Protozoan predation drives the adaptive evolution of Vibrio cholerae
- Publication Type:
- Thesis
- Issue Date:
- 2022
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Protozoa are unicellular eukaryotic organisms that play an important role in controlling bacterial population structure and composition in the environment. Heterotrophic protozoa survive by feeding on bacteria. Many pathogenic bacteria are capable of resisting predation and some are able to multiply inside of these hosts. To resist predation, bacteria have evolved many mechanisms or defensive traits and often these traits contribute to the persistence of the pathogen in the environment and give rise to virulence upon encounter with human and animal hosts.
The waterbourne bacterium, 𝘝𝘪𝘣𝘳𝘪𝘰 𝘤𝘩𝘰𝘭𝘦𝘳𝘢𝘦, is the etiological agent of the disease cholera and shares an ecological niche with the free-living amoeba, 𝘈𝘤𝘢𝘯𝘵𝘩𝘢𝘮𝘰𝘦𝘣𝘢 𝘤𝘢𝘴𝘵𝘦𝘭𝘭𝘢𝘯𝘪𝘪. Here, the experimental evolution of the model pathogen 𝘝. 𝘤𝘩𝘰𝘭𝘦𝘳𝘢𝘦 with 𝘈. 𝘤𝘢𝘴𝘵𝘦𝘭𝘭𝘢𝘯𝘪𝘪 was performed for three months with the aim to increase our understanding of the effects of long-term protozoan predation on the evolution of virulence-related traits and how that impacts environmental persistence.
Long-term adaptation with the amoeba host leads to phenotypic and genetic variability in 𝘝. 𝘤𝘩𝘰𝘭𝘦𝘳𝘢𝘦. Late-stage amoeba adapted 𝘝. 𝘤𝘩𝘰𝘭𝘦𝘳𝘢𝘦 showed trade-offs among multiple phenotypic traits that contribute to their enhanced intracellular survival and fitness in amoeba. Whole-genome sequencing and mutational analysis revealed that these altered phenotypes and improved fitness were linked to non-synonymous mutations in conserved regions of the flagellar transcriptional regulator, 𝘧𝘭𝘳𝘈. Transcriptomic analysis of the ∆𝘧𝘭𝘳𝘈 mutant revealed that increased iron acquisition, oxidative stress resistance, and metabolic coordination are also associated with improved intracellular survival and fitness. Additionally, adaptation with the amoeba host result in 𝘝. 𝘤𝘩𝘰𝘭𝘦𝘳𝘢𝘦 isolates that exhibited an increased capacity to colonise zebrafish, establishing a connection between protozoan predation and enhanced environmental persistence.
The results presented here highlight multiple adaptation strategies acquired by the pathogen when under intense grazing pressure. Predation pressure drives the accumulation of beneficial mutations that serve as key drivers of the adaptation process and enhance commensalism with the host protozoa. Further, this study provides an important contribution to the understanding of the adaptive traits that evolve in pathogens under predatory pressure, and how these adaptive traits impact colonisation of eukaryotic hosts.
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