MECHANISMS OF STAPHYLOCOCCUS AUREUS PERSISTENCE AND ERADICATION OF CHRONIC STAPHYLOCOCCAL INFECTIONS

Bacteria can exist in different phenotypic states depending on environmental conditions. Under stressed conditions, such as antibiotic exposure, bacteria can develop into persister cells that allow them to stay dormant until the stress is removed, when they can revert back to a growing state. The interconversion of non-growing persister cells and actively growing cells is the underlying basis of relapsing and chronic persistent infections. Eradication and better treatment of chronic, persistent infections caused by Staphylococcus aureus requires a multifaceted approach, including a deeper understanding of how the bacteria persist under stressed conditions, regulate its cell death pathways, and development of novel drug therapies. Persisters were first discovered in the 1940s in a staphylococcal culture in which penicillin failed to kill a small subpopulation of the cells. Despite the discovery many decades ago, the specific mechanisms of Staphylococcus aureus persistence is largely unknown. Recently renewed interest has emerged due to the rise of chronic infections caused by pathogens such as M. tuberculosis, B. burgdorferi, S. aureus, P. aeruginosa, and E. coli. Treatments for chronic infections are lacking and antibiotic resistance is becoming a bigger issue. The goal of our research is to define the mechanisms involved in S. aureus persistence and cell death to improve our knowledge of genes and molecular pathways that can be used as targets for drugs to eradicate chronic infections. Using several high-throughput genetic screens, we identified and confirmed several core regulators of bacterial persistence and cell death upon exposure to bactericidal antibiotics and environmental stresses such as heat and acid stress. Currently, drug combinations approved for clinical usage do not target persister cells. One exception is the treatment regimen for tuberculosis that includes pyrazinamide, an antipersister drug that targets unconventional drug targets such as proteins involved in energy metabolism and trans-translation. Upon the addition of pyrazinamide, which kill persister bacteria to the combination of rifampin and isoniazid which kill growing bacteria, the duration of tuberculosis treatment was shortened from 9-12 months to 6 months. Here, using the treatment for tuberculosis as an example that demonstrates the powerful activity of an anti-persister drug such as pyrazinamide, we screened for drugs with high activity against growing and non-growing forms of S. aureus to formulate drug combinations that can effectively kill the heterogeneous population of bacteria in biofilms. To test this approach in a clinically-relevant animal model, we established a chronic, skin infection mouse model of S. aureus to validate the improved efficacy of drug combinations in clearing persistent skin infections than currently-approved regimens used in the clinics. Our findings reveal that drug combinations consisting of drugs targeting both actively growing bacteria and persister cells, such as the combination of meropenem + daptomycin + clinafloxacin, can eradicate biofilms more effectively than doxycycline + rifampin, a combination used currently on patients. Our in vivo mouse model studies further validate the efficacy of such drug combinations in clearing the bacteria in the lesions, reducing the pathology, and completely healing the chronic lesions formed on the skin. These findings have important implications for treating other persistent bacterial infections