Staphylococcus aureus small colony variants are susceptible to light activated antimicrobial agents

Antibiotic therapy can select for small colony variants of Staphylococcus aureus that are more resistant to antibiotics and can result in persistent infections, necessitating the development of more effective antimicrobial strategies to combat small colony variant infections. Photodynamic therapy is an alternative treatment approach which utilises light in combination with a light-activated antimicrobial agent to kill bacteria via a non-specific mechanism of action. In this study, we investigated whether the combination of 665 nm laser light and the light-activated antimicrobial agent methylene blue was able to successfully kill S. aureus small colony variants. S. aureus and isogenic stable small colony variant were exposed to varying doses (1.93 to 9.65 J/cm2) of 665 nm laser light in the presence of varying concentrations (1 to 20 μM) of methylene blue.

The combination of 665 nm laser light and methylene blue was found to be an effective strategy for the killing of small colony variants. At the highest light dose (9.65 J/cm2) and methylene blue concentration (20 μM) tested, the number of viable bacteria decreased by approximately 6.9 log10 for the wild type and approximately 5 log10 for the small colony variant.

These results suggest that photodynamic therapy has potential for use in the treatment of superficial infections caused by small colony variants of S. aureus and supports further research in this field.