Effect of light exposure on growth rate of veterinary clinical dermatophyte isolates

Abstract

Background -Veterinary textbooks and literature suggest that exposure to light is inhibitory to growth of clinical dermatophyte isolates.Hypothesis/objectives -We hypothesized that this idea was derived from experiments that examined the effect of high doses of ultraviolet and visible light exposure on dermatophyte growth, and that exposure to typical room lighting would not adversely affect dermatophyte growth rate.Methods and materials -Isolates of common veterinary dermatophytes (three each of Microsporum canis, Nannizia gypsea and Trichophyton benhamiae) were exposed to typical fluorescent room lighting, incubated in a closed drawer, or exposed at close range to fluorescent wide-spectrum light. Dermatophytes were grown on Sabouraud Dextrose Agar (SAB) and Dermatophyte Test Medium (DTM). Colony diameter was measured and growth rate (expressed as colony diameter increase mm/day) calculated at the linear portion of the culture growth curve. Statistical analyses compared growth rates across the various incubation conditions and among dermatophyte isolates.Results -There was little difference in growth rate between cultures incubated under typical fluorescent room lighting and those placed in the dark. Exposure to the close-range light increased growth rate as a consequence of the elevated incubation temperature created by the lamp. Significant differences in growth rate were noted among strains of the same dermatophyte species. Dermatophytes grew more rapidly on SAB than DTM agar.Conclusions and clinical relevance -Exposure to typical lighting conditions in a clinical environment does not inhibit growth of dermatophyte colonies. Veterinary clinicians may conduct routine dermatophyte cultures without incubating them in the dark.

Survey of dermatophytes in stray dogs and cats with and without skin lesions in Puerto Rico and confirmed with MALDI-TOF MS

“…All samples were inoculated by gently pressing the toothbrush on medium surface (20 repetitions) [19] alternating between sections I and II at random order. The Petri dishes were incubated upside down in a room with normal laboratory lighting and at a constant temperature of 23˚C and examined daily for 21 days for evidence of growth [20]. If dermatophyte growth was detected via microscopic examination, the isolates were subcultured onto a Sabouraud's dextrose agar (SDA) plate (Hardy Diagnostics, Santa Maria, CA).…”