The effects of salt concentration and growth phase on MRSA solar and germicidal ultraviolet radiation resistance.

The extensive use of antimicrobial drugs has led to the widespread emergence of resistant bacterial strains. One such organism, methicillin-resistant Staphylococcus aureus, is now found extensively in both healthcare facilities and diverse community settings such as households, correctional facilities, and athletic teams. The importance of ultraviolet radiation as an adjunctive therapy to reduce bioburden and improve wound status in patients has been documented. An in vitro study to assess the effects of different types of ultraviolet radiation on antibiotic-resistant strains was conducted to provide information that will aid in the development of rational UV irradiation medical protocols. Methicillin-resistant Staphylococcus aureus was found to be sensitive to both germicidal (ultraviolet C) and solar (ultraviolet A and B) ultraviolet radiation (ultraviolet C substantially more lethal). For both types of ultraviolet radiation, as the medium concentration of sodium chloride increased, the methicillin-resistant Staphylococcus aureus cells exhibited increased sensitivity. It also was shown for both types of ultraviolet radiation that kill curves were comparable for log and stationary phase methicillin-resistant Staphylococcus aureus cells. Photoreactivation was observed for Pseudomonas aeruginosa PAO-1 but not for methicillin-resistant Staphylococcus aureus when ultraviolet C was applied to log phase cells. The Gram-negative Pseudomonas aeruginosa PAO-1 was considerably more sensitive than the Gram-positive methicillin-resistant Staphylococcus aureus to ultraviolet C radiation. The experiments reveal that medium composition exerts a substantial effect on methicillin-resistant Staphylococcus aureus ultraviolet resistance and that this species lacks photoreactivation capacity. This suggests that in a clinical setting, eradication of the bacterium may be achieved at far lower doses of ultraviolet radiation than would be indicated by treatment protocols that do not account for ionic conditions.