We report the identification and sequence from Escherichia coli and Salmonella enterica strains of the cld gene, encoding the chain-length determinant (CLD) which confers a modal distribution of chain length on the O-antigen component of lipopolysaccharide (LPS). The distribution of chain lengths in the absence of this gene fits a model in which as the chain is extended there is a constant probability of 0.165 of transfer of growing chain to LPS core, with termination of chain extension. The data for E. coli O111 fit a model in which the CLD reduces this probability for short chains and increases it to 0.4 for longer chains, leading to a reduced number of short chain molecules but an increase in numbers of longer molecules and transfer of essentially all molecules by chain length 21. We put forward a model for O-antigen polymerase which resembles the ribosome and fatty acid synthetase in having two sites, with the growing chain being transferred from a D site onto the new unit at the R site to extend the chain and then back to the D site to repeat the process. It is proposed that the CLD protein and polymerase form a complex which has two states: 'E' facilitating extension and 'T' facilitating transfer to core. The complex is postulated to enter the E state as O-antigen polymerization starts, and to shift to the T state after a predetermined time, the CLD acting as a molecular clock. The CLD is not O-antigen or species-specific but the modal value does depend on the source of the cld gene.

Previous studies have shown that Shigella flexneri bacteriophage X (SfX) encodes a glucosyltransferase (GtrX, formerly Gtr), which is involved in 0 antigen modification (serotype Y to serotype X). However, GtrX alone can only mediate a partial conversion. More recently, a three-gene cluster has been identified next to the attachment site in the genome of two other 5. flexneri bacteriophages (i.e. S N and Sfll). This gene cluster was postulated to be responsible for a full 0 antigen conversion. Here it is reported that besides the gtrX gene, the other two genes in the gtr locus of SfX were also involved in the 0 antigen modification process. The first gene in the cluster (gtrA) encodes a small highly hydrophobic protein which appears to be involved in the translocation of lipid-linked glucose across the cytoplasmic membrane. The second gene in the cluster (gtrl3) encodes an enzyme catalysing the transfer of the glucose residue from UDP-glucose to a lipid carrier. The third gene (gtrx) encodes a bacteriophage-specif ic glucosyltransferase which is largely responsible for the final step, i.e. attaching the glucosyl molecules onto the correct sugar residue of the 0 antigen repeating unit. A three-step model for the glucosylation of bacterial 0 antigen has been proposed.

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The O antigen of Escherichia coli O111 is identical in structure to that of Salmonella enterica serovar adelaide. Another O-antigen structure, similar to that of E. coli O111 and S. enterica serovar adelaide is found in both E. coli O55 and S. enterica serovar greenside. Both O-antigen structures contain colitose, a 3,6 dideoxyhexose found only rarely in the Enterobacteriaceae. The O-antigen structure is determined by genes generally located in the rfb gene cluster. We cloned the rfb gene cluster from an E. coli O111 strain (M92), and the clone expressed O antigen in both E. coli K-12 and a K-12 strain deleted for rfb. Lipopolysaccharide analysis showed that the O antigen produced by strains containing the cloned DNA is polymerized. The chain length of O antigen was affected by a region outside of rfb but linked to it and present on some of the plasmids containing rfb. The rfb region of M92 was analysed and compared, by DNA hybridization, with that of strains with related O antigens. The possible evolution of the rfb genes in these O antigen groups is discussed.

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Shigella is the cause of shigellosis or bacillary dysentery, the occurrence of which is estimated to be 165 million cases per year worldwide, resulting in 1.1 million deaths. Rapid and reliable assays for detecting and identifying Shigella in food, environmental and clinical samples are therefore necessary. Shigella species are traditionally identified by their O antigens. This study developed a DNA microarray targeting O-serotype-specific genes to detect all 34 distinct O-antigen forms of Shigella, including Shigella boydii types 1–18, Shigella dysenteriae types 1–13, Shigella flexneri types 1–5 and 6, and Shigella sonnei. A total of 282 strains were used to test the specificity of the microarray, including 186 Shigella and Escherichia coli representative strains, 86 Shigella clinical isolates and ten strains of other bacterial species that are commonly isolated from food or clinical stool specimens. The oligonucleotide probes were printed on the microarray in concentrations from 1 to 100 μM, and 10 μM proved to be the optimal probe concentration. The detection sensitivity for each serotype was 50 ng genomic DNA or 1 c.f.u. in 25 g milk powder sample following a 6 h enrichment in broth. The microarray is specific, sensitive and reproducible, and, to our knowledge, is the first report of a microarray for serotyping all O-antigen forms of Shigella.

Shigella and pathogenic Escherichia coli are major causes of human infectious diseases and are responsible for millions of cases of diarrhea worldwide every year. A convenient and rapid method to identify highly pathogenic serotypes of Shigella and E. coli is needed for large-scale epidemiologic study, timely clinical diagnosis, and reliable quarantine of the pathogens. In this study, a DNA microarray targeting O-serotypespecific genes was developed to detect 15 serotypes of Shigella and E. coli, including Shigella sonnei; Shigella flexneri type 2a; Shigella boydii types 7, 9, 13, 16, and 18; Shigella dysenteriae types 4, 8, and 10; and E. coli O55, O111, O114, O128, and O157. The microarray was tested against 186 representative strains of all Shigella and E. coli O serotypes, 38 clinical isolates, and 9 strains of other bacterial species that are commonly present in stool samples and was shown to be specific and reproducible. The detection sensitivity was 50 ng genomic DNA or 10 4 CFU per ml in mock stool specimens. This is the first report of a microarray for serotyping Shigella and pathogenic E. coli. The method has a number of advantages over traditional bacterial culture and antiserum agglutination methods and is promising for applications in basic microbiological research, clinical diagnosis, food safety, and epidemiological surveillance.