Exploring Gut Microbial Dynamics and Symbiotic Interaction in Blattella germanica Using Rifampicin

SIMPLE SUMMARY: The German cockroach Blattella germanica harbours two types of helpful bacteria: the obligate endosymbiont Blattabacterium, with a well-known function in the host metabolism, and complex gut microbiota, which are acquired mainly through faeces, whose functions still need to be fully understood. Our goal was to understand if and how these two spatially isolated symbionts communicate and interact, by disturbing them with the antibiotic rifampicin. The treatment produced deep changes in the gut microbiota, regardless of faeces addition. After their removal, most, but not all the taxa from the control population were recovered. On the other hand, it was impossible to obtain aposymbiots after treatment with rifampicin during two generations, highlighting the essential role played by the endosymbiont. Thus, quasi-aposymbiotic individuals with a reduced load of endosymbiont was obtained in the second generation and mixed with control individuals in the same environment. The microbiota were not affected by the reduction in endosymbiont. Overall, our results indicate that the gut microbiota cannot replace the essential endosymbiont and that there is no interaction between the two symbiotic systems. ABSTRACT: Blattella germanica harbours two cohabiting symbiotic systems: an obligate endosymbiont, Blattabacterium, located inside bacteriocytes and vertically transmitted, which is key in nitrogen metabolism, and abundant and complex gut microbiota acquired horizontally (mainly by coprophagy) that must play an important role in host physiology. In this work, we use rifampicin treatment to deepen the knowledge on the relationship between the host and the two systems. First, we analysed changes in microbiota composition in response to the presence and removal of the antibiotic with and without faeces in one generation. We found that, independently of faeces supply, rifampicin-sensitive bacteria are strongly affected at four days of treatment, and most taxa recover after treatment, although some did not reach control levels. Second, we tried to generate an aposymbiotic population, but individuals that reached the second generation were severely affected and no third generation was possible. Finally, we established a mixed population with quasi-aposymbiotic and control nymphs sharing an environment in a blind experiment. The analysis of the two symbiotic systems in each individual after reaching the adult stage revealed that endosymbiont’s load does not affect the composition of the hindgut microbiota, suggesting that there is no interaction between the two symbiotic systems in Blattella germanica.