The impact of strain-level diversity on microbiota-mediated protection against pathogens
All multicellular organisms live in association with microorganisms that form a complex microbial network consisting of bacteria, archaea, viruses, and fungi, the so-called microbiome (Turnbaugh et al., 2007). In recent years, the human microbiota and its impact on human health have increasingly become a focus of biological and medical research (Bäckhed et al., 2005; Hooper et al., 2002). The protective effects of the microbiota are widely spread across animal-microbe associations and are also present in the nematode Caenorhabditis elegans and its symbiont, Pseudomonas lurida MYb11 (Kissoyan et al., 2019). C. elegans is a widely studied model organism, and its natural microbiota has been characterized in recent years (Dirksen et al., 2020). P. lurida is a natural and beneficial member of the nematode microbiota and was studied in an evolutionary experiment (Obeng et al., 2023). During a biphasic life cycle, comprising a free-living and a host-associated phase, P. lurida diversified into different isolates with diverse morphologies and altered characteristics of these so-called morphotypes. This thesis subsequently investigated the consequences of the bacterial lifestyle evolution of P. lurida on microbiota-mediated protection. I was able to show that the morphotypes differ in their microbiota-mediated protection of the worm against infection with the pathogen Bacillus thuringiensis, with two trends: increased and decreased protection compared to the wildtype P. lurida. Furthermore, I demonstrated that properties that can be associated with protection, such as colonization and growth capacity, differ between morphotypes. This shows that evolutionary adaptation processes of bacteria can alter protective properties and thereby influence the health and survival of the host.
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