Abstract
Quantitative PCR analysis shows that the virulence plasmid of Salmonella enterica serovar Typhimurium (pSLT) is a low-copy-number plasmid, with 1–2 copies per chromosome. However, fluorescence microscopy observation of pSLT labeled with a lacO fluorescent tag reveals cell-to-cell differences in the number of foci, which ranges from 1 to 8. As each focus must correspond to ≥1 plasmid copy, the number of foci can be expected to indicate the minimal number of pSLT copies per cell. A correlation is found between the number of foci and the bacterial cell volume. In contrast, heterogeneity in the number of foci appears to be independent of the cell volume and may have stochastic origin. As a consequence of copy number heterogeneity, expression of a pSLT-bone reporter gene shows high levels of cell-to-cell variation, especially in actively dividing cultures. These observations support the notion that low-copy-number plasmids can be a source of gene expression noise in bacterial populations.
Highlights
Plasmid-encoded genes of bacterial pathogens can influence multiple aspects of the pathogenhost interaction, such as adhesion to host surfaces, invasion of host cells, intracellular survival, colonization of organs, and modulation of host immune responses (Pilla and Tang, 2018)
We provide evidence that the plasmid of Salmonella enterica serovar Typhimurium (pSLT) copy number is heterogeneous, especially among dividing cells
Oligonucleotide primers were designed to amplify a gene located relatively close to the origin of replication of the S. enterica chromosome, a gene located near the replication terminus, and the traJ and ccdB genes of pSLT
Summary
Plasmid-encoded genes of bacterial pathogens can influence multiple aspects of the pathogenhost interaction, such as adhesion to host surfaces, invasion of host cells, intracellular survival, colonization of organs, and modulation of host immune responses (Pilla and Tang, 2018). In Salmonella enterica, plasmids that encode virulence determinants were described several decades ago (Gulig et al, 1993). In serovar Typhimurium, most strains carry a plasmid of 50–100 kb known as the Salmonella virulence plasmid (Gulig et al, 1993; Guiney et al, 1994; Baumler et al, 1998). This plasmid was designated virulence plasmid of S. enterica serovar Typhimurium (pSLT) in strain LT2 (Jones et al, 1982), and the acronym was later extended to sibling plasmids of other Salmonella Typhimurium strains (Rotger and Casadesus, 1999)
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