The oxidizing agent, paraquat, is more toxic to Wolbachia than to mosquito host cells

Cultured cells provide an important in vitro system for examining metabolic interactions between the intracellular bacterium, Wolbachia pipientis, and its insect hosts. To test whether Wolbachia-associated changes in antioxidant activities could provide a tool to select for infected cells, we tested the effects of paraquat (PQ) on Aedes albopictus mosquito cells. Like mammalian cells, mosquito cells tolerate PQ over a wide range of concentrations, and for considerable lengths of time, depending on cell density at the time of treatment. When mosquito cells were plated at low density and allowed to grow in the presence of PQ, we measured an LC₅₀ of approximately 1–2 μM. Unexpectedly, cells persistently infected with Wolbachia strain wStr, from the planthopper Laodelphax striatellus, grew to higher densities in the presence of 1.5 μM PQ than in its absence. This effect of PQ was similar to the improved growth of host cells that occurs in the presence of antibiotics that suppress the Wolbachia infection. A more detailed examination of growth and metabolic sensitivity indicated that wStr is about 10-fold more sensitive to PQ than the mosquito host cells. Microscopic examination confirmed that Wolbachia levels were reduced in PQ-treated cells, and DNA estimates based on the polymerase chain reaction (PCR) indicated that Wolbachia abundance decreased by approximately 100-fold over a 10-d period. Although Wolbachia genomes encode superoxide dismutase, inspection of annotated genomes indicates that they lack several genes encoding products that ameliorate oxidative damage, including catalase, which converts the PQ byproduct, hydrogen peroxide, to molecular oxygen and water. We suggest that loss of multiple genes that participate in repair of oxidative damage accounts for increased sensitivity of Wolbachia to PQ, relative to its host cells.