Detection of salmonellae from fish in a natural river system

Sediment, water, and fish gut samples taken at three sites near the headwaters of the San Marcos River, Texas, were analyzed for salmonellae Salmonella spp. by culture and molecular techniques. While enrichment cultures from sediment and water samples from the two uppermost sites were negative for salmonellae in polymerase chain reaction analyses, both sediment and water samples were positive at the downstream site. At all sites, salmonellae were present in the guts of different fishes (e.g., largemouth bass Micropterus salmoides, channel catfish Ictalurus punctatus, common carp Cyprinus carpio, and suckermouth catfish Hypostomus plecostomus). The highest percentage of detection (33% of analyzed fish) occurred at the downstream site, whereas detection percentages at the upper two sites were 18% and 17%. Detection of salmonellae was usually limited to one segment of the gut (i.e., upper or lower part). Serovars were highly variable among individuals and differed between the upper and lower gut in the only individual (a common carp) that had salmonellae in both gut segments. In situ hybridization demonstrated that salmonellae were normally associated with particulate material in the gut and occurred in highly variable numbers ranging from an occasional organism to a majority of the gut microbe population. These results demonstrate the presence of different serovars of potentially human pathogenic salmonellae among four ecologically distinct fishes within natural environments. They also suggest that salmonellae are not components of the indigenous microbial community in fish intestines but rather are ingested with particulate material.