Therefore the aim of this study was to determine the capacity of the cationic light activated antimicrobial agent methylene blue in combination with 665 nm laser light to kill S. aureus SCVs. Results and discussion As mentioned small colony variants of S. aureus have been reported to have increased resistance to conventional antimicrobials such as aminoglycosides. In this study we determined that the minimum inhibitory concentration of the aminoglycoside kanamycin against the hemB and menD small colony variants was 8-fold higher (128 μg/ml) than the isogenic parent strains (16 μg/ml). The hemB SCV and its isogenic parent were both found to be susceptible to photodynamic see more killing using methylene blue and 1.93 J/cm2 of 665 nm

laser light in a methylene blue concentration-dependent

manner SB-715992 (Figure 1). Neither laser light nor photosensitiser alone had any effect on bacterial viability (data not shown). The menD SCV and its wild-type parent were also susceptible to photodynamic SAR302503 chemical structure killing by methylene blue (20 μM) and 1.93 J/cm2 of 665 nm laser light, with reductions in cell viability of 3.5 log10 and 4.1 log10, respectively (data not shown). Increasing the light dose was found to significantly increase the killing of both the hemB SCV and its parent strain; the highest light dose examined (9.65 J/cm2) resulted in reductions in viable cells of approximately 6.9 log10 and 5 log10 respectively (Figure 2). There was no significant difference between the kills observed for both strains when a light dose of 9.65 J/cm2 was used for the experiments. Figure 1 Number of viable bacteria recovered following exposure to 1.93 J/cm 2 of 665 nm laser light and different concentrations of methylene blue. The clear bars represent recovery of the wild type strain

LS-1 and the grey bars the isogenic hemB SCV. Error bars represent the standard deviation from the mean. **P < 0.01, ***P < 0.001 (ANOVA). Figure 2 Number of viable bacteria recovered following exposure to methylene blue and different doses of 665 nm laser light. The clear bars represent recovery of the wild type strain LS-1 and the grey bars the isogenic hemB SCV. Error bars represent the standard deviation from the mean. ***P < 0.001 (ANOVA). Small colony variants of S. aureus represent a serious challenge to clinicians treating infections caused by these Docetaxel microorganisms [2] due to the increased antibiotic resistance and persistent infections that are characteristic of SCVs [1, 3, 4]. It would therefore be advantageous to develop a therapeutic strategy with a differing mode of action to those antibiotics for which lower susceptibility is observed. We have previously shown that light-activated antimicrobial agents, which have a non-specific mode of action, are highly effective at killing S. aureus[6–8]. To investigate the capacity of the light-activated antimicrobial agent methylene blue in combination with laser light for eradicating SCVs of S.