Varying activity of chlorhexidine-based disinfectants against Klebsiella pneumoniae clinical isolates and adapted strains

Background: Control of multi-drug-resistant (MDR) organisms relies increasingly on the use of biocides, including chlorhexidine, to limit the risk of infection. The concentration and formulation of chlorhexidine can vary hugely between products. Aim: To establish the activity of chlorhexidine and in-use chlorhexidine formulations against 14 clinical Klebsiella pneumoniae strains isolated before and since the use of chlorhexidine became routine, and strains that have adapted following sublethal chlorhexidine exposure. Methods: Minimum inhibitory concentrations (MICs) and minimum bactericidal concentrations (MBCs) of five chlorhexidine-containing formulations were measured at 5. min, 15. min, 30. min and 24. h for the panel of K. pneumoniae strains. Findings: After 5. min, MBCs of five formulations varied from 0.006 to >50% working concentration (WC) or from 78 to 2500. μg/mL chlorhexidine. For one formulation, MBCs were >50% WC for five of the 14 strains, and for another formulation, four of the 14 strains could resist 25% WC. NCTC 13368 was consistently most tolerant to chlorhexidine, whereas the strains isolated before the use of chlorhexidine became routine were more sensitive. One pre-chlorhexidine era and five modern strains increased MICs up to 16-fold following exposure to sublethal concentrations of chlorhexidine. A hand disinfectant with MBCs of 0.39% WC for all six of the wild-type strains, had MBCs of 50% WC for the chlorhexidine-adapted strains. Conclusion: Not all chlorhexidine formulations kill MDR K. pneumoniae after the recommended exposure time. Activity, especially against chlorhexidine-adapted strains, depends on additional ingredients. Careful formulation of chlorhexidine products is therefore important to maintain and enhance the activity of chlorhexidine products, and avoid potential breakdown in infection control.