Optimization of media and temperature for enhanced antimicrobial production by bacteria associated with Rhabditis sp

Entomopathogenic nematodes, belonging to the family heterorhabditis and steinernematidae, are reported to be symbiotically associated with specific bacteria and the secondary metabolites produced by these bacteria possess antimicrobial activity. In this study, bacteria were isolated from nematodes belonging to the family rhabditidae, and the antimicrobial activity was tested against four bacteria viz. Bacillus subtilis MTCC 2756, Staphylococcus aureus MTCC 902, Escherichia coli MTCC 2622, and Pseudomonas aeruginosa MTCC 2642 and five fungi viz. Aspergillus flavus MTCC 183, Candida albicans MTCC 277, Fusarium oxysporum MTCC 284, Rhizoctonia solani MTCC 4634 and Penicillium expansum MTCC 2006.

The isolated bacteria were cultured in nutrient broth (NB), Luria broth (LB) and Tryptic soya broth (TSB) at 25, 30 and 35°C. Cell free culture filtrate was prepared by centrifugation and was separated into organic and aqueous fractions. Organic fraction was concentrated and tested for antimicrobial activity.

The culture filtrate of the bacteria isolated from the entomopathogenic Rhabditis sp. was found to possess antimicrobial activity against the four bacteria and five fungi tested. The bacterium grew well in TSB, LB and NB media though in TSB yield and activity were higher. Antimicrobial activity was higher at 30°C as compared with 25 or 35°C. HPLC analysis indicated major differences in peak areas and retention times at different temperatures. Increased number of peaks with higher peak areas was obtained at 30°C.

The study suggests that the bacteria could produce more bioactive molecules effective against medically and agriculturally important bacteria and fungi depending on culture media and temperature. Modified media could yield different types of molecules effective against diseases/disorders of plant, animals and humans.