25Probiotics are recognized to outcompete pathogenic bacteria by receptor-mediated 26 colonizing and secreting functional metabolites which have direct antimicrobial activities 27 towards pathogens and/or improving host's gut health and immunity. We have constructed 28 a Lactobacillus casei (LC) probiotic strain, LC +mcra , by inserting mcra (myosin cross-29 reactive antigen) gene, which stimulates the conversion of conjugated linoleic acids. In this 30 study, we evaluated the protective roles of LC +mcra against pathogenic Salmonella enterica 31 serovar Typhimurium (ST) and enterohaemorrhagic E. coli (EHEC) infection in BALB/cJ 32 mice. Through a series of in vivo investigation, we observed that LC +mcra colonized 33 efficiently in mice gut and competitively reduced the infection with ST and EHEC in 34 various locations of small and large intestine, specifically cecum, jejunum, and ileum 35 (p<0.05). The cecal microbiota in ST-challenged mice with LC +mcra protection were 36 positively modulated with higher relative abundances Firmicutes but lower Proteobacteria 37 plus increased bacterial species diversity/richness based on 16S metagenomic sequencing. 38 Based on cytokine gene expression analysis by qRT-PCR, mice pretreated with LC +mcra 39 were found with attenuated bacterial pathogen-induced gut inflammation. Furthermore, 40 mice fed LC +mcra daily for one week could protect themselves from the impairments caused 41 by enteric infections with ST or EHEC. These impairments include weight loss, negative 42 hematological changes, intestinal histological alterations, and potential death. This in vivo 43 study suggests that daily consumption of novel conjugated linoleic acids over-producing 44 probiotic might be efficient in improving gut intestinal microbiome composition and 45 preventing/combating foodborne enteric bacterial infections with pathogenic Salmonella 46 and diarrheagenic E. coli. 3 47 48Author summary 49 Numerous bacteria colonize throughout the gastrointestinal tract and form a complex 50 microbial ecosystem known as gut microbiota. A balanced microbial composition is crucial 51 for maintaining proper gut health and host defense against pathogenic microbes. However, 52 enteric bacterial infections could cause illness and even lead to death of host when 53 foodborne pathogens like Salmonella and enterohaemorrhagic E. coli (EHEC) invade gut 54 intestine and cause imbalance of gut microbiota. Beneficial microbes in gastrointestinal 55 tract such as Lactobacillus and their secreted bio-active metabolites, are potential bio-56 agents to improve gut immunity and outcompete bacterial pathogens. In this study, to 57 evaluate roles of novel Lactobacillus strain LC +mcra which produce higher amount of a 58 group of beneficial secondary metabolites called conjugated linoleic acids, we have shown 59 that daily oral administration of this LC +mcra for one-week in mice lead to higher proportion 60 of beneficial bacterial colonization in different locations of intestine and a significant 61 reduction of path...