In Vitro Vehicle Effects on Borrelia burgdorferi as Indicated by ATP-Dependent Metabolic Activity and Cell Counts

The causative agent of Lyme disease, Borrelia burgdorferi, poses significant health and economic problems in the United States and elsewhere. Developing borreliastatic or borreliacidal agents requires the use of solvent vehicles. Vehicle tolerance can affect the interpretation of antimicrobial screening tests. We examined the effects of dimethyl sulfoxide, sodium hydroxide, and ethanol on B. burgdorferi growth. Spirochaetes were exposed to vehicle concentrations ranging from 0.15% to 2.4% v/v. Cultures were evaluated with dark-field microscopy and a commercial cell viability assay. There were no significant decreases in cell counts or ATP-dependent luminescence with dimethyl sulfoxide. Sodium hydroxide was shown to be unsuitable as an unbuffered alkaline vehicle. Ethanol produced discordant ATP-dependent luminescence and cell-count endpoints at moderate concentrations, although detectable cell presence persisted through 1.2%. Differences between endpoints may be due to cellular stress from exposure. These results show that the identity and concentration of these vehicles differentially affect B. burgdorferi growth and ATP-dependent metabolic activity. The applicability of high-throughput methods may be affected by conditions that cause stress-induced changes. Future studies should explore how molecular effects relate to observed differences in cell counts and metabolic activity.