Antibacterial activity of bacteriocinogenic Enterococcus faecium R9 and the effect of bacteriocins on the microbial population of raw camel milk

Foodborne outbreaks pose a serious risk to public health and constitute an economic burden on the health care system. Bacteriocins are recognized as potential substitutes for chemical preservatives that prevent and inhibit the growth of pathogens in food. This work aims to study the antibacterial activity and the mode of action of the cell-free supernatant (CFS) and the multiple enterocins produced by Enterococcus faecium R9 strain using the microplate inhibition assay and scanning electron microscopy (SEM). Additionally, the effect of the enterocins on the microbial population of raw camel milk was studied using Illumina amplicon sequencing. The CFS and the enterocins displayed antibacterial activity against a broad spectrum of gram-positive and gram-negative foodborne pathogens, including Listeria monocytogenes, Salmonella Enteritidis, Escherichia coli, Escherichia coli O157:H7, and multi-drug resistant Salmonella and Enterococcus faecium. The CFS completely inhibited the growth of Listeria monocytogenes by exerting a bacteriostatic effect in the exponential phase and bactericidal effect in the stationary phase. The enterocins treatment reduced the growth of Listeria monocytogenes by up to 80 % through a bacteriostatic effect. SEM revealed that the CFS inhibits the gram-negative bacteria by inducing bacterial cell wall damage and pore formation on the cell surface, and inhibits gram-positive bacteria by cell membrane lysis. The enterocins exert their activity on pathogens through pore formation and bacterial cell wall damage or cell lysis. The enterocins treatment increased the richness and diversity of the bacterial genera in raw camel milk compared to the untreated condition. They inhibited important pathogenic bacteria, namely, Rothia, Paenibacillus, Escherichia-Shigella, Moraxella, Pseudomonas, and Salmonella, while stimulating the growth of lactic acid bacteria with potential technologically relevant characteristics, namely, Lactococcus and Streptococcus. This study demonstrated that the CFS and the enterocins produced by Enterococcus faecium R9 exhibit broad-spectrum antibacterial activity, and the enterocins modulate the microbial population of raw camel milk. These findings suggest that enterocins could be potential natural preservatives for controlling foodborne pathogens.