Characterization of serotype 6 strains from geographically diverse low- and middle-income countries.

serotype 6 (6) is one of the most common serotypes recovered from surveillance studies of moderate to severe diarrhea. Despite the clinical significance of 6, this serotype is understudied. In this work, we have performed both serotype-specific genomic and phenotypic comparisons of 6 isolates to one another and non-. serotypes. Comparative genomic analyses identified significant nucleotide homology between 6 strains ( = 325), despite a broad range of collection timeframes and geographic locations. We identified 6 specific factors, including a potential novel virulence factor (type II secretion system). Additionally, we identified established virulence genes () and metabolic genes () that were absent in 6 strains while present in the majority of 728 non-6 strains. Complete sequencing of 11 clinical 6 strains, demonstrated that the 6 virulence plasmid (pINV) is ~38 kb smaller than the average non-6 pINV (~228 kb). Comparisons of species level antibiotic susceptibility highlighted that clinical 6 isolates from Africa in the Global Enteric Multicenter Study (GEMS) and Vaccine Impact on Diarrhea in Africa (VIDA) study demonstrated geographic, serotype-specific susceptibility pattern. Phenotypic analyses of 6 identified reduced intracellular invasion and cytokine induction from HT-29 cells, as well as reduced Ipa protein effector secretion, compared with serotype 2a strain 2457T. Together these data highlight conserved and unique serotype-specific genotypic and phenotypic features for 6. This level of conservation has not been noted for other serotypes and is promising for vaccine and diagnostic assays to provide global 6-specific coverage.IMPORTANCEShigellosis is an ongoing global public health crisis with >270 million annual episodes among all age groups; however, the greatest disease burden is among children in low- and middle-income countries (LMIC). The lack of a licensed vaccine and the observed rise in antimicrobial-resistant spp. highlights the urgency for effective preventative and interventional strategies. The inclusion of serotype 6 (6) is a necessary component of a multivalent vaccine strategies based on its clinical and epidemiological importance. Given the genomic diversity of 6 compared with other serotypes and 6 unique O-antigen core structure, serotype-specific characterization of 6 is a critical step to inform -directed vaccine and alternative therapeutic designs. Herein, we identified conserved genomic content among a large collection of temporally and geographically diverse 6 clinical isolates and characterized genotypic and phenotypic properties that separate 6 from non-6 serotypes.