The endosymbiont Wolbachia rebounds following antibiotic treatment

Antibiotic treatment has emerged as a promising strategy to sterilize and kill filarial nematodes due to their dependence on their endosymbiotic bacteria, Wolbachia. Several studies have shown that novel and FDA-approved antibiotics are efficacious at depleting the filarial nematodes of their endosymbiont, thus reducing female fecundity. However, it remains unclear if antibiotics can permanently deplete Wolbachia and cause sterility for the lifespan of the adult worms. Concerns about resistance arising from mass drug administration necessitate a careful exploration of potential Wolbachia recrudescence. In the present study, we investigated the long-term effects of the FDA-approved antibiotic, rifampicin, in the Brugia pahangi jird model of infection. Initially, rifampicin treatment depleted Wolbachia in adult worms and simultaneously impaired female worm fecundity. However, during an 8-month washout period, Wolbachia titers rebounded and embryogenesis returned to normal. Genome sequence analyses of Wolbachia revealed that despite the population bottleneck and recovery, no genetic changes occurred that could account for the rebound. Clusters of densely packed Wolbachia within the worm’s ovarian tissues were observed by confocal microscopy and remained in worms treated with rifampicin, suggesting that they may serve as privileged sites that allow Wolbachia to persist in worms while treated with antibiotic. To our knowledge, these clusters have not been previously described and may be the source of the Wolbachia rebound.
Author summary Onchocerciasis (river blindness) and lymphatic filariasis (elephantiasis) are two neglected tropical diseases caused by filarial nematodes, which harbor the endosymbiotic bacteria, Wolbachia. Major efforts to discover new drugs to treat these diseases have led to the discovery of novel compounds including those that target Wolbachia. We investigated the long-term effects of rifampicin on the filarial nematode, Brugia pahangi, and its endosymbiont, Wolbachia, in an in vivo rodent model of infection. Initially, Wolbachia titers were significantly reduced by 95% and female fecundity was impaired shortly after treatment. 8 months later however, Wolbachia rebounded and embryogenesis returned to normal. Sequence analysis of the Wolbachia genome revealed that despite the population bottleneck and recovery, no genetic changes occurred that could account for the rebound. Clusters of Wolbachia were observed within the ovaries of female worms throughout the entire 8-month study. These clusters may sequester Wolbachia and allow the bacteria to persist during antibiotic treatment, thereby enabling them to repopulate ovarian tissues and ensuring their vertical transmission to future generations of microfilariae.