Partial Purification and Characterization of Bacteriocin‐Like Inhibitory Substances Produced by <i>Streptomyces</i> sp. Isolated from the Gut of <i>Chanos chanos</i>

Abstract

Bacteriocin-like inhibitory substances (BLIS) have sparked great interest because of their promising use in food as natural antimicrobial agents. In this work, six <i>Streptomyces</i> isolates obtained from the gut of <i>Chanos chanos</i> demonstrated their ability to produce extracellular metabolites with inhibitory activity against <i>Salmonella enterica</i> serovar Typhimurium, <i>Escherichia coli</i>, <i>Listeria monocytogenes</i>, and <i>Staphylococcus aureus</i>. Exposure of the extracellular metabolites to proteolytic enzymes (i.e., proteinase-K, trypsin, and pepsin) revealed high sensitivity and confirmed their proteinaceous nature. The metabolites were stable at high temperatures (up to 100°C for 30 min) and a wide range of pH (pH 2.0-7.0). Fractionation of the crude BLIS by filtration yielded three fractions based on molecular weight: <3 kDa, 3-10 kDa, and >10 kDa. Analysis of the antibacterial activity of these fractions showed increased specific activity, especially in the fraction with a molecular weight (MW) of <3 kDa, relative to the crude sample. The fraction with MW < 3 kDa had minimum inhibitory and bactericidal concentrations in ranges 0.04-0.62 mg·mL^-1 and 0.08-1.25 mg·mL^-1, respectively. This fraction also showed better temperature and pH stability compared with crude BLIS. Brine shrimp toxicity assay revealed that this fraction has moderate toxicity with a 50% lethal concentration of 226.975 <i>μ</i>g·mL^-1 (i.e., moderate toxicity) to <i>Artemia salina</i>. Identification of the peptide sequences of this fraction by liquid chromatography-tandem mass spectrometry yielded 130 proteins with retention times of 15.21-19.57 min. Eleven proteins with MWs of 1345.66-2908.35 Da and composed of less than 30 amino acid residues with high hydrophobicity (15.34-26.22 kcal·mol^-1) appeared to be responsible for the antibacterial activity of the fraction. This study revealed the potential application of BLIS from <i>Streptomyces</i>, especially BLIS SCA-8, as antibacterial agents.