Antifungal activity of a soil isolate of Pseudomonas chlororaphis against medically important dermatophytes and identification of a phenazine-like compound as its bioactive metabolite

Abstract

The increasing importance of dermatophytoses and emerging resistance of dermatophytes to current synthetic antifungals have stimulated the search for safer and more effective alternative drugs from natural sources. The present study was carried out to identify antagonistic bacteria of soil origin with strong inhibitory activities on the growth of major human pathogenic dermatophytes. Antifungal activity of isolated soil bacteria was screened against the dermatophytes from three genera Microsporum (M.canis, M.gypseum), Epidermophyton (E.floccosum) and Trichophyton (T.mentagrophytes, T.rubrum, T.violaceum, T.tonsurans) by using visual plate agar assay method. A Pseudomonas chlororaphis isolate S105, identified at the species level by 16S ribosomal RNA sequence analysis, was reported as the strongest antagonistic bacterium. P.chlororaphis S105 culture supernatant (PCCS) was examined against tested dermatophytes by GY (glucose-yeast extract) broth bioassay in 6-well microplates. Antifungal compound of the bacterium was partially purified from the culture supernatant through a purification scheme of methanol extraction, Diaion HP20 ion-exchange chromatography and preparative thin layer chromatography. P.chlororaphis S105 was the most potent inhibitor of fungal growth for all tested dermatophytes with a percent inhibition ranged from 57.1% to 99.8%. The PCCS suppressed the growth of all fungi tested in the range of 18.5% to 84.8%. Partially purified antifungal compound of the bacterium was identified as a phenazine-like compound with an Rf value of 0.51. The compound inhibited fungal growth by 73.6% to 97.9% on GY broth. Fungal growth inhibition was significant for all dermatophytes tested in comparison with the controls (Anova, P<0.05). With respect to the strong inhibitory activity of P.chlororaphis against pathogenic dermatophytes reported here, it may be considered as a rich source of useful metabolites with potential application in antifungal drug discovery.