Susceptibility of MRSA to octenidine dihydrochloride

Abstract number: p893

Al-Doori  Z., Morrison  D., Goroncy-Bermes  P., Edwards  G.F.S., Gemmell  C.G.

Methicillin-resistant S. aureus (MRSA) strains are a major cause of sepsis in hospitals and they can lead to skin infections, septicaemia and death. Antiseptics, such as octenidine dihydrochloride, are used for the treatment of MRSA infected patients. The ability of bacteria and especially MRSA to develop resistance to antimicrobials, especially antibiotics is well documented and resistance to biocides has also been reported. It is important that products should be tested for any resistance arising from repeated and continued exposure. 76 MRSA (including several clonal variants of the two dominant UK MRSA: EMRSA-15 and EMRSA-16) and 24 MSSA from Scottish hospitals were tested for MIC to octenidine dihydrochloride (OCT) according to NCCLS methodology. Adaptation/Tolerance studies were performed on representatives of the five major international MRSA clones (CC5, CC8, CC22, CC30 and CC45). Isolates were grown in BHI broth in the presence of increasing concentrations of OCT for a period of up to 3 months. Octenidine dihydrochloride MIC of the parent and adapted isolates were determined. The MIC50 for the 76 MRSA and 24 MSSA was 4 mg/ml and 2 mg/ml, respectively and MIC90 for both was 4 mg/ml (range 24 mg/ml). The parent isolates of all five clones tested in the adaptation studies had an MIC of 4 mg/ml to OCT. Following continuous exposure to increasing concentrations of OCT over a three months period some differences were observed between clones in their ability to grow at increasing concentrations. The highest OCT concentrations allowing growth in broth were as follows: CC5 (8 mg/ml), CC8 (4 mg/ml), CC22 (6 mg/ml), CC30 (7 mg/ml), and CC45 (8 mg/ml). Although growth occurred at concentrations higher than the MIC of the "parent" strain the MIC of the adapted isolates was identical to that of the parent (4 mg/ml). In conclusion, the data indicates that, under these experimental conditions, the five epidemic MRSA clones tested failed to acquire resistance/reduced susceptibility following continuous exposure to increasing concentrations of octenidine dihydrochloride.