Chlorhexidine bigluconate resistance in Methicillin Resistant Staphylococcus aureus Open Access

Evans, Eric (Spring 2020)

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Background: With increased chlorhexidine use in infection control practices, concerns of bacterial tolerance has grown. Methicillin-resistant Staphylococcus aureus (MRSA) are likely frequently exposed to chlorhexidine. Reduced chlorhexidine susceptibility in staphylococci is associated with quaternary ammonium compound (qac) efflux proteins. Mechanisms of tolerance to chlorhexidine beyond the qac proteins are also not well understood. Surveillance conducted by the Georgia EIP provided data to test patient factors contributing to increased chlorhexidine resistance. We serially exposed MRSA to increasing concentrations of chlorhexidine in a qac-negative MRSA strain to induce chlorhexidine tolerance. 

Methods: 90 MRSA strains collected by the Georgia EIP were tested for MIC of chlorhexidine using standardized test provided by the CDC. Patient information was used to analyze whether any specific variable contributed to increased resistance. Additional laboratory experimentation, seeking to induce chlorhexidine tolerance, was completed. Using both liquid and plate cultures, colonies were selected and exposed to serial passaging in BHI broth with increasing concentrations (0.1 -10 µg/mL) of chlorhexidine for 24-72 hours. After each passage to a higher concentration of chlorhexidine, colonies were examined and confirmed to be SA, tested by broth microdilution to confirm the minimum inhibitory concentration (MIC) to chlorhexidine and frozen.  

Results: No significant relationships between any of the patient variables and chlorhexidine tolerance were identified. Notably, samples collected before and after widespread chlorhexidine usage did not show increased chlorhexidine tolerance. All in vitro experiments trying to induce chlorhexidine tolerance failed to produce a tolerant mutant. 

Discussion: Despite growing concerns about chlorhexidine tolerance in MRSA, this study does not show an increased chlorhexidine tolerance in MRSA despite widespread use in their respective healthcare facilities. In vitro evolution of increased tolerance to chlorhexidine susceptibility was not successful. These combined experiments suggest development of chlorhexidine resistance in MRSA is likely not as simple as previously assumed.

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