The genus Aeromonas consists of gram-negative rods (Fig.1) widely distributed in freshwater, estuarine, and marine environments. The organisms cause a wide spectrum of disease syndromes among warm and cold-blooded animals.
Diarrheal disease is the most common manifestation of Aeromonas infection. The organism has also been associated with a variety of extraintestinal presentations, including wound infections, bacteremia and rarely meningitis. Necrotizing fasciitis has also been reported with species such as A. hydrophila, A. veronii biovar sobria, A. schubertii, and A. caviae. Aeromonads are not routinely identified in most microbiology laboratories as part of the normal protocol for isolating stool pathogens unless there’s a high index of clinical suspicion.
For cases in which Aeromonas is clinically suspected, the laboratory must be advised to look for this organism. Automated identification systems can readily identify most true Aeromonas isolates to the level of A. hydrophila group or A. hydrophila/A. caviae. However, these identifications are often incomplete or erroneous due to insufficient discriminatory markers to detect interspecies differences.
Aeromonads grow quite readily on most media used for both routine cultures and stool cultures. Hemolysis is variable on blood agar media; most species display beta hemolysis (Figure 2 - Aeromonas on a blood plate). Although aeromonads grow on nearly all enteric media, they often are overlooked on MacConkey agar.
Aeromonas spp are oxidase-positive, polar flagellated, glucose-fermenting, facultatively anaerobic, gram-negative rods that are unable to grow in 6.5 percent NaCl. The presumptive identification of an isolate involves the initial separation from other oxidase-positive genera such as Vibrio and Plesiomonas to avoid misidentification. This can be accomplished with simple tests such as O/129 susceptibility, tolerance to various NaCl broth concentrations, and the ability to ferment inositol.
Antimicrobial resistance markers and susceptibility studies should be determined by either the standard agar dilution method or by Kirby-Bauer disk diffusion method using the CLSI Standard M45-A2 for Aeromonas species (1). Some automated MIC systems, such as BioMerieux Vitek, may be used.
Antimicrobial susceptibility — Clinical studies have demonstrated differences in antimicrobial susceptibility between species, highlighting the importance of both species identification and susceptibility testing for all isolates, particularly in the setting of serious infection. Most Aeromonas species produce an inducible chromosomal beta-lactamase, which may not be detected by rapid commercial susceptibility systems. Aeromonads produce beta-lactamases from three different classes; a class C cephalosporinase, a class D penicillinase, and a class B metallo-beta-lactamase (MBL). Two other MBLs (VIM and IMP) in strains of A. hydrophila and A. caviae have also been detected. Most Aeromonas strains are resistant to penicillin, ampicillin, carbenicillin, and ticarcillin; most are susceptible to trimethoprim-sulfamethoxazole (TMP-SMX), fluoroquinolones, and third generation cephalosporins, aminoglycosides, carbapenems.
You can read more about posttraumatic brain abscesses caused by Aeromonas hydrophilia in this article: Posttraumatic Brain Abscess Caused by Aeromonas hydrophila, by Yesholata Mahabeer, Amanda Khumalo, Erastus Kiratu and Koleka Mlisana, Journal of Clinical Microbiology.
Ref (1): CLSI. Methods for Antimicrobial Dilution and Disk Susceptibility Testing of Infrequently Isolated Fastidious Bacteria; Approved Guideline—Second Edition. CLSI document M45-A2. Wayne, PA: Clinical and Laboratory Standards Institute.
This article was first published on Global Health Microbiology.