Antimicrobial Activity against Some Saprophytic and Pathogenic Microorganisms of Bacillus species Strains Isolated from Natural Spring Waters in Bulgaria

It is well known that many members of genus Bacillus possess an antimicrobial activity against a variety of microorganisms. In this study we present a primary screening for antimicrobial activity against some saprophytic and pathogenic microorganisms of nineteen strains identified as Bacillus cereus, Bacillus thuringiensis and Bacillus subtilis, isolated from eight natural thermal springs in two districts - Haskovo and Stara Zagora, Bulgaria. The inhibitory activities of the Bacillus sp. strains were determined by agar-well diffusion method. Test microorganisms were preliminarily included into the agar medium, whereas the Bacillus sp. strains were added to the wells. After 48 hours of incubation, the antimicrobial effects were determined by measuring the diameter of zones of inhibition around the wells. Most of Bacillus sp. strains showed high antimicrobial activitiy against the molds Penicillium sp., Aspergillus oryzae, Aspergillus niger, Aspergillus awamori, Fusarium moliniforme and Rhizopus sp. Inhibitory activity against the mold Mucor sp. and bacterium Enterococcus faecalis ranged between low and moderate. Two of the strains (Bacillus cereus 52/GI1 and Bacillus thuringiensis 56/H3) possessed moderate antimicrobial activity against bacterium Pseudomonas aeruginosa and yeasts Saccharomyces cerevisiae. One strain (Bacillus subtilis 47/YA1) had high activity against the mold Mucormucedo; one strain (Bacillus cereus 36/GI6) showed moderate activity against yeasts Candida utilis, while bacterium Escherichia coli was not inhibited at all.