Antimicrobial activity of Lactobacillus plantarum strains and antimicrobial properties of L. plantarum small heat shock proteins

The aim of this study was to investigate Lactobacillus plantarum strains possessing antimicrobial features to be proposed as potential bioagents and biopreservant stains that could prevent the growth of pathogenic bacteria in fermented food. A total of 85 Lactobacillus plantarum strains, including strains overproducing small heat shock proteins, were investigated for their antimicrobial activity against the pathogenic bacteria Escherichia coli O157:H7, Staphylococcus aureus, Listeria monocytogenes and Salmonella enterica. Firstly, the antimicrobial effect was evaluated using over-night cultures (ONC) of lactobacilli by agar spot test. The antagonistic effects of L. plantarum strains against pathogens were detected measuring the growth inhibition halos on plates. All strains showed significant antibacterial activity and we classified L. plantarum strains as no, mild, strong and very strong inhibiting strain when inhibitions zone of 1 mm, 2 mm, 2-5 mm and more than 5 mm were produced. Subsequentially, 15 strains showing strong and very strong antimicrobial activity were selected and their cell-free supernatant (CFS) was harvested and used for agar diffusion and microdilution bioassays over an incubation time of 24 h at 30 °C, in order to investigate the substances determining the antimicrobial ability. Overall, the results showed an interesting antagonism depending both on L. plantarum strain analysed and pathogens bacteria tested. Moreover, antimicrobial activity was observed by L. plantarum strains overproducing small heat shock proteins. The applications of L. plantarum strains exhibiting antimicrobial activity need to be considered as safety bio-alternative to traditional antibiotics used against human infection and to chemical substances used in the food processing.