Aeromonas

Serotyping is based upon somatic (O) antigen determinants (Sakazaki and Shimada 1984).

Several typing schema have been proposed (Fricker 1987; Cheasty et al.,,, 1988; Thomas et al.,,, 1990), but only one comparison study of two of these schema has been published (Shimada and Kosako 1991). The schema of Sakazaki and Shimada recognizes 44 serogroups, with an additional 52 provisional serogroups (Albert et al.,, (1995). Aeromonas spp. are found to be serologically heterogeneous, with individual serogroups found in more than one species (Janda et al.,,, 1996). Most type and reference strains were not serologically representative of a genomospecies. Three serotypes predominate in clinical specimens, O:11 (24%), O:16 (14%), and O:34 (10%). Korbsrisate et al.,, (2002) characterized the distribution of A. hydrophila serogroups in clinical specimens and developed polyclonal antibodies for rapid identification of clinical isolates by direct agglutination. Only 50% of strains fell into the common serogroups O:11, O:16, O:18, O:34, or O:83. Rough strains (15.2%) and untypable strains (2.3%) reduced the effectiveness of serotyping for identification of clinical strains. A polyvalent antiserum was produced that resulted in positive agglutination of 102 or 105 strains, for a calculated sensitivity of 97.1% and specificity of 90.7%. This test could be useful in rapid identification of Aeromonas to genus where they are isolated from samples that may also contain vibrios.

1.2.3. Classification: The genus includes at least 13 genospecies, among which are the mesophilic A. hydrophila, A. caviae, A. sobria, A. veronii, and A. schubertii, and the non-motile, psychrophilic A. salmonicida infection. By contrast, the mesophilic species have been associated with a wide range of infections in humans (Janda & Abbott, 1996). Although members of the genus have classically been divided into three biochemically differentiated groups (typified by A. hydrophila, A. caviae, and A. sobria), these contain a number of genospecies, to which new species have been added (Carnahan & Altwegg, 1996). Currently the genus is made up of 17 DNA hybridization groups representing a range of genospecies and phenospecies The mesophilic Aeromonas have been commonly isolated from patients with gastroenteritis although their role in disease causation remains unclear. They are also associated with sepsis and wounds, and with eye, respiratory tract, and other systemic infections (Janda & Duffey, 1988; Janda & Abbott, 1996; Nichols et al.,,, 1996); Many of the systemic infections arise following contamination of lacerations and fractures with Aeromonas-rich waters The species principally associated with gastroenteritis are A. caviae, A. hydrophila, and A. veronii biovar sobria (Joseph, 1996); A. caviae is particularly associated with young children (under 3 years of age).

1.2.4. Sources of Aeromonas spp: Aeromonas are ubiquitous in aquatic environments and readily isolated from both nutrientrich and nutrient-poor environments (Holmes, Niccolls & Sartory, 1996). As they are autochthonous to fresh and marine waters their recovery is to be expected. However, increasing levels of pollution may result in substantially greater populations, and may also affect distribution of the organisms (Holmes, Niccolls & Sartory, 1996). Different physicochemical parameters of water may affect the growth and distribution of Aeromonas in aquatic environment .Two decades from nineteen seventies, effect of physicochemical parameters of water on Aeromonas in the aquatic ecosystem was extensively studied by different investigators. These studies mostly focused on the effect of dissolved oxygen (Seidler et al.,, 1980), temperature (Hazen et al.,,1978), pH (Hazen et al.,,1978) and conductivity (Burke et al.,,1984a) etc. Aeromonas spp. are also found in soil and the feces of