| dc.description.abstract | Predator-prey interactions pervade all ecosystems at all levels of biological organization. Ample research using higher eukaryotes has demonstrated the role of predator-prey interactions on the evolution of ecological and physiological processes, but recently the importance of predation within the microbial world has been increasingly recognized. In addition to eukaryotic nematodes, protists and amoebae, a variety of bacteria that differ greatly in their modes of predation and phylogenetic breadth of their prey feed on bacterial cells.
M. xanthus is a bacterial predator frequently present in the soil microbiome. It uses both contact-dependent (secretion systems) and contact-independent (antibiotics, enzymes, secondary metabolites) modes of prey-killing strategies. I hypothesised that the use of diversity of antimicrobial strategies -including antibiotics- by M. xanthus might influence the evolution and abundance of antibiotic resistance in nature. Additionally, the effect of M. xanthus predation on the evolution of prey bacteria in their local environment is likely to be strongly affected by abiotic factors.
Thus the questions answered during my PhD revolved around two distinct themes. First (Objective 1), the effect of predation on the evolution and ecology of antibiotic resistance. Second (Objective 2) , the effects of abiotic factors on the evolution and ecology of microbial predator-prey interactions. To do so, using a combination of microbiological assays and laboratory evolution experiment, I studied both natural multispecies bacterial communities and synthetic two species bacterial communities.
Objective 1: To demonstrate the effects of M. xanthus predation on the evolution and ecology of antibiotic resistance in prey bacteria
Since the emergence and spread of drug resistance is a major threat to the healthcare system, globally. We investigated whether the predatory bacterium M. xanthus (which uses antibiotics as one of the prey-killing mechanisms) can also influence the abundance of antibiotic-resistant isolates in its local soil communities. We observed an association between the presence of M. xanthus and a higher frequency of resistant isolates in natural microbial communities. Interestingly, we show that this enrichment of resistance in non-myxobacterial species is brought about by the death of M. xanthus within these communities. This is because starvation-induced death in M. xanthus population results in the release of growth-inhibitory molecules, that further result in the toxification of the environment, thereby enriching the resistant isolates. Therefore, the soil communities that harbour M. xanthus also have a higher abundance of antibiotic-resistant bacteria.
In addition to the enrichment of pre-existing resistances, our results demonstrate that pre-adaptation to M. xanthus can constrain the evolvability of antibiotic resistance in prey bacterium, E. coli. Furthermore, coevolution with M. xanthus also influences the de novo evolution of antibiotic resistance in prey when the environment is contaminated with sub-inhibitory concentrations of antibiotics. Although the outcome of this effect is idiosyncratic and largely depends on both the nature of antibiotics used as well as the prey bacterium, our results suggest that coevolution with M. xanthus has either a negative or neutral effect on the de novo evolution of antibiotic resistance in prey bacteria.
Objective 2: To demonstrate that ecological factors can strongly influence bacterial predator-prey interactions dynamics and determines their evolutionary outcomes
Microbial populations frequently undergo mixing events and such population mixing strongly influences microbial interaction dynamics. However, the effects of repeated population mixing on the evolutionary outcome of prey-predator interaction dynamics are largely unexplored. Hence, we conducted a laboratory evolution experiment with bacterial predator-prey communities under two transfer regimens: repeated mixing (horizontal transfer) versus no mixing (vertical transfer). For this, we used Myxococcus xanthus as the generalist predator and Escherichia coli as prey. We show that prey populations from the vertical regimen were less resistant to predation than the ones from the horizontal regimen. This is because of the two distinct evolutionary trajectories of the vertically and horizontally evolved prey population. The differences in the outcomes of the two treatments were because the variants better at the intraspecies competition can only be maintained in the vertical treatment, whereas in horizontal treatment the benefits of superior intraspecies competitive fitness are diminished because of population mixing. Moreover, we show asymmetrical evolution between the prey and predator population, where contrary to our expectation, we observed the evolution of M. xanthus isolates with lesser predatory efficiencies. Therefore, we demonstrate that mixing strongly influences the evolution of prey populations when coevolved with a social predatory bacteria.
Finally, we show that the nature of the interaction dynamics between M. xanthus and its prey is strongly influenced by abiotic conditions like nutrient availability. Where in nutrient-rich conditions the prey can alter the environment as a result of accumulation of metabolic by-products. This altered environment can further result in the inhibition of the predator’s growth. Thus, reversing the identities of the exploited partner in the interaction dynamics.
Together, my work shows that bacterial predators play an essential role in shaping the eco-evolutionary trajectories of their prey in microbial communities. Importantly, the work presented in this thesis highlights that understanding the microbial predator-prey interactions is essential for our understanding of the evolution and maintenance of antibiotic resistance in natural microbial communities. | en_US |
