The genus Aeromonas consists of at least 14 species and 13 DNA hybridization groups (HG) or genospecies (Table 1). Aeromonas species can be grouped into two subdivisions:
1. Psychrophilic group: A. salmonicida . The only species in this group. It is a fish pathogen, nonmotile and do not grow at 37 o C. A. salmonicida have not been isolated from humans and, therefore, is not important in clinical microbiology.
2. Mesophilic group: A. hydrophila group. The organisms are motile and grow at 37 o C. It can be divided into 3 principal phenotypic groups (Phenons) which are equivalent to the species A. hydrophila , A. caviae and A. sobria (Table 1).
A number of putative virulence factors have been implicated in the patho-genesis of Aeromonas species. Genes that code for such factors have been described. These virulence markers are important to distinguish between potentially pathogenic and non-pathogenic strains. Aeromonas virulence factors include:
Motility and lateral (polar) flagella:
Recent studies have shown that lateral flagella production by Aeromonas is a pathogenic feature due to its enhancement of cell adherence, invasion and biofilm formation.
Septicaemia: Mainly in immunocompromised individuals (e.g. patients with hepatobiliary disease).
Cellulitis and wound infections: Infections associated with exposure to contaminated water or after alligator bite. The infection usually results in gangrene-like syndrome. Also, in patients who went treatment with medicinal leech.
Diarrhoea: Most commonly watery in consistency and sometimes cholera-like of short duration. Occur in all ages but mainly in children less than 3 years. Several reports associated Aeromonas species with travelers’ and chronic diarrhoea.
Food poisoning: A number of food poisoning outbreaks have been reported.
Others: A wide range of infections that include: meningeal, sore throat, urinary tract, ear, endocarditis, septicemia, etc..
A wide range of other media are available commercially.
A loopful of liquid stool specimen is added to 10 ml APW and incubated at 37 ° C. After overnight incubation a loopful from APW is plated onto BA and another loopful onto ABA and incubated at 37 ° C overnight.
10 ml of blood is inoculated into blood bottles and incubated at 37 ° C for up to 2 weeks. When growth is observed a loopful of culture is plated on BA and incubated at 37 ° C overnight.
A wound swab is plated directly on blood agar and on ampicillin blood agar (15mg/L) and the swab is then placed in APW. After incubation overnight a loopful from APW is plated on BA and another loopful onto ABA. Plates are incubated overnight at 37 ° C.