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

Abstract

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 5min, 15min, 30min and 24h for the panel of K. pneumoniae strains.

Findings: After 5min, MBCs of five formulations varied from 0.006 to >50% working concentration (WC) or from 78 to 2500μg/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.

Keywords: Adaptation; Antiseptic; Biocide; Chlorhexidine; Disinfectant; Formulation; Klebsiella pneumoniae; Reduced susceptibility.