Transposon sequencing reveals the essential core gene set and genes enabling gut symbiosis in the insect gut symbiont Caballeronia insecticola
Caballeronia insecticola is a bacterium belonging to the Burkholderia genus sensu lato, able to colonize multiple environments like soils and the gut of the bean bug Riptortus pedestris. To identify the essential genome of a bacterium is a first step in the understanding of its lifestyles. We constructed a saturated Himar1 mariner transposon library and revealed by transposon-sequencing (Tn-seq) that 501 genes (7.9% of the genome) constitute the essential core genome of C. insecticolla for growth in free-living conditions. By comparing essential genome sets of C. insecticolla and five related Burkholderia s.l. species, only 126 common essential genes were identified indicating that a large part of the core essential genome is strain-specific. In order to reproduce specific nutritional conditions that are present in the gut of R. pedestris, we grew the mutant library in minimal media supplemented with various candidate gut nutrients and we identified several condition-dependent fitness-defect genes by Tn-seq. To validate the robustness of the Tn-seq approach, insertion mutants of 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 R. pedestris gut colonization, confirming that gluconeogenic carbon sources, taurine and inositol are nutrients catabolized by the symbiont in the gut.