Inhibiting sulfate-reducing bacteria in biofilms on steel with antimicrobial peptides generated in situ

In batch and continuous fermentations, the reduction in corrosion of SAE 10 18 mild steel and 304 stainless steel caused by inhibition of the reference sulfate-reducing bacterium (SRB) Desulfovibrio vulgaris by a protective, a ntimicrobial-producing Bacillus brevis biofilm was investigated. The presen ce of D. vulgar is produced a thick black precipitate on mild steel and a h igher corrosion rate in batch cultures than that seen in a mono-culture of non-antimicrobial-producing Pseudomonas fragi K upon the addition of SRB to the aerobic P. fragi K biofilm. In continuous reactors, the polarization r esistance R-p decreased for stainless steel and increased for mild steel up on the addition of SRB to a P. fragi K biofilm. Addition of either 200 mu g /ml ampicillin, chloramphenicol, or ammonium molybdate to batch and continu ous reactors after SRB had colonized the metal was ineffective in killing S RB, as inferred from the lack of change in both R, and the impedance spectr a. However, when ampicillin was added prior to SRB colonization, the growth of SRB was completely inhibited on stainless steel in continuous reactors. Prior addition of ampicillin was only able to delay the growth of SRB on m ild steel in continuous reactors. External addition of the purified peptide antimicrobial agent gramicidin S prior to the addition of SRB also inhibit ed the growth of SRB on stainless steel in continuous reactors, and the SRB were also inhibited on stainless steel in both batch and continuous reacto rs by producing gramicidin S in situ in a protective biofilm when the grami cidin-S-overproducing strain Bacillus brevis 18 was used.