Abstract
The long co-existence of bacteria and protozoa has led to the development of bacterial protozoa resistance strategies, which are suggested to serve as drivers for the evolution of pathogenic bacteria. However, the ecological mechanisms underpinning selection for protozoa-resistance in aquatic bacteria are poorly known. To assess the role of nutrient availability and predation-pressure on selection for protozoa-resisting bacteria (PRB), an enrichment-dilution experiment was designed using laboratory microcosms containing natural lake water. PRB was monitored by screening 16S rRNA amplicon sequence data for reads assigned to bacteria that previously has been shown to resist degradation by amoebae. To estimate the effects of the microbial food web dynamics (microscopy of; heterotrophic bacteria, phytoplankton, protozoa and rotifers) and physicochemical variables on the PRB abundance in the study system, a joint species distribution modelling approach was used. The predation-pressure (ratio between predator and bacterial biomass) had a positive effect on the abundance of the PRB genus Mycobacterium, while perturbation (enrichment and dilution) favored the PRB genus Pseudomonas that dominated the bacterial community in the disturbed systems. Our results show that PRB with different ecological strategies can be expected in water of high and intermediate nutrient levels and after major disturbances of an aquatic system.
Highlights
The potential for transmission of pathogenic bacteria from environmental reservoirs to susceptible hosts depends on the pathogens’ environmental distributions and the hosts’ behaviour
The dissolved organic carbon (DOC) and total nitrogen (TN) concentrations for the highest and lowest nutrient levels differed ~20–30-fold, while TP concentrations differed by a factor of ~10 (Fig. 1)
At nutrient level 3, the DOC and TN concentrations decreased over time, but at lower levels (2 and 1), DOC and TN were stable throughout the experiment
Summary
The potential for transmission of pathogenic bacteria from environmental reservoirs to susceptible hosts depends on the pathogens’ environmental distributions and the hosts’ behaviour. Pathogenic bacteria contain virulence factors that enable them to colonize a niche in their hosts, evade or suppress the host’s immune response, enter and exit host cells, or obtain nutrition from the host Several of these abilities are similar to those of PRB; it is widely believed that many traits that render bacteria pathogenic in susceptible hosts were shaped by evolutionary forces outside the context of human-pathogen interactions, and should be seen as colonization factors that produce “accidental virulence”[15]. In addition to protozoan predation and viral lysis, aquatic bacterial communities are exposed to bottom-up forces such as limited nutrient availability[16,17,18] These communities experience highly variable conditions, creating fluctuations in their selection dynamics i.e. variation in the bottom-up and top-down control of the microbial communities[19,20]. We hypothesized that increased nutrient load would increase the predation-pressure on the bacteria, leading to selection for PRB
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