agar), because the fermentation process results in acidification of the medium and a false-negative result may result. Biochemical testing can help to distinguish Vibrio spp. from Aeromonas spp. as some commercially available kits have trouble separating these two genera.


There are few studies that have focused on the susceptibility of Aeromonas species to antimicrobial agents. Fluoroquinolones have been shown to be active against isolates of A. hydrophila, A. caviae, A. veronii with MICs of less than 1 g/ml for 90% of the isolates tested. Susceptibility of Aeromonas species to cefotaxime, nalidixic acid, and trimethoprim/sulfamethoxazole have been reported as well. Patients with travelers diarrhea showed varying resistance to chloramphenicol, tetracycline, and cotrimoxazole. Aeromonads also show complete resistance to ampicillin by chromosomally-mediated inducible beta-lactamases.

Wrap-up of the Case:

The Vitek 2 was not able to differentiate between A. hydrophila and A. caviae. In order to resolve the organism further, a Voges-Proskauer test would need to be performed. Aeromonas hydrophila would test positive, while Aeromonas caviae wold not. Sensitivities of the organism showed resistance to ampicillin and sulfamethoxazole/trimethoprim and was sensitive to ceftriaxone, ciprofloxacin, ertapenem, and piperacillin/tazobactam.


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Contributed by Waseem Anani, MD and Rahman Hariri, MD

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