Antimicrobial activity and storage stability of cell-free supernatants from lactic acid bacteria and their applications with fresh beef

This study aimed to evaluate the effectiveness of cell-free supernatants (CFS) produced by selected strains of lactic acid bacteria (LAB) as antimicrobials in vitro against Escherichia coli, Staphylococcus aureus, Shigella sonnei, Pseudomonas fluorescens, Salmonella Typhimurium, or Listeria monocytogenes. The agar-well diffusion method was performed using CFS from sixteen LAB. Neutralization of CFS pH as well as treatment with proteinase K were utilized to determine the nature of CFS’ antimicrobial compounds. Then stability of the three most effective CFS during storage at various temperatures (15, 25, or 35 °C) was determined through agar-well diffusion assays each week for 20 weeks. In addition, antimicrobial activity of CFS from Lb. plantarum was tested in inoculated (E. coli, Staph. aureus, S. Typhimurium, or L. monocytogenes) beef pieces; using this CFS to marinate beef pieces. Furthermore, the effect on beef color (raw or grilled beef, marinated or not with Lb. plantarum CFS) was determined. CFS from Lb. plantarum, Lb. sakei, and Lb. rhamnosus were found to be the most effective, with inhibition halos greater than 20.2 ± 2.0, 20.8 ± 2.9, and 17.1 ± 3.6 mm, respectively. Antimicrobial properties were eliminated when pH was adjusted to 6.5 for most of tested CFS while only Lb. plantarum CFS maintained their antimicrobial activity, which was lost when treated with proteinase K; according to these results, the antimicrobial activity of tested CFS can therefore be mainly attributed to organic acids. Antibacterial activity was significantly (p < 0.05) reduced between time zero and after 20 weeks of storage at three studied temperatures; the greatest reductions in antibacterial activity were observed at 35 °C. Antimicrobial activity against L. monocytogenes was less affected by time and temperature for the three most effective CFS. Lb. plantarum CFS was effective in reducing the microbial load of inoculated bacteria in beef, mainly S. Typhimurium and L. monocytogenes. Meat color differences were important in raw beef; while on grilled meat, changes were scarcely detected.