Chlorhexidine Gluconate (CHG) Susceptibility of Methicillin-Resistant Staphylococcus aureus (MRSA) Isolates from a Multi-State Study of Adult Intensive Care Unit (ICU) Patients

Background: CHG is a topical disinfectant with broad antimicrobial activity that has been used for over 50 years for a range of healthcare applications. Skin cleansing with CHG has been a component of successful targeted MRSA decolonization programs; universal bathing of ICU patients with CHG has been associated with a reduction in cross-transmission of MRSA. Decreased susceptibility to CHG and reduced clinical effectiveness have been described in some strains of MRSA in association with the presence of qacA/B genes, which encode multidrug antiseptic efflux pumps. To our knowledge, the prevalence of reduced susceptibility to CHG (MIC > 4 µg/mL) and qacA/B genes in MRSA strains in the US is unknown.

Objective: To estimate the prevalence of reduced susceptibility to CHG and qacA/B genes in a multistate sample of MRSA isolates collected from adult ICU patients.

Methods: MRSA isolates were collected prospectively from ICU patients at the 44 hospitals (75 ICUs) in 16 states participating in an ongoing 3-way cluster randomized trial to assess the impact of targeted versus universal strategies to reduce MRSA in ICUs (REDUCE – MRSA; http://clinicaltrials.gov/ct2/show/NCT00980980). All ICU clinical and anterior nares MRSA isolates (one per patient) were collected during the baseline, pre-intervention period (8/23/09-3/27/10). CHG was used for central line insertion site preparation in 94% of ICUs; CHG-impregnated catheters and CHG catheter dressings were used in 46% and 44% of ICUs, respectively. Up to 64 isolates per hospital that were confirmed as MRSA by the central laboratory were tested for susceptibility to CHG by a microdilution method. QacA/B carriage was determined for up to 15 isolates per hospital by a user-developed real-time PCR assay.

Results: 936 isolates were tested for CHG susceptibility (27% clinical, 73% nares screening). The MIC₅₀ and MIC₉₀ of CHG were 2 µg/ml and 4 µg/ml, respectively (MIC range, 0.5 µg/ml – 4 µg/ml). 12/522 (2.3%) isolates carried qacA/B: 4 qacA, 3 qacB, 1 qacA and qacB. QacA or B could not be distinguished in 4 isolates. MIC₅₀ and MIC₉₀ for qac-positive isolates were 2 µg/ml and 4 µg/ml, respectively. Qac-positive isolates were found in 9 hospitals in 8 states.

Conclusions: Reduced susceptibility to CHG was not detected in a large sample of nosocomial MRSA isolates collected from ICU patients in multiple states in the US. The prevalence of qacA/B in the sample was low.  Although the clinical relevance of these findings remains to be determined, the apparently high barrier to CHG resistance in MRSA suggests that CHG will continue to be an effective component of MRSA decolonization regimens in US hospitals.