Molecular identification of antagonistic bacteria from Tehran soils and evaluation of their inhibitory activities toward pathogenic fungi

BACKGROUND AND OBJECTIVES To find antagonistic bacteria with potential antifungal activity against some pathogenic fungi, including Aspergillus niger, A. flavus, Fusarium moniliforme and Penicillium marneffei, a total of 148 agricultural soil samples from different sites of Tehran were examined. MATERIALS AND METHODS Antagonistic soils were selected by screening against A. niger on glucose-yeast extract (GY) agar using a visual agar plate assay method. All growing bacteria were examined for antifungal activity, and antagonistic bacteria identified based on 16S rRNA sequence analysis. Among a total number of 97 bacteria isolated form inhibitory soils (36 samples), 16 bacteria were reported as strong growth inhibitors in co-cultures on GY agar with all tested fungi at variable degrees. Fungal growth inhibitory bacteria were cultured against all fungi and growth inhibition was measured and analyzed between test and control groups by statistical analysis (ANOVA). RESULTS Molecular identification of antagonistic bacteria indicated that most bacterial isolates belonged to the genus Bacillus (81.25%), including B. subtilis (5 isolates), B. amyloliquefaciens (6 isolates) and B. valismortis (2 isolates), followed by one isolate (6.25%) from each Streptomyces sp., Pseudomonas chlororaphis and Acinetobacter baumannii. Based on the visual plate assay results, total fungal growth inhibition of all bacteria was reported in the range of 13.2 to 68.3%. P. chlororaphis S105 was reported as the most potent antagonistic bacterium which inhibited the growth of A. niger by 68.3%, followed by F. moniliforme (66.4%), A. flavus (64.7%) and P. marneffei (57.1%). CONCLUSION P. chlororaphis and some other inhibitory bacteria reported in the present study, they may be considered not only as a rich source of useful metabolites with potential application in antifungal drug discovery, but also as potential candidates for biological control programs.