Endosymbionts mediate the effects of antibiotic exposure in the tramp ant Cardiocondyla obscurior

1. Bacterial endosymbionts play a fundamental role in insect ecology. Ants host a large diversity of bacterial symbionts, but comparably little is known about how the loss or reduction of symbionts affects ant fitness. 2. We investigated the effects of the rifampicin, a commonly used antibiotic, on colonies from several populations of the globally distributed tramp ant Cardiocondyla obscurior, which differ in their endosymbiont communities. 3. We found that rifampicin treatment negatively affected queen fecundity and colony productivity, even when there was a delay of 3 months between treatment and productivity assessment. In addition, the viability of sperm from males produced in rifampicin-treated colonies was significantly reduced, pointing towards a trans-generational effect of antibiotics on male ant fitness. As expected, rifampicin treatment also led to a significant decrease in the titres of Candidatus Westeberhardia cardiocondylae and Wolbachia sp., the main bacterial endosymbionts of this ant. 4. The negative effects of antibiotic exposure on ant and symbiont fitness were modulated by the presence and strain of symbiotic bacteria, revealing a complex relationship between the microbiome and ant fitness.