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 loci 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.

定量PCR分析表明，大肠杆菌的毒力质粒。 肠沙门氏菌鼠伤寒血清型（pSLT）是低拷贝数质粒，每个染色体有1-2个拷贝。但是，荧光显微镜观察到的pSLT标记有乳胶荧光标签揭示了焦点之间的细胞间差异，范围从1到8。由于每个焦点必须对应于≥1个质粒拷贝，因此可以预期焦点数目表明每个细胞中pSLT拷贝的最小数目。在病灶数量和细菌细胞体积之间发现相关性。相反，基因座数目的异质性似乎与细胞体积无关，并且可能具有随机起源。由于拷贝数异质性，pSLT骨报道基因的表达显示出高水平的细胞间差异，尤其是在主动分裂培养物中。这些观察结果支持以下观点：低拷贝数的质粒可能是细菌种群中基因表达噪声的来源。