ABSTRACT Fungus-growing termites rely on symbiotic microorganisms to help break down plant material and to obtain nutrients. Their fungal cultivar, Termitomyces , is the main plant degrader and food source for the termites, while gut bacteria complement Termitomyces in the degradation of foodstuffs, fixation of nitrogen, and metabolism of amino acids and sugars. Due to the community complexity and because these typically anaerobic bacteria can rarely be cultured, little is known about the physiological capabilities of individual bacterial members of the gut communities and their associations with the termite host. The bacterium Trabulsiella odontotermitis is associated with fungus-growing termites, but this genus is generally understudied, with only two described species. Taking diverse approaches, we obtained a solid phylogenetic placement of T. odontotermitis among the Enterobacteriaceae , investigated the physiology and enzymatic profiles of T. odontotermitis isolates, determined the localization of the bacterium in the termite gut, compared draft genomes of two T. odontotermitis isolates to those of their close relatives, and examined the expression of genes relevant to host colonization and putative symbiont functions. Our findings support the hypothesis that T. odontotermitis is a facultative symbiont mainly located in the paunch compartment of the gut, with possible roles in carbohydrate metabolism and aflatoxin degradation, while displaying adaptations to association with the termite host, such as expressing genes for a type VI secretion system which has been demonstrated to assist bacterial competition, colonization, and survival within hosts.