About 10-20% of Lyme disease patients treated with antibiotics continue to suffer debilitating symptoms, known as post-treatment Lyme disease. It is not understood whether this is due to an inappropriate immune response or continued Borrelia bacterial replication. In vitro laboratory experiments have showed that persister forms of bacteria are difficult to eliminate with antibiotics. This is because persisters are very slow-growing, and most antibiotics target actively-replicating bacteria. Limited clinical observations suggested some efficacy of dapsone, a sulfa drug, which belongs to a class of antibiotics not commonly used to treat Lyme disease. To test this drug against persister forms, Dr. Ying Zhang’s group studied sulfamethoxazole, dapsone, and sulfachlorpyridazine singly as well as in combination. By themselves, these drugs had similar efficacy as doxycycline, with 59-92% of stationary bacteria still alive after treatment. Their bacteria-killing activity was weak compared to daptomycin, which when used at the same concentrations, resulted in 26-52% of remaining cell viability. As combinations, they could not completely eradicate persister bacteria, with 74-77% bacteria remaining viable after treatment. But when they were combined with non-sulfa drugs like doxycycline, cefuroxime, ciprofloxacin, rifampin, and azithromycin, the rate of bacterial killing was higher. The most effective combinations included daptomycin. Further research will identify effective combinations that will more completely eradicate persister bacteria in vitro and in patients.