Comparative genomics of subsp. serovar 61:k:1,5,(7) reveals lineage-specific host adaptation of ST432.

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.