Original article/Article originalAntifungal activity of a soil isolate of Pseudomonas chlororaphis against medically important dermatophytes and identification of a phenazine-like compound as its bioactive metaboliteActivité antifongique d’un isolat tellurique de Pseudomonas chlororaphis vis-à-vis des dermatophytes pathogènes et l’identification d’un composé de type phénazine comme son métabolite actif
Introduction
Dermatophytes are a group of specialized filamentous fungi classified in three genera including Trichophyton, Epidermophyton and Microsporum that invade the keratinized tissues of skin, hair and nails. They are considered as an important group of zoonotic fungi with worldwide distribution due to causing different types of superficial infections named “dermatophytoses” [34]. Increasing frequency of dermatophytosis, rapid emergence of resistant pathogenic dermatophytes to currently available antibiotics and life-threatening side effects of chemical antifungal drugs especially in long-term treatments of chronic dermatophytoses provoke the researchers to find alternative management strategies focusing on natural biodiversity [8]. Primary resistance of T. rubrum, the main agent of chronic dermatophytosis to terbinafine as one of the most used anti-dermatophytic drugs, shows the possibility of cross-resistance to various other inhibitors of squalene epoxidase, including naftifine, butenafine, tolnaftate and tolciclate, among human pathogenic dermatophytes [11], [20]. These aspects are becoming a health problem prompting testing of novel therapeutic drugs from natural sources possessing less toxicity and high bioactivity with new modes of action.
The primary current means for the identification of new antifungal agents are represented by screening of the vast biodiversity prevalent in natural resources such as soil samples, marine waters, insects, and tropical plants [10], [13], [19], [25], [26], [27], [28], [29]. In recent years, a large number of studies focused on the treatment of fungal infections by using bioactive metabolites identified in various microorganisms, especially bacteria existing as an important part of biodiversity in natural habitats. Antagonistic bacteria are in the first line of investigation not only for their extreme population diversity, but also for capability to producing a wide array of bioactive metabolites with antimicrobial properties [2], [10], [17], [23], [24], [25], [33].
Among antagonistic bacteria, members of the genus Pseudomonas have been recognized as one of the most important groups producing structurally different natural antibiotics like bactericins and phenazines capable of inhibiting the growth of a diverse range of fungi either plant or human pathogenic [14], [18], [22], [32]. As another important role, some species of Pseudomonas have been reported to present antagonistic activity against some medically important dermatophytes belonging to the genera Trichophyton and Microsporum [3], [4]. It has been suggested that P. aeruginosa may exert its growth inhibitory effect on dermatophytes and non-dermatophyte molds within the infected nail [9].
Despite a large data that has now been published about antifungal activity of various bacteria, little has been documented about the effect of bacterial antagonists and their bioactive metabolites on the growth of human pathogenic fungi especially dermatophytes as major public health hazards. In the present study, antifungal activity of a novel soil isolate of P. chlororaphis strain S105 was evaluated against major human pathogenic dermatophytes from three genera Microsporum, Trichophyton and Epidermophyton. A phenazine-like compound was purified from the bacterium culture supernatant as its bioactive metabolite.
Section snippets
Fungal strains and preparation of inoculums
Fungal strains used in the present work including Trichophyton rubrum PFCC51431, T. mentagrophytes PFCC50541, T. tonsurans PFCC88-1352, T. violaceum PFCC80-137, Microsporum canis PFCC50691, M. gypseum PFCC50701, Epidermophyton floccosum PFCC88-1437 were provided from the Pathogenic Fungi Culture Collection of the Pasteur Institute of Iran (http://en.pasteur.ac.ir/pages.aspx?id=586). For preparing of fungal spores, all the fungi were cultured on Potato Dextrose Agar (PDA; E. Merck, Darmshdt,
Identification of antagonistic bacteria
Antagonistic bacteria with anti-dermatophytic activity selected by agar plate bioassay on GY medium were identified by 16S ribosomal RNA (rRNA ∼1 kb) sequencing using ABI prism Big Dye Terminator Cycle Sequencing Apparatus. Bacterial sequences were compared with the sequences of databases in NCBI using BLAST. They were identified as B. amyloliquefaciens (6 isolates), B. subtilis (5 isolates), B. valismortis (2 isolates) and one isolate of each Acinetobacter baumannii, Streptomyces sp. and
Discussion
In the present study, a total of 16 antagonistic bacteria with strong inhibitory activity against the major human pathogenic dermatophytes were isolated from 148 soil samples and identified in the genera Bacillus, Pseudomonas, Acinetobacter and Streptomyces using 16S rRNA sequence analysis. P. chlororaphis was reported as the most potent inhibitory bacterium affecting the growth of all fungi tested. A phenazine-like compound was purified from the bacterium as its inhibitory component. There are
Disclosure of interest
The authors declare they have no conflicts of interest concerning this article.
Acknowledgments
This work was supported financially by a grant from Deputy of Research of the Tarbiat Modares University, Tehran-Iran.
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