Bacterial Microflora of Wild Brown Trout (Salmo trutta), Wild Pike (Esox lucius), and Aquacultured Rainbow Trout (Oncorhynchus mykiss)

Abstract

Initial numbers of bacteria associated with wild (brown trout and pike) and cultured (rainbow trout) freshwater fish as well as with the water in which they were caught were determined. Subsequently, a total of 979 randomly selected 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 samples, the highest level corresponding to psychrotrophs in pike environments (4.23 × 103 CFU/ml). Overall, the skin and intestinal content of brown trout 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 findings in other temperate geographical environments. Biological characteristics (feeding and skin properties) and the use of antimicrobials in aquaculture 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 bacteria, Bacillus, and coryneforms were also found. The groups represented in psychrotrophic isolates from the outer surfaces do not reflected those detected in water, so it was common that those organisms recovered in significant numbers from fish were not detected in surrounding habitat of the fish. Motile aeromonads and Carnobacterium were the dominant psychrotrophs in the guts of pike and brown trout, respectively. The intestinal content of reared fish gave a high incidence of Bacillus and coryneforms, while Enterobacteriaceae was absent. Again, rearing practices could have influenced this finding. Listeria monocytogenes was not detected in any of the examined samples. Two strains of Salmonella, which belonged to the same serovar and lysotype, were recovered from pond-water samples taken from one facility on different sampling days. From the gut of a pike specimen and from the pike's environment, two Plesiomonas shigelloides strains of different serovars were recovered. These latter four strains were resistant to a considerable number of antimicrobial compounds (multiple antibiotic resistance indices > 0.2).