Department of Biological Safety, German Federal Institute for Risk Assessment (BfR), Max-Dohrn-Straße 8-10, 10589 Berlin, Germany.
Unit for Enteropathogenic Bacteria and Legionella (FG11)/National Reference Centre for Salmonella and Other Bacterial Enteric Pathogens, Robert Koch Institute (RKI), Burgstraße 37, 38855 Wernigerode, Germany.
Microb Genom. 2021 Aug;7(8). doi: 10.1099/mgen.0.000604.
Unlike most subsp. , which are predominantly associated with cold-blooded animals such as reptiles the serovar IIIb 61:k:1,5,(7) (termed SASd) is regarded as host-adapted to sheep. The bacterium is rarely associated with disease in humans but, nevertheless, SASd isolates are sporadically obtained from human clinical samples. It is unclear whether these transmissions are directly linked to sheep or whether transmissions may, for example, occur through other domestic animals also carrying SASd. For this reason, we utilized whole-genome sequencing to investigate a set of 119 diverse SASd isolates, including sheep-associated and human-associated isolates, as well as isolates obtained from other matrices. We discovered that serovar IIIb 61:k:1,5,(7) is composed of two distinct lineages defined by their sequence types ST432 and ST439. These two lineages are distinguished by a number of genetic features, as well as their prevalence and reservoir. ST432 appears to be the more prevalent sequence type, with the majority of isolates investigated in this study belonging to ST432. In contrast, only a small number of isolates were attributed to ST439. While ST432 isolates were of sheep, human or other origin, all ST439 isolates with source information available, were obtained from human clinical samples. Regarding their genetic features, lineage ST432 shows increased pseudogenization, harbours a plasmid that encodes a type IV secretion system (T4SS) and does not possess the gene cluster, which encodes a salmochelin siderophore for iron acquisition. These characteristics likely contribute to the ability of ST432 to persistently colonize the intestines of sheep. Furthermore, we found isolates of the lineage ST432 to be highly clonal, with little variation over the sampling period of almost 20 years. We conclude from the genomic comparisons that SASd underlies a microevolutionary process and that it is specifically lineage ST432 that should be considered as host-adapted to sheep.
与大多数主要与冷血动物（如爬行动物）相关的亚种不同，血清型 IIIb 61:k:1,5,(7)（称为 SASd）被认为适应于绵羊宿主。该细菌很少与人类疾病相关，但仍然偶尔从人类临床样本中分离到 SASd 分离株。目前尚不清楚这些传播是否直接与绵羊有关，或者传播是否可能例如通过携带 SASd 的其他家畜发生。出于这个原因，我们利用全基因组测序技术对一组 119 种不同的 SASd 分离株进行了研究，包括绵羊相关和人类相关的分离株，以及从其他基质中分离出的分离株。我们发现血清型 IIIb 61:k:1,5,(7) 由两个不同的谱系组成，它们由序列类型 ST432 和 ST439 定义。这两个谱系在许多遗传特征以及它们的流行程度和储存库方面存在差异。ST432 似乎是更为流行的序列类型，本研究中调查的大多数分离株都属于 ST432。相比之下，只有少数分离株属于 ST439。虽然 ST432 分离株来自绵羊、人类或其他来源，但具有来源信息的所有 ST439 分离株均来自人类临床样本。关于它们的遗传特征，谱系 ST432 显示出增加的假基因化，携带编码 IV 型分泌系统（T4SS）的质粒，并且不具有编码用于铁摄取的salmochelin 铁载体的基因簇。这些特征可能有助于 ST432 持续定植绵羊肠道的能力。此外，我们发现 ST432 谱系的分离株高度克隆，在近 20 年的采样期间几乎没有变化。我们从基因组比较中得出结论，SASd 存在微观进化过程，并且应该将 ST432 谱系视为适应绵羊的宿主。
