Evaluation of the effects of chlorhexidine digluconate with and without cBD103 or cCath against multidrug-resistant clinical isolates of Staphylococcus pseudintermedius.

BACKGROUND: Because of the increased incidence of multidrug-resistant (MDR) bacteria, the use of disinfectants over antibiotics has been encouraged. However, the interactions between disinfectants and host local immunity are poorly understood. OBJECTIVE: To assess the effects of chlorhexidine digluconate (Chx), with and without selected host defence peptides (HDPs), against MDR Staphylococcus pseudintermedius (MDR-SP). METHODS AND MATERIALS: Ten clinical isolates of MDR-SP were tested, using a modified microbroth dilution method. Four two-fold dilutions of 2% Chx and 1 μg/mL the HDPs synthetic canine β-defensin 103 (cBD103) or cathelicidin (cCath) were tested alone or in combination. Colony counts after 5, 15, 30 and 60 min, and a minimum inhibitory concentration (MIC) after 24 h were recorded. Friedman followed by Dunn's multiple comparison tests with significance of P < 0.05 were used for statistical analysis. Synergy, additivity/neutrality or antagonism were calculated. RESULTS: Growth was not inhibited by either HDP alone. An MIC of 0.312 μg/mL Chx was achieved for nine of the isolates. One isolate had an MIC of 0.078 μg/mL Chx. A MIC(in nine of 10 isolates) of 0.312 µg/mL was seen for Chx in combination with either HDP. Synergy was seen in the combination Chx/cCath used at the highest concentrations of Chx (0.624 µg/mL and 0.312 µg/mL) after 30 and 60 min incubation. Additivity/neutrality was seen for most of the other concentrations and times of incubation. CONCLUSIONS AND CLINICAL IMPORTANCE: These results suggest a synergistic/additive effect between Chx and HDPs in dogs. Further studies evaluating the mechanisms behind this effect are needed.