Predation, Competition, and Nutrient Levels Affect the Survival of Escherichia coli, Enterococci and Enteric Pathogens in Aquatic Habitats

Fecal indicator bacteria (FIB) such as Escherichia coli and enterococci are used to assess microbiological water quality in recreational waters worldwide. FIB are used with the assumption that their presence correlates with that of fecal-associated pathogens in recreational waters. In aquatic habitats, several factors can interfere with the predictive relationship between FIB and pathogens including extended survival of FIB in secondary habitats such as sediment, vegetation and sand. Furthermore, many biotic (e.g. predation from bacterivorous protozoa and competition from indigenous bacteria) and abiotic factors (e.g. temperature, salinity, ultraviolet (UV) light irradiation, and nutrient availability) can influence the fate of FIB and pathogens associated with gastrointestinal tracts of animals (enteric pathogens) in secondary habitats. The relative importance of these factors is not well characterized, thus limiting our knowledge on the efficacy of FIB as indicators of fecal contamination and microbial pathogens in water. The studies presented in this dissertation investigated the influence of biotic (predation from bacterivorous protozoa and competition from indigenous bacteria) and abiotic factors (e.g. nutrient availability) on the survival of FIB (E. coli and Enterococcus faecalis) and pathogens (E. coli O157 and Salmonella enterica) in aquatic habitats. Water and sediment samples were collected from a fresh water river source (Hillsborough River, Tampa, FL) and used to prepare a series of outdoor mesocosm experiments. In each experiment, biota treatments were varied to include various combinations of predation and competition, both or neither. Manipulation of biota treatments involved disinfection of water and baking of sediments to remove indigenous microbiota, or addition of cycloheximide or kanamycin to diminish the effect of predation from natural protozoa or competition from indigenous bacteria respectively. Bacterial levels in all experiments were monitored over a five day period. In the mesocosms investigating the effect of predation and competition on FIB (E. coli and Ent. faecalis) and a pathogen (E. coli O157:H7), predation had a detrimental effect on the survival of the FIB and pathogen in the water column but only influenced the survival of the FIB in the sediment. Unlike predation, competition from indigenous bacteria influenced the survival of E. coli but not Ent. faecalis in both water and sediment. The second set of mesocosms investigated the effect of predation on two motile and non-motile enteric bacteria types (E. coli O157 and S. enterica), each with a motile and non-motile counterpart. An allochthonous predation source (Tetrahymena pyriformis) was added into the mesocosms to supply a consistent level of predation. Motility had a significant positive effect on the survival of S. enterica in the water and sediment but had negative significant effect for E. coli O157 in sediment only. Motility also played a more important role in the sediment compared to predation while predation played a more important role in the water column for both bacteria types. The third study compared the relative effects of predation, competition and nutrients on the survival of E. coli. Natural waters (not amended with nutrients) served as a baseline condition to which organic nutrients were added in two increments. Significant interactions among predation, nutrients and competition (all possible combinations) were observed. Interactions between predation and nutrients as well as competition and predation also accounted for the greatest effects (10% and 8% respectively). The interaction between predation and competition was particularly pronounced at the highest nutrient level. These studies reveal that predation, competition and nutrients are all important factors in the survival of FIB and enteric bacteria in water and sediment, and provide new observations on the relative magnitude of these effects. I show that survival characteristics of FIB and enteric bacteria in secondary habitats can vary depending on bacteria type (FIB or pathogen), location (water or sediment), prey characteristics (motile or non-motile) and specific environmental stressor present (predation, competition or nutrients). The findings of this dissertation provide new insights on the ecology of FIB and enteric bacteria in secondary habitats and underscore the importance of biotic and abiotic factors as determinants of the fate of FIB and enteric bacteria in secondary habitats