Detection and distribution of salmonellae in the intestine of warmouth (Lepomis gulosus)
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Members of the genus Salmonella are enteric bacteria that can cause salmonellosis (a disease associated with the gastrointestinal tract, yielding diarrheal symptoms) in both humans and animals. Salmonellosis continues to be a problem for industrialized countries such as the United States where consumption of contaminated, cooked or uncooked food is generally assumed to be the cause of infection with salmonellae. The intestinal tract of vertebrates is generally assumed to be the native habitat of salmonellae from which the feces then contaminate environments such as fresh- or marine waters, estuarine environments, vegetables, compost, or soils and sediments. The occurrence of salmonellae in these environments is therefore frequently linked to environmental contamination through, e.g., manure or wastewater discharges. Salmonellae, however, have also been detected in water and biofilms in pristine aquatic systems, and in the intestine of animals such as turtles, crayfish and fish living in these systems. The detection of salmonellae in the intestine of turtles and fish, and especially in biofilms on the carapace of turtles, however, could not be linked to runoff, and this failure opens the door for speculations on the dissemination and on the fate of salmonellae with respect to short- and long-term population establishment in aquatic ecosystems. In this thesis, I re-evaluated the detection of salmonellae in the intestine of fish, with the specific focus on warmouth (Lepomis gulosus). This evaluation was performed as a seasonal study in order to test three hypotheses:
- Salmonellae can be detected infrequently in the intestine of warmouth, with their detection most likely linked to terrestrial runoff during rainfall events
- Salmonellae are not part of the indigenous intestinal flora, but their presence is associated to food sources
- Food sources generally promote bacterial cell growth in the intestine, with fast-growing copiotrophic bacteria (including salmonellae) dominating the flora in the intestine of warmouth
The study took advantage of the availability of molecular tools, i.e. PCR, and in situ hybridization that allowed detection and quantification of salmonellae directly in the environmental samples (i.e. the intestine content). Salmonellae were detected randomly and associated to food sources in the intestine content of individual fish indicating that they were not established members of the indigenous flora, but rather pass through the intestine with the food. Detection of salmonellae was related to precipitation profiles, as much higher precipitation during fall compared to the remainder of the season resulted in their detection. Cell size of salmonellae associated with food material in the intestine was larger than in pure culture, but reflected cell sizes of the major groups of bacteria present that were dominated by members of the fast-growing copiotrophic Proteobacteria and the CFB phylum.