Bacterial microflora of wild brown trout (Salmo trutta), wild pike (Esox lucius), and aquacultured rainbow trout (Oncorhynchus mykiss)

Initial numbers of bacteria associated with wild (brown trout and pike) and cultured (rainbow trout) freshwater fish as well as with the water in whic h they were caught were determined. Subsequently, a total of 979 randomly s elected isolates were characterized and identified to the genus level. For all counts performed (aerobes, psychrotrophs, anaerobes, Enterobacteriaceae , and enterococci), no significant differences were observed in water sampl es, the highest level corresponding to psychrotrophs in pike environments ( 4.23 X 10(3) CFU/ml). Overall, the skin and intestinal content of brown tro ut were the most contaminated, while rainbow trout specimens (gills and gut ) yielded the lowest numbers. For all bacterial groups, pike gills had the highest numbers. Counts for all of the sampling sites compare well with fin dings in other temperate geographical environments. Biological characterist ics (feeding and skin properties) and the use of antimicrobials in aquacult ure might have influenced these results. Motile and nonmotile aerobic gram- negative bacteria together with Enterobacteriaceae accounted for 50 to 70% of the psychrotrophs isolated from water. Micrococcaceae, lactic acid bacte ria, Bacillus, and coryneforms were also found. The groups represented in p sychrotrophic isolates from the outer surfaces do not reflected those detec ted in water, so it was common that those organisms recovered in significan t numbers from fish were not detected in surrounding habitat of the fish. M otile aeromonads and Carnobacterium were the dominant psychrotrophs in the guts of pike and brown trout, respectively. The intestinal content of reare d fish gave a high incidence of Bacillus and coryneforms, while Enterobacte riaceae was absent. Again, rearing practices could have influenced this fin ding. Listeria monocytogenes was not detected in any of the examined sample s. Two strains of Salmonella, which belonged to the same serovar and lysoty pe, were recovered from pond-water samples taken from one facility on diffe rent sampling days. From the gut of a pike specimen and from the pike's env ironment, two Plesiomonas shigelloides strains of different serovars were r ecovered. These latter four strains were resistant to a considerable number of antimicrobial compounds (multiple antibiotic resistance indices > 0.2).