Asiatic acid, a novel ciprofloxacin adjuvant inhibits Shigella flexneri infection

Bacterial infection caused by intracellular pathogens such as Shigella flexneri is a rapidly increasing global health concern that requires urgent and necessary action. The dearth of licensed vaccines against shigellosis and the decline in susceptibility to conventional antibiotics has encouraged the development of new antibiotic principles and drugs. The treatment options are decreasing faster than the discovery rate of new antibacterial agents. Combinatorial approach of antibiotics with non-antibiotic adjuvants is a promising aspect to treat resistant bacterial infections. Asiatic acid, a membrane-disrupting triterpenoid with wide antimicrobial and immunomodulatory properties, can potentiate antibiotics, but the exact mechanisms remain broadly unexplored. Therefore, in this study, we screened the interaction of asiatic acid with several antibiotics. The results showed synergistic interactions of asiatic acid with antibiotics against susceptible and multidrug-resistant S. flexneri clinical isolates. Particularly important was the interaction of asiatic acid with the quinolone antibiotics ciprofloxacin and nalidixic acid. A detailed study showed that combined treatment of asiatic acid with ciprofloxacin inhibited S. flexneri biofilm formation and resistance development. An increase in membrane disruption and depolarization upon co-treatment was evident by surface electron and confocal microscopy. In addition, asiatic acid and ciprofloxacin synergism was identified to inhibit efflux activity and intracellular bacterial viability. However, asiatic acid showed no synergistic toxicity with ciprofloxacin towards mammalian cells. The antibacterial activity was further verified in a S. flexneri infected mice model. Therapeutic benefits were evident with reduced bacterial burden, recovery from intestinal tissue damage and increase in mice survivability. The results showed that this combination can target the bacterial membrane, efflux pump proteins and biofilm formation, thereby preventing resistance development. The combination treatment offers a proof of concept in targeting essential bacterial activities and might be developed into a novel and efficient treatment alternative against S. flexneri.