Shigella is the major cause of endemic bacillary dysentery (shigellosis) in developing countries. It is estimated that there are about 164.7 million cases of shigellosis annually worldwide, of which 163.2 million were in developing countries, resulting in 1,1 million deaths, most of which were children under 5 years of age . Among the four Shigella species, S. dysenteriae, S. flexneri, S. boydii, and S. sonnei, S. flexneri is the predominant species.
Based on the combination of antigenic determinants present in the O-antigen of the cell envelope lipopolysaccharide (LPS), S. flexneri is further divided into various serotypes. To date, at least 16 serotypes have been recognized [2–4]. Except for serotype 6, all share a basic repeating tetrasaccharide unit, comprised of one GlcNAc and three rhamnoses . Modifications to the side chain of the tetrasaccharide by the addition of glucosyl and/or O-acetyl groups give rise to various antigenic determinants . The genes responsible for the O-antigen modification are always either the gene cluster gtrABC for glucosyl groups or the single oac gene for the O-acetyl group; all encoded by serotype-converting bacteriophages [3, 5–10]. In all glucosylation modification phages, the gtrABC gene cluster is always located immediately upstream of the attP site, followed by the int and xis genes .
Up to now, four S. flexneri serotype-converting bacteriophages, SfV, SfX, Sf6 and SfII, have been induced and purified by different groups [8, 11–13]. Morphologically, SfV and SfII, which have an isometric head and a long tail, belong to Group A in the family of Myoviridae[8, 11]; while SfX and Sf6, which possess a short tail linked to an isometric head, belong to the family of Podovirida[12, 13]. The complete genome sequences of phage SfV and Sf6 have been obtained by directly sequencing the phage DNA purified from phage particles, and their genetic features have been well characterized [9, 10]. Recently, the prophage genome of SfX was determined from the sequenced S. flexneri serotype Xv strain 2002017; which is presumably the whole genome of phage SfX, because a SfX phage particle can be induced and isolated from 2002017 . The SfX genome is 37,355 bp length, encoding 59 ORFs (unpublished data). The genome of SfII has not yet been sequenced from free phage particles, but prophage genomes can be derived from sequenced S. flexneri serotype 2a strains Sf301 and 2457T [14, 15], which show considerable variation with one or both being prophage remnants.
S. flexneri serotype 1 is defined by reaction with type I antisera. A total of 4 subtypes, 1a, 1b, 1c and 1d have been recognized [16–18]. In serotype 1, a glucosyl group is attached to the GlcNac residue of the repeating unit by an alpha-1, 4 linkage, which results in the presence of serotype 1-specific I antigen. The type I modification is mediated by an O-antigen glucosylation locus (gtrI, gtrA, gtrB) encoded on the SfI prophage genome . The glucosylation genes and flanking partial SfI sequences were previously obtained from a serotype 1a strain Y53 . However, the free phage particle of SfI had not been isolated, and its full genomic characteristics have not yet been elucidated .
In this study, we induced and purified the free SfI phage particles from S. flexneri serotype 1a clinical strain 019 and characterized its morphology, host range and genomic features.