Reduced efficacy of chlorhexidine against clinical MRSA residues compared to standard strains

Abstract number: 1733_205

Davies S., Vali L., Amyes S.

Objectives: To determine whether sub-inhibitory concentrations of chlorhexidine acetate (CHX) are effective against surface dried MRSA and whether there are differences in the susceptibility of clinical strains compared to control EMRSA 16 and Staphylococcus aureus strains.

Methods: A surface disinfectant test was performed as follows: Washed bacterial suspensions (overnight growth in nutrient broth, washed three times in saline) of control strains (EMRSA 16 and S. aureus NCTC 6571) and clinical MRSA were dried onto surfaces for 2 and 24 hours. After drying, a sub-inhibitory concentration of chlorhexidine diacetate (CHX) was added. Following a 5 minute contact time, neutraliser was added and the cells resuspended for 5 minutes using magnetic followers. Enumeration was performed using the drop-counting method (serial dilution in SDW followed by 3×10 ul drops of each dilution on nutrient agar plates). Microbiocidal effect (log reduction in CFU/mL following CHX exposure) was determined by evaluating triplicate counts against a control without CHX for each isolate, enabling the results from different isolates to be compared.

Results: CHX was more effective after longer bacterial drying times (figure).

Mean microbiocidal effect of CHX on bacterial residues.

This difference was more pronounced in the control EMRSA 16 (C) and NCTC 6571 (S) strains than in the clinical isolates (LF26, LF67 and LF93), in which the effectiveness of CHX after 24 hours bacterial drying time was not greatly increased. At both drying times, CHX was less effective on the clinical isolates than the control strains. Even after 24 hours bacterial drying, there was less than a 3-log CFU/mL reduction in the clinical isolates following exposure to CHX.

Conclusions: These results suggest that sub-inhibitory concentrations of CHX, as might occur during misuse of biocides in hospitals, are minimally effective against clinical isolates of MRSA, and may therefore lead to selection of reduced susceptibility MRSA isolates in the clinical environment. The difference between CHX efficiency against control strains compared to clinical isolates has implications for the tested efficacy of biocides, as it may be that clinical strains are less susceptible than efficacy testing on standard strains would imply.

Session Details

Date: 31/03/2007
Time: 00:00-00:00
Session name: European Society of Clinical Microbiology and Infectious Diseases
Location: ICC, Munich, Germany
Presentation type:
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