The present study was investigated to determine the competitive interaction between Gram positive (B. subtilis) and Gram negative (P. aeruginosa and E. coli) bacteria in vitro. Bacteria of interest were isolated from soil sample and were identified through morphological and biochemical tests. Bacterial cultures were grown alone and as co- culture in test tubes containing sterile broth. Bacterial growth and their growth inhibition in co- culture tubes were analyzed till 96 h through spectrophotometric assay and colony forming unit (CFU) assay. Spectrophotometric analysis and CFU/mL assay showed that the effect of B. subtilis on P. aeruginosa and E. coli in co- culture tubes was growth inhibitory. This is accompanied by the reduction in absorbance value and CFU/mL of the co-culture tubes. The bactericidal activity of B. subtilis was measured on P. aeruginosa and E. coli using agar well diffusion method. B. subtilis showed zone of inhibition of 8 mm and 6 mm against P. aeruginosa and E. coli, respectively. The antagonistic activity test of B. subtilis favoured the spectrophotometric and colony forming assay results. These results suggest that B. subtilis develop adaptational pathways by extracellular signaling molecules and antibacterial factors in order to compete with bacterial neighbors.
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