Shigella sonnei

Abstract

Shigella sonnei is a rod-shaped, Gram-negative facultative intracellular pathogen. It was named ‘Sonne’s bacillus’ after Carl Olaf Sonne who described it as a causative agent of bacillary dysentery. S. sonnei is distributed worldwide and represents the most common cause of shigellosis in industrialized regions in Europe, North America, and Australia. It is currently undergoing expansion in middle-income countries across Asia, Latin America, and the Middle East. S. sonnei evolved from Escherichia coli to specialize in intracellular infection of the human gut epithelium, and its genome comprises a 4.99 Mbp circular chromosome and a 216 kbp invasion plasmid (pINV) required for virulence. The chromosome is ~6% smaller than other E. coli and is punctuated by >300 copies of insertion sequence (IS) elements, whose expansion has degraded the genome through disruption and deletion of genes. Here we describe the key and disease facts allowing bacteria to evade host immune defences and to establish infection. A chromosomally encoded type VI secretion system (T6SS), involved in bacterial competition, is a key determinant for niche occupancy. pINV-encoded virulence determinants include a type III secretion system (T3SS) involved in host cell invasion and phagosomal escape, the actin-polymerizing factor IcsA, a g4c capsule, and an unusual O-antigen encoded by genes horizontally acquired from Plesiomonas shigelloides. The g4c capsule and O-antigen of S. sonnei protect the bacteria from a wide variety of host defence mechanisms, including phagocytosis, phagolysosome degradation, and complement-mediated lysis. S. sonnei causes acute, self-limiting disease characterized by bloody diarrhea, fever, and abdominal pain. Infections can be life-threatening for children under 5 years and can stunt growth. S. sonnei spreads via the fecal–oral route, as ingested bacteria can survive the gastric acidity and be released in stool. Outbreaks are often centered around large events or care facilities. S. sonnei is also sexually transmitted, particularly among men who have sex with men. The main treatment for bacillary dysentery is rehydration therapy. Fluoroquinolones, cephalosporins, and azithromycin are recommended when antimicrobials are required. However, resistance is increasingly common and new antimicrobials are needed. There is no licensed vaccine. O-antigen-based immunization has been proposed; it holds particular promise as S. sonnei has only one serotype. KINGDOM: Bacteria PHYLUM: Proteobacteria CLASS: Gammaproteobacteria ORDER: Enterobacteriales FAMILY: Enterobacteriaceae GENUS: Shigella (whole-genome comparative analysis does not support distinct genus designation and suggests placement of Shigella spp. as lineages within the species E. coli) SPECIES: Shigella sonnei SUBSPECIES: S. sonnei is monoclonal. All circulating strains originated from a common ancestor in Europe ~1500 AD. There is only one serogroup (Shigella serogroup D) consisting of one serotype. Five distinct subtypes (lineages I–V) have been identified by whole-genome sequencing; the common laboratory strain 53G belongs to lineage II; however, most clinical isolates now belong to lineage III. Work in the S.M. laboratory is supported by a European Research Council Consolidator Grant ( 772853 – ENTRAPMENT), Wellcome Trust Senior Research Fellowship ( 206444/Z/17/Z ), and the Lister Institute of Preventive Medicine . K.E.H. is supported by a Senior Medical Research Fellowship from the Viertel Foundation of Australia . Download .docx (.05 MB) Help with docx files Table S1Country-specific references for S. sonnei resistance to first- and second- line antibiotics. (Listed in alphabetical order by country)

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