Mesophilic aeromonads in wild and aquacultured freshwater fish

Abstract

Numbers and species of motile Aeromonas were determined in freshly caught freshwater fish, in the surrounding environment, and also during iced chilled storage of fish specimens. Although no significant differences were observed in water samples, initial levels for skin, gill, and intestines were significantly lower in farmed rainbow trout (Oncorhynchus mykiss) than in wild brown trout (Salmo trutta) and pike (Esox lucius). During storage of wild specimens, naturally occurring aeromonads grew fairly well on the surfaces of skin and body cavity. Of 171 strains assigned to the genus Aeromonas, 88% were identified to phenospecies and putative genospecies level by using comprehensive biochemical schemes. The isolates were allocated to putative hybridization groups (HGs) 1 and 3 Aeromonas hydrophila (29%); putative HG 8 Aeromonas veronii biovar sobria (19%); putative HG 2 Aeromonas bestiarum (18%); putative HG 9 Aeromonas jandaei (16%); putative HGs 4 and 5a Aeromonas caviae (2%); putative HG 12 Aeromonas schubertii (2%); and putative HG 11 (unnamed, 0.6%). The remaining 20 isolates (12%) resembled A. schubertii but could not be allocated to currently recognized phenospecies or to putative HGs. Although cultured rainbow trout yielded strains of putative HGs 1, 4, and 8, which appear to be of major clinical importance, most isolates assigned to putative HGs 1 and 8 were recovered from pike. Differences among HGs found in wild animals could be related to their origin (unpolluted rivers for brown trout and urban rivers for pike). The recovery of these aeromonads species was not related to sampling site. The initial levels of motile aeromonads, their behavior during storage, and the strong potential spoilage activity of most isolates confirm that these bacteria can contribute to deterioration of iced wild freshwater fish. Although adequate cooking would inactivate motile aeromonads, the high incidence of isolates belonging to gastroenteritis-associated HGs should be regarded as a potential health concern, particularly for susceptible populations when there is a possibility of cross-contamination.