Spatial Segregation of Virulence Gene Expression during Acute Enteric Infection with Salmonella enterica serovar Typhimurium

Abstract

The pathogenic bacterium Salmonella enterica serovar Typhimurium invades and persists within host cells using distinct sets of virulence genes. Genes from Salmonella pathogenicity island 1 (SPI-1) are used to initiate contact and facilitate uptake into nonphagocytic host cells, while genes within SPI-2 allow the pathogen to colonize host cells. While many studies have identified bacterial virulence determinants in animal models of infection, very few have focused on virulence gene expression at the single-cell level during an in vivo infection. To better understand when and where bacterial virulence factors are expressed during an acute enteric infection of a natural host, we infected bovine jejunal-ileal loops with S. Typhimurium cells harboring fluorescent transcriptional reporters for SPI-1 and -2 (PinvF and PssaG, respectively). After a prescribed time of infection, tissue and luminal fluid were collected and analyzed by microscopy. During early infection (≤2 h), bacteria within both intact and compromised membrane-bound vacuoles were observed within the epithelium, with the majority expressing SPI-1. As the infection progressed, S. Typhimurium displayed differential expression of the SPI-1 and SPI-2 regulons, with the majority of tissue-associated bacteria expressing SPI-2 and the majority of lumen-associated bacteria expressing SPI-1. This underscores the finding that Salmonella virulence gene expression changes as the pathogen transitions from one anatomical location to the next.