Bactericidal effects of polyhexamethylene biguanide against intracellular Staphylococcus aureus EMRSA-15 and USA 300

Abstract

Objectives: The treatment of skin infections caused by Staphylococcus aureus is limited by acquired antibiotic resistance and poor drug delivery into pathogen and host cells. Here, we investigated the antibacterial activities of six topically used antimicrobials and a cationic polymer, polyhexamethylene biguanide (PHMB), against intracellular MSSA strain RN4420 and MRSA strains EMRSA-15 and USA 300.

Methods: The MICs of antimicrobials were determined for MSSA and MRSA strains, and the bactericidal activities of nadifloxacin and PHMB against intracellular MRSA were determined using infected keratinocytes. Fluorescein-tagged PHMB (PHMB-FITC) was used to study PHMB uptake, co-localization with intracellular EMRSA-15 and retention in keratinocytes. The mechanism(s) of PHMB uptake into keratinocytes were studied using a dynamin inhibitor, dynasore.

Results: Gentamicin, nadifloxacin and PHMB showed the lowest MICs for MRSA. Nadifloxacin at 10 mg/L killed 80% of intracellular EMRSA-15, but was not effective against USA 300. PHMB at 4 mg/L killed almost 100% of intracellular EMRSA-15 and USA 300. PHMB entered keratinocytes, co-localized with intracellular EMRSA-15 and was retained by the cells for over 5 h. PHMB uptake and its intracellular antibacterial activities were inhibited by the dynamin inhibitor, dynasore.

Conclusions: PHMB kills intracellular MRSA via direct interaction with pathogens inside keratinocytes and host cell entry is dynamin dependent.