Transcriptional Profiling of Salmonella Montevideo Exposed to the Probiotic Lactobacillus animalis NP51

Introduction: Non-typhoid Salmonella is one of the most important causes of foodborne illness worldwide. In the United States, between 800,000 and 1.4 million cases of Salmonella infections occur annually; a common source of salmonellosis is direct contact with animals carrying the pathogen. Salmonella Montevideo has been commonly isolated from feedlot cattle, and it is among the top ten serotypes that cause human disease. Lactobacillus animalis NP51 is a direct-fed microbial (DFM) used as a pre-harvest intervention to reduce Salmonella colonization of the bovine gastrointestinal tract and subsequent fecal shedding that contributes to carcass contamination by this foodborne pathogen.

Purpose: This study used RNA-Seq to elucidate the gene expression profile of Salmonella Montevideo grown in the presence of Lactobacillus animalis NP51.

Methods: Bacterial strains were grown overnight at 37°C in media shown to support co-culture of both Salmonella spp. and Lactobacillus spp. Overnight cultures were diluted into fresh media contained in dialysis tubes. Tubes were, then, placed in falcon tubes containing media with or without NP51 (control). Samples were incubated at 37°C until mid-logarithmic phase was reached. Total RNA was extracted from three biological replicates; treatment and control samples were rRNA depleted, followed by bar-coding of individual samples. RNA-Seq libraries were prepared and sequenced on a MiSeq instrument. Raw data sets were assembled de novo; DNAStar Array Star was used to analyze gene expression profiles, and Blast2go software was used to annotate differentially expressed genes.

Results: A total of 339 genes were found to be differentially expressed at two-fold change and 95% confidence intervals; 50.1% (n=170) genes showed reduced expression, while 49.85% (n=169) increased their expression. Transporter activity and binding were among the molecular functions up-regulated; motility and virulence-associated genes were found to be down-regulated.

Significance: This study provided important insights into probiotic-pathogen interactions and mechanisms by which Lactobacillus animalis NP51 prevented Salmonella Montevideo from colonizing the host.