Production and partial purification of thermostable bacteriocins from Bacillus pumilus ZED17 and DFAR8 strains with antifungal activity

The bacteria which are members of the genus Bacillus are known to produce a wide variety of antimicrobial substances and bacteriocins. The main objective of this study was to investigate the effect of these bacteriocins on eukaryotic cells such as fungi, yeast and plant seeds. Several strains were screened for antifungal activities and identified by the means of polymerase chain reaction (PCR) of the 16s rRNA gene and sequencing. Our experiments showed that the Bacillus pumilus ZED17 and DFAR8 strains, had antifungal activities against Rhizoctonia solani and selected for further investigations. Extracellular peptides produced by these strains were purified by ammonium sulfate precipitation and dialysis. Addition of these peptides to Potato Dextrose Agar (PDA) medium inoculated with R. solani indicated significant inhibition of the fungal growth. The antifungal peptides were thermo-stable and remained active after boiling at 100˚C for 15 min. The molecular weight of the peptide with antifungal activity was estimated by electrophoresis on the Sodium Dodecyl Sulfate Poly Acrylamide Gel (SDS-PAGE) as about 5 KDa. Structural nature of this peptide was determined after gel extraction by Fourier-transform infrared spectroscopy (FTIR). Moreover, this peptide showed inhibiting effect on seeds germination of some herbs.  This peptide could be applied to control herbal fungal disease induced by R. solani which is a broad host range plant pathogen fungus and its inhibition is very important. The peptide also prevents seed germination. Hence, it can be appropriate for inhibiting weeds growth. No significant effect against Saccharomyces cerevisiae and Candida albicans was observed.