Fig. 6

Quantification and antimicrobial activity assay of subtilin production. Strains TMB3044 (W168 spa ΔabrB) and B. subtilis ATCC6633 were cultured in mixed media (50% LB + 50% Medium A) or in Medium A. The liquid culture was collected at defined time points as indicated on the x-axis and filtered to obtain cell-free subtilin supernatant. The subtilin supernatants were tested for subtilin production based on the induction of the reporter strain TMB1617 (W168 P_liaI-lux) (left panels). Values are reported as RLU/OD at peak induction point of the reporter as detailed in Additional file 6: Fig. S5. The control was treated with the same amount of sterile water. The antimicrobial activity of the respective subtilin supernatant was tested against the Gram-positive species M. luteus (the yellowish soft agar layer), and indicated by the inhibition halo (right panels). The control (labeled as capital letter C) on the left top corner of each plate represents the antimicrobial activity of subtilin supernatant harvested from B. subtilis ATCC6633 culture at the 20th hour of growth. Subtilin supernatant under each condition was collected three times and all tested for their antimicrobial activity. The results shown here are representatives for the triplicates of measurement. The error bars in the bar charts represent the standard deviation of triplicates test