Combinatorial effects of resazomycins and azithromycin against Neisseria gonorrhoeae

Gonorrhea is a sexually transmitted disease caused by the bacterium Neisseria gonorrhoeae. The WHO estimates 87 million new gonococcal infections occur each year making it the second most common sexually transmitted bacterial infection. When left untreated, gonorrhea can cause pelvic inflammatory disease and infertility in women. Treatment is further complicated by the fact that N. gonorrhoeae has acquired resistance to all antibiotics approved for treatment including the gold standard ceftriaxone. One way to address the growing problem of antibiotic resistance is to administer drugs in combination in the hope the two are more effective together than alone. Resazomycins, a novel family of resazurin (Rz) based antibiotics, exhibit antimicrobial activity against N. gonorrhoeae and Francisella tularensis. Here, we used E-test assays to determine if there was a synergistic bactericidal effect between resazomycins and azithromycin, which is commonly used to treat gonorrhea. Multiple clinical isolates of N. gonorrhoeae were more sensitive to azithromycin in the presence of Rz. However, additional testing in a checkerboard assay revealed no synergistic effect between azithromycin and Rz or another resazomycin, resorufin pentyl ether (RPE). Upon further investigation, we determined the conflicting results between the E-test and checkerboard method were due to differences in bacterial load. When the E-test assays were repeated using a lower inoculum of N. gonorrhoeae, increased sensitivity to azithromycin was not observed in the presence of Rz. In the future, we plan to test resazomycins in combination with other antibiotics against N. gonorrhoeae, particularly extensively drug-resistant strains. (This research was made possible by NIH Grant P20GM103434 to the West Virginia IDeA Network for Biomedical Research Excellence and the NASA West Virginia Space Grant Consortium, Grant # 80NSSC20M0055)