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ABSTRACTShigella flexneriis the major cause of shigellosis in developing countries. All serotypes except for serotype 6 share an O-antigen backbone composed of a ?2)-a-l-RhapIII-(1?2)-a-l-RhapII-(1?3)-a-l-RhapI-(1?3)-ß-d-GlcpNAc-(1? tetrasaccharide repeat. It can be modified by the addition of a glucosyl group to one or more sugar residues and/or an O-acetyl group to RhaIand/or a phosphoethanolamine to RhaIIand/or RhaIII. These modifications give rise to type I-, IC-, II-, IV-, and V- and to group 6-, 7,8-, and MASF IV-1-specific antigenic determinants, which comprise the current serotyping scheme of S. flexneri. Recently, another O-antigen modification created by adding an O-acetyl group to RhaIIIat position 3 or 4 (3/4-O-acetylation) has been found in S. flexneriserotypes 1a, 1b, 2a, 5a, Y, and 6. A new O-acyltransferase gene named oacBhas been shown to mediate the 3/4-O-acetylation in serotypes 1a, 1b, 2a, 5a, and Y but not in 6. In this work, we studied the distribution of the 3/4-O-acetylation in S. flexneriand the antigenicity that resulted from this modification. PCR screening of the oacBgene in clinical isolates of S. flexneridemonstrated that the oacB-mediated 3/4-O-acetylation is widespread in serotypes 1a, 1b, 2a, 5a, and Y. Serological analysis indicated that this modification confers the host with a novel antigenic determinant that is provisionally named group O factor 9. These findings enhance our understanding of the varieties of O-antigenic determinants related to O-antigen modification in S. flexneriand will assist epidemiological studies and vaccine development.

ABSTRACTShigella flexneriis the major cause of shigellosis in developing countries. A new S. flexneriserotype, Xv, appeared in 2000 and replaced serotype 2a as the most prevalent serotype in China. Serotype Xv is a variant of serotype X, with phosphoethanolamine modification of its O antigen mediated by a plasmid that contained the optgene. Serotype Xv isolates belong to sequence type 91 (ST91). In this study, whole-genome sequencing of 59 S. flexneriisolates of 14 serotypes (serotypes 1 to 4, Y, Yv, X, and Xv) indicated that ST91 arose around 1993 by acquiring multidrug resistance (MDR) and spread across China within a decade. A comparative analysis of the chromosome and opt-carrying plasmid pSFXv_2 revealed independent origins of 3 serotype Xv clusters in China, with different divergence times. Using 18 cluster-dividing single-nucleotide polymorphisms (SNPs), SNP typing divided 380 isolates from 3 provinces (Henan, Gansu, and Anhui) into 5 SNP genotypes (SGs). One SG predominated in each province, but substantial interregional spread of SGs was also evident. These findings suggest that MDR is the key selective pressure for the emergence of the S. flexneriepidemic clone and that Shigellaepidemics in China were caused by a combination of local expansion and interregional spread of serotype Xv.

Shigella flexneri is the major Shigella species that causes diarrheal disease in developing countries. It is further subdivided into 15 serotypes based on O-antigen structure. Serotyping of S. flexneri is important for epidemiological purposes. In this study, we developed a multiplex PCR assay targeting the O-antigen synthesis gene wzx and the O-antigen modification genes gtrI, gtrIC, gtrII, oac, gtrIV, gtrV, and gtrX for molecular serotyping of S. flexneri. The multiplex PCR assay contained eight sets of specific PCRs in a single tube and can identify 14 of the 15 serotypes (the exception being serotype Xv) of S. flexneri recognized thus far. A nearly perfect concordance (97.8%) between multiplex PCR assay and slide agglutination was observed when 358 S. flexneri strains of various serotypes were analyzed, except that 8 strains were carrying additional cryptic and/or defective serotype-specific genes. The multiplex PCR assay provides a rapid and specific method for the serotype identification of S. flexneri.

ABSTRACTShigellaspp. are the causative agent of shigellosis with Shigella flexneriserotype 2a being the most prevalent in developing countries. Epidemiological surveillance in China found that a new serotype of S. flexneriappeared in 2001 and replaced serotype 2a in 2003 as the most prevalent serotype in Henan Province. The new serotype also became the dominant serotype in 7 of the 10 other provinces under surveillance in China by 2007. The serotype was identified as a variant of serotype X. It differs from serotype X by agglutination to the monovalent anti-IV type antiserum and the group antigen-specific monoclonal antibody MASF IV-I. Genome sequencing of a serotype X variant isolate, 2002017, showed that it acquired a Shigellaserotype conversion island, also as an SfX bacteriophage, containing gtrgenes for type X-specific glucosylation. Multilocus sequence typing of 15 genes from 37 serotype X variant isolates and 69 isolates of eight other serotypes, 1a, 2a, 2b, 3a, 4a, 5b, X, and Y, found that all belong to a new sequence type (ST), ST91. Pulsed-field gel electrophoresis revealed 154 pulse types with 655 S. flexneriisolates analyzed and identified 57 serotype switching events. The data suggest that S. flexneriepidemics in China have been caused by a single epidemic clone, ST91, with frequent serotype switching to evade infection-induced immunity to serotypes to which the population was exposed previously. The clone has also acquired resistance to multiple antibiotics. These findings underscore the challenges to the current vaccine development and control strategies for shigellosis.

Shigellaspp. are a leading cause of human diarrheal disease worldwide, with Shigella flexneribeing the most frequently isolated species in developing countries.