In nature, organisms are constantly associated with a variety of microorganisms. The interactions between these microbes and their host can vary along a continuum of different types, with outcomes ranging from obligate symbiosis to lethal infection. In this spectrum, the microbiota occupies the central and balanced part, covering microbial species that establish commensalistic or mutualistic relationships with each other and with the host organism. There is increasing evidence that the microbiota plays a crucial role in different aspects of the host physiology, and disruption in its composition has been linked to several types of pathological situations.
In order to elucidate the fine-tuned dialogue governing the relationships between the microbiota and its host, we use the model Drosophila melanogaster associated with one of its natural commensals, Lactobacillus plantarum. This rather simple gnotobiotic system allowed us to reveal a growth-promoting effect mediated by L. plantarum in nutritionally challenged drosophila larvae. In case of nutrient scarcity, larvae associated with L. plantarum develop twice faster than the germ-free ones, giving rise to adults of similar weight and size. However, adjusting development to environmental cues is key to organismal fitness, and yet here animals are growing fast even though the nutritional conditions are poor. What seems like an advantage could then be deleterious at later stages, and the L. plantarum-mediated growth acceleration upon nutritional challenge could in turn adversely impact adult fitness.
In this light, we studied different life history traits and showed that adults emerging from the fast-growing, L. plantarum-associated larvae perform as well as their slow-growing axenic siblings. They were able to resist starvation equally well and produced equally numerous and fit progeny. In addition, males kept on a low-yeast diet and associated with L. plantarum had a markedly increased median lifespan in comparison to the germ-free males. We are now investigating the physiological basis of this differential response, by analysing the metabolic state of these males. In summary, L. plantarum acts as a true mutualist of D. melanogaster, as it allows the precocious emergence of mature and fertile adults without fitness drawbacks. Furthermore, in certain conditions, this commensal can even increase the lifespan of nutritionally challenged males.