Accessory genetic content in Campylobacter jejuni ST21CC isolates from feces and blood

Campylobacter jejuni is an important foodborne pathogen and the most commonly reported bacterial cause of gastroenteritis. C. jejuni is occasionally found in blood, although mechanisms important for invasiveness have remained unclear. C. jejuni is divided into many different lineages, of which the ST21 clonal complex (CC) is widely distributed. Here, we performed comparative genomic and in vitro analyses on 17C. jejuni ST21CC strains derived from human blood and feces in order to identify features associated with isolation site. The ST21CC lineage is divided into two large groups; centered around ST-21 and ST-50. Our clinical strains, typed as ST-50, showed further microevolution into two distinct clusters. These clusters were distinguished by major differences in their capsule loci and the distribution of accessory genetic content, including C. jejuni integrated elements (CJIEs) and plasmids. Accessory genetic content was more common among fecal than blood strains, whereas blood strains contained a hybrid capsule locus which partially consisted of C. jejuni subsp. doylei-like content. In vitro infection assays with human colon cell lines did not show significant differences in adherence and invasion between the blood and fecal strains. Our results showed that CJIEs and plasmid derived genetic material were less common among blood isolates than fecal isolates; in contrast, hybrid capsule loci, especially those containing C. jejuni subsp. doylei-like gene content, were found among many isolates derived from blood. The role of these findings requires more detailed investigation.