The effects of UVC irradiation on group A streptococcus in vitro

Streptococcus pyogenes (group A streptococcus--GAS) is a common cause of necrotizing fasciitis (NF)--a severe infection of the subcutaneous soft tissue. The purpose of this study was to determine if the topical therapy ultraviolet light C (UVC) is effective in killing GAS in vitro and to evaluate the most effective treatment parameters for use with UVC therapy. Five replications of GAS at 10(8) organisms/mL were plated. The cultures were treated with a UVC light 1 inch from the surface. Irradiation times were as follows: 0, 2, 3, 4, 5, 15, 30, 45, 60, 90, 120, and 180 seconds. Bacterial cultures were incubated and colony counts performed. A second set of GAS cultures were exposed to UVC for 30, 90, and 120 seconds either once daily (qd) or twice daily (bid). Kill rates were 99.9% for GAS at 4 seconds to 180 seconds. Kill rates of 99.9% were also obtained at 30 seconds and 90 seconds when UVC treatment was given either qd or bid. This data indicates that UVC is bactericidal for GAS at times as short as 4 seconds. In addition, UVC treatment was not effective when administered through thin film dressings.