Transposon sequencing reveals the essential gene set and genes enabling gut symbiosis in the insect symbiont .

is a bacterium belonging to the genus , which is able to colonize multiple environments like soils and the gut of the bean bug . We constructed a saturated mariner transposon library and revealed by transposon-sequencing that 498 protein-coding genes constitute the essential genome of for growth in free-living conditions. By comparing essential gene sets of and seven related strains, only 120 common genes were identified, indicating that a large part of the essential genome is strain-specific. In order to reproduce specific nutritional conditions that are present in the gut of , we grew the mutant library in minimal media supplemented with candidate gut nutrients and identified several condition-dependent fitness-defect genes by transposon-sequencing. To validate the robustness of the approach, insertion mutants in six fitness genes were constructed and their growth deficiency in media supplemented with the corresponding nutrient was confirmed. The mutants were further tested for their efficiency in gut colonization, confirming that gluconeogenic carbon sources, taurine and inositol, are nutrients consumed by the symbiont in the gut. Thus, our study provides insights about specific contributions provided by the insect host to the bacterial symbiont.