Establishing Sodalis species as a laboratory model of endosymbionts

Symbiosis is defined as the close and long relationship between two organisms. Establishment of new symbioses, or redefining relationships underpins much of the ecological diversity found in the natural world. Microbial symbionts, being some of the longest living organisms on the planet with the largest distribution, offer the best opportunity to understand the complex mechanisms behind host-symbiont interactions and evolutionary processes. The insect kingdom, comprised of over 1.2 million described species, is an ideal sample group in symbiont research as over 50% of them harbour microbial symbionts. Glossina spp., the viviparous, obligate blood feeding (tsetse) flies that populate sub Saharan Africa, are of interest within symbiont research as they play host to at least four bacterial symbionts, with diverse phenotypes: Wigglesworthia, Wolbachia, Spiroplasma and Sodalis. Sodalis glossinidius – a secondary endosymbiont - is interesting as sequencing of its genome suggests S. glossinidius has undergone less genome reduction than its primary symbiont counterparts such as Wigglesworthia, and therefore has a more recent association with its host than the other symbionts. The benefit of this reduced rate of genome reduction is the ability to culture S. glossinidius in vitro, a feature that most bacterial symbionts lack. Culture of two Sodalis species – S. glossinidius and a related species, S. praecaptivus, was performed to compare the viability of S. glossinidius to free living bacteria to determine its potential as a laboratory model of symbiosis. This was studied via growth curves in different laboratory media, resistance to oxidative stress, antibiotic susceptibility and survival in an experimental host – Galleria mellonella. The difficulties in culturing bacterial endosymbionts is highlighted; the ability, with care, to culture S. glossinidius, and the potential to compare to closely-related, free-living species such as Sodalis praecaptivus is vital as a research model for studying symbiosis and host interaction