Sodalis glossinidius is a Gram-negative, nonspore-forming, rod-shaped, filamentous bacteria. It has flagella expressed during immature host developmental stages, though it is non-motile(10). It is one of three endosymbionts for the tsetse fly (Glossina spp.), which are all maternally transmitted to progeny(6,7). It has a mutualistic relationship with the tsetse fly. This secondary endosymbiont resides inter- and intracellularly in the midgut, fat body and haemolymph of the insect. S. glossinidius was the first true insect endosymbiont to be isolated and cultured, achieved in 1999 using a mosquito (Aedes albopictus) feeder cell culture system and a sample from the haemolymph of Glossinia morsitans morsitans(7). The type strain M1T was isolated(7). S. glossinidius can grow intracellularly in Aedes albopictus or axenically in semidefined solid medium containing already enzimatically digested proteins as a nitrogen source. This microaerophilic bacterium grows optimally with 5% oxygen and 95% carbon dioxide at 25 °C. Colony morphology is uniform, with defined edges, and off-white(7).
The tsetse fly is a vector for Trypanosoma brucei gambiense and Trypanosoma brucei rhodesiense, parasitic protozoa that cause African trypanosomiasis (commonly known as African Sleeping Sickness), an epidemic fatal disease plaguing sub-Saharan Africa(9). S. glossinidius is important enough to have its genome sequenced because this it is a key player involved in the host-parasite relationship, possibly enabling the creation of tsetse flies that are incapable of transimtting trypanosomes and thus bringing an end to African sleeping sickness (See Ecology for more details)(6). The genome of S. glossinidius also provides an example of an organism that has undergone degenerative adaptations in order to adopt a mutualistic relationship with its host (See Genome structure for more details)(3).
Three phylogenetically distinct type-III secretion systems (TTSS) referred to as Sodalis symbiosis regions (SSRs) designated SSR-1, SSR-2, and SSR-3(10). When assembled on the surface of the bacterial cell, the multicomponent TTSS functions as a molecular syringe so the bacteria can inject effector proteins into the host cell and then to facilitate their its entry(3). While core components of the needle structure were conserved, regions coding for some effectors or regulators have been modified or eliminated(10). SSR-1 is similar to the pathogenicity island ysa from Yersinia enterocolitica, which is involved in entry into the host cell. SSR-2 is similar to the Salmonella pathogenicity island SPI-1 and Mxi-Spa from Shigella flexneri, which are involved in post-infection processes once the bacteria has already entered. SSR-3 is similar to SPI-2, which is found in both Salmonella spp. and Yersinia pestis, and is crucial for survival and virulence(5,10).
S. glossinidius has retained functional pathways for glycolysis, gluconeogenesis, the TCA cycle and the pentose phosphate pathway of free-living bacteria(10). Many energy metabolism and carbon compound assimilation genes are missing in the genome. This may indicate an adaptation to the low concentration and lack of diversity of carbohydrates available due to vertebrate blood being the only nutrient of the tsetse host(2,5). The pathways to synthesize all amino acids except alanine are present(10). Most genes involved in energy production and conversion via anaerobic respiration have been lost. Which is expected in the constant microaerophilic environment of the tsetse midgut and haemolymph.(10) In its iron-rich environment, iron transport across the cytoplasmic membrane is achieved through active transport using an ABC transporter(10). It also has an intact FeoA ferrous iron importer. In addition, plasmid pSG1 has a system of siderophores to transport and store iron(4).