Type VI secretion system-mediated antagonism in the honey bee gut microbiota
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Gram-negative bacteria, including animal symbionts, often use type VI secretion systems (T6SSs) to inject toxins into nearby competitors. The honey bee gut microbiota consists of a small number of highly coevolved species, which may use T6SSs to interact competitively with one another and with pathogens. This dissertation explores the diversity, evolution, and function of T6SSs and associated toxins in the bee gut. Chapter 1 identifies genes encoding two T6SSs in bee symbiont Snodgrassella alvi, which are more highly expressed in the bee gut. S. alvi strains encoding Sa-T6SS-1 also encode many Rhs toxin genes, which have been horizontally transferred between S. alvi and co-occurring symbiont Gilliamella apicola. These toxins diversify through recombination, or “C-terminal displacement,” resulting in long arrays of orphaned 3 ends, which lack secretion domains, but are expressed and encode functional proteins. Chapter 2 provides a broader perspective on the diversity and evolution of T6SSs within bee gut symbionts, which have co-diversified with each other and their hosts for more than 80 million years. A survey of 198 isolate genomes identified 5 unique T6SS loci, which are likely to have been present within the microbiota of the common ancestor of social bees, and revealed that co-occurring species have adopted different strategies for toxin diversification. Furthermore, S. alvi uses a T6SS to antagonize a wide range of bee gut symbionts. Chapter 3 shows that opportunistic pathogen Serratia marcescens is rapidly eliminated from the guts of bees with a conventional gut microbiota, an effect which is not fully attributable to any single gut symbiont. From microscopy images, S. alvi may also serve as a physical barrier against invasion. S. marcescens encodes a T6SS used to antagonize E. coli and other S. marcescens strains, but not bee gut isolates, and T6SSs appear not to benefit S. marcescens in the bee gut. Together, these findings indicate that multiple bee gut symbionts engage in T6SS-mediated antagonism using conserved T6SSs and diverse toxins. An opportunistic pathogen is excluded, regardless of whether or not it has a T6SS, suggesting that coevolution may be important for effective antagonism.