Public Health Microbiology, Forensic and Scientific Services, Queensland Department of Health, Coopers Plains, Queensland, Australia.
PLoS One. 2019 Apr 11;14(4):e0215207. doi: 10.1371/journal.pone.0215207. eCollection 2019.
The virulence plasmid pSLT as exemplified by the 94 Kb plasmid in Salmonella Typhimurium strain LT2 is only found in isolates of serovar Typhimurium. While it occurs commonly among such isolates recent genotyping methods have shown that it is mostly confined to certain genotypes. Although pSLT plasmids are capable of self-transmissibility under experimental conditions their confinement to certain host genotypes suggests that in practice they are maintained by vertical rather than by horizontal transmission. This would imply that evolution of the pSLT plasmid proceeds in parallel with evolution of its host. The development of a phylogenetic evolutionary framework for genotypes of S. Typhimurium based on single-nucleotide-polymorphism (SNPs) typing provided an opportunity to test whether the pSLT plasmid coevolves with its host genotype. Accordingly SNPs analysis was applied to the pSLT plasmids from 71 strains S. Typhimurium of Australian and international origins representing most of the genotypes which commonly have a pSLT. The phylogenetic tree showed that pSLT sequences clustered into almost the same groups as the host chromosomes so that each pSLT genotype was associated with a single host genotype. A search for tandem repeats in pSLT plasmids showed that a 9 bp VNTR in the traD gene occurred in the pSLT from all isolates belonging to Clade II but not from isolates belonging to Clade I. Another 9 bp repeat occurred only in three Clade I genotypes with a recent common ancestor. The evidence relating to both of these VNTRs supports the proposition that the pSLT plasmid is only transmitted vertically. Some isolates belonging to one S. Typhimurium genotype were found to have pSLTs which have lost a large block of genes when a resistance gene cassette has been acquired. Examples were found of pSLT plasmids which have recombined with other plasmids to form fusion plasmids sometimes with loss of some pSLT genes. In all cases the underlying genotype of the modified pSLT was the same as the genotype of regular pSLTs with the same host genotype implying that these changes have occurred within the host cell of the pSLT plasmid.
毒力质粒 pSLT 以鼠伤寒沙门氏菌 LT2 株的 94 Kb 质粒为例，仅存在于血清型鼠伤寒沙门氏菌的分离株中。虽然它在这些分离株中很常见，但最近的基因分型方法表明，它主要局限于某些基因型。虽然 pSLT 质粒在实验条件下能够自我传播，但它们局限于某些宿主基因型表明，实际上它们是通过垂直传播而不是水平传播来维持的。这意味着 pSLT 质粒的进化与宿主的进化是并行的。基于单核苷酸多态性 (SNP) 分型的鼠伤寒沙门氏菌基因型的系统发育进化框架的发展提供了一个机会，以测试 pSLT 质粒是否与其宿主基因型共同进化。因此，对来自澳大利亚和国际来源的 71 株鼠伤寒沙门氏菌的 pSLT 质粒进行了 SNP 分析，这些菌株代表了通常具有 pSLT 的大多数基因型。系统发育树显示，pSLT 序列几乎与宿主染色体聚类成相同的组，因此每个 pSLT 基因型都与单个宿主基因型相关联。在 pSLT 质粒中搜索串联重复序列表明，traD 基因中的 9 bp VNTR 存在于属于 II 类群的所有分离株的 pSLT 中，但不存在于属于 I 类群的分离株中。另一个 9 bp 重复仅存在于具有最近共同祖先的三种 I 类群基因型中。这两个 VNTR 相关的证据都支持这样的观点，即 pSLT 质粒仅通过垂直传播进行传播。属于一个鼠伤寒沙门氏菌基因型的一些分离株发现，当获得抗性基因盒时，pSLT 已经失去了一大块基因。发现了一些 pSLT 质粒与其他质粒重组形成融合质粒的例子，有时会丢失一些 pSLT 基因。在所有情况下，修饰后的 pSLT 的基本基因型与具有相同宿主基因型的常规 pSLT 的基因型相同，这意味着这些变化是在 pSLT 质粒的宿主细胞内发生的。
