Characterization of six flagellin genes in the H3, H53 and H54 standard strains of Escherichia coli

Diversification of Escherichia albertii H-Antigens and Development of H-Genotyping PCR

Escherichia albertii is a recently recognized human enteropathogen that is closely related to Escherichia coli. As E. albertii sometimes causes outbreaks of gastroenteritis, rapid strain typing systems, such as the O- and H-serotyping systems widely used for E. coli, will be useful for outbreak investigation and surveillance. Although an O-genotyping system has recently been developed, the diversity of E. albertii H-antigens (flagellins) encoded by fliC genes remains to be systematically investigated, and no H-serotyping or genotyping system is currently available. Here, we analyzed the fliC genes of 243 genome-sequenced E. albertii strains and identified 73 sequence types, which were grouped into four clearly distinguishable types designated E. albertii H-genotypes 1–4 (EAHg1–EAHg4). Although there was a clear sign of intraspecies transfer of fliC genes in E. albertii, none of the four E. albertii H-genotypes (EAHgs) were closely related to any of the 53 known E. coli H-antigens, indicating the absence or rare occurrence of interspecies transfer of fliC genes between the two species. Although the analysis of more E. albertii strains will be required to confirm the low level of variation in their fliC genes, this finding suggests that E. albertii may exist in limited natural hosts or environments and/or that the flagella of E. albertii may function in a limited stage(s) in their life cycle. Based on the fliC sequences of the four EAHgs, we developed a multiplex PCR-based H-genotyping system for E. albertii (EAH-genotyping PCR), which will be useful for epidemiological studies of E. albertii infections.

Identification of Two New Sequences of Flagellin-Encoding Gene in Escherichia coli Using PCR and Sequencing-Based Methods

Escherichia coli has traditionally been serotyped using antisera against the O and H antigens. However, a proportion of E. coli isolates are nonmotile and, in addition, some isolates do not react with the currently available H-typing sera. Alternative molecular methods have been developed based on the detection of genes encoding for H antigens. In this study, we studied 13 serologically nontypable H antigen E. coli strains using polymerase chain reaction (PCR) and sequencing-based methods. We found two new sequences of flagellin-encoding gene, for each of which a specific antiserum was produced to confirm their expression. Sequencing of the flagellin gene offers a rapid determination of E. coli H antigens and could be used to detect potential novel flagellar antigens.

A Targeted Sequencing Assay for Serotyping Escherichia coli Using AgriSeq Technology

The gold standard method for serotyping Escherichia coli has relied on antisera-based typing of the O- and H-antigens, which is labor intensive and often unreliable. In the post-genomic era, sequence-based assays are potentially faster to provide results, could combine O-serogrouping and H-typing in a single test, and could simultaneously screen for the presence of other genetic markers of interest such as virulence factors. Whole genome sequencing is one approach; however, this method has limited multiplexing capabilities, and only a small fraction of the sequence is informative for subtyping or identifying virulence potential. A targeted, sequence-based assay and accompanying software for data analysis would be a great improvement over the currently available methods for serotyping. The purpose of this study was to develop a high-throughput, molecular method for serotyping E. coli by sequencing the genes that are required for production of O- and H-antigens, as well as to develop software for data analysis and serotype identification. To expand the utility of the assay, targets for the virulence factors, Shiga toxins (stx₁, and stx₂) and intimin (eae) were included. To validate the assay, genomic DNA was extracted from O-serogroup and H-type standard strains and from Shiga toxin-producing E. coli, the targeted regions were amplified, and then sequencing libraries were prepared from the amplified products followed by sequencing of the libraries on the Ion S5™ sequencer. The resulting sequence files were analyzed via the SeroType Caller™ software for identification of O-serogroup, H-type, and presence of stx₁, stx₂, and eae. We successfully identified 169 O-serogroups and 41 H-types. The assay also routinely detected the presence of stx_1a,c,d (3 of 3 strains), stx_2c−e,g (8 of 8 strains), stx_2f (1 strain), and eae (6 of 6 strains). Taken together, the high-throughput, sequence-based method presented here is a reliable alternative to antisera-based serotyping methods for E. coli.

Koutsoumanis K, Allende A, Alvarez-Ordóñez A, Bolton D, Bover-Cid S, Chemaly M, Davies R, De Cesare A, Hilbert F, Lindqvist R, Nauta M, Peixe L, Ru G, Simmons M, Skandamis P, Suffredini E, Jenkins C, Malorny B, Ribeiro Duarte AS, Torpdahl M, da Silva Felício MT, Guerra B, Rossi M, Herman L. Whole genome sequencing and metagenomics for outbreak investigation, source attribution and risk assessment of food-borne microorganisms

This Opinion considers the application of whole genome sequencing (WGS) and metagenomics for outbreak investigation, source attribution and risk assessment of food‐borne pathogens. WGS offers the highest level of bacterial strain discrimination for food‐borne outbreak investigation and source‐attribution as well as potential for more precise hazard identification, thereby facilitating more targeted risk assessment and risk management. WGS improves linking of sporadic cases associated with different food products and geographical regions to a point source outbreak and can facilitate epidemiological investigations, allowing also the use of previously sequenced genomes. Source attribution may be favoured by improved identification of transmission pathways, through the integration of spatial‐temporal factors and the detection of multidirectional transmission and pathogen–host interactions. Metagenomics has potential, especially in relation to the detection and characterisation of non‐culturable, difficult‐to‐culture or slow‐growing microorganisms, for tracking of hazard‐related genetic determinants and the dynamic evaluation of the composition and functionality of complex microbial communities. A SWOT analysis is provided on the use of WGS and metagenomics for Salmonella and Shigatoxin‐producing Escherichia coli (STEC) serotyping and the identification of antimicrobial resistance determinants in bacteria. Close agreement between phenotypic and WGS‐based genotyping data has been observed. WGS provides additional information on the nature and localisation of antimicrobial resistance determinants and on their dissemination potential by horizontal gene transfer, as well as on genes relating to virulence and biological fitness. Interoperable data will play a major role in the future use of WGS and metagenomic data. Capacity building based on harmonised, quality controlled operational systems within European laboratories and worldwide is essential for the investigation of cross‐border outbreaks and for the development of international standardised risk assessments of food‐borne microorganisms.

Escherichia coli H-Genotyping PCR: a Complete and Practical Platform for Molecular H Typing

In Escherichia coli, more than 180 O groups and 53 H types have been recognized. The O:H serotyping of E. coli strains is an effective method for identifying strains with pathogenic potential and classifying them into clonal groups. In particular, the serotyping of Shiga toxin-producing E. coli (STEC) strains provides valuable information to evaluate the routes, sources, and prevalence of agents in outbreak investigations and surveillance. Here, we present a complete and practical PCR-based H-typing system, E. coli H-genotyping PCR, consisting of 10 multiplex PCR kits with 51 single PCR primer pairs. Primers were designed based on a detailed comparative analysis of sequences from all H-antigen (flagellin)-encoding genes, fliC and its homologs. The specificity of this system was confirmed by using all H type reference strains. Additionally, 362 serotyped wild strains were also used to evaluate its practicality. All 277 H-type-identified isolates gave PCR products that corresponded to the results of serological H typing. Moreover, 76 nonmotile and nine untypeable strains could be successfully subtyped into any H type by the PCR system. The E. coli H-genotyping PCR developed here allows broader, rapid, and low-cost subtyping of H types and will assist epidemiological studies as well as surveillance of pathogenic E. coli.

Strain-Level Discrimination of Shiga Toxin-Producing Escherichia coli in Spinach Using Metagenomic Sequencing

Consumption of fresh bagged spinach contaminated with Shiga toxin-producing Escherichia coli (STEC) has led to severe illness and death; however current culture-based methods to detect foodborne STEC are time consuming. Since not all STEC strains are considered pathogenic to humans, it is crucial to incorporate virulence characterization of STEC in the detection method. In this study, we assess the comprehensiveness of utilizing a shotgun metagenomics approach for detection and strain-level identification by spiking spinach with a variety of genomically disparate STEC strains at a low contamination level of 0.1 CFU/g. Molecular serotyping, virulence gene characterization, microbial community analysis, and E. coli core gene single nucleotide polymorphism (SNP) analysis were performed on metagenomic sequence data from enriched samples. It was determined from bacterial community analysis that E. coli, which was classified at the phylogroup level, was a major component of the population in most samples. However, in over half the samples, molecular serotyping revealed the presence of indigenous E. coli which also contributed to the percent abundance of E. coli. Despite the presence of additional E. coli strains, the serotype and virulence genes of the spiked STEC, including correct Shiga toxin subtype, were detected in 94% of the samples with a total number of reads per sample averaging 2.4 million. Variation in STEC abundance and/or detection was observed in replicate spiked samples, indicating an effect from the indigenous microbiota during enrichment. SNP analysis of the metagenomic data correctly placed the spiked STEC in a phylogeny of related strains in cases where the indigenous E. coli did not predominate in the enriched sample. Also, for these samples, our analysis demonstrates that strain-level phylogenetic resolution is possible using shotgun metagenomic data for determining the genomic relatedness of a contaminating STEC strain to other closely related E. coli.

In silico serotyping of E. coli from short read data identifies limited novel O-loci but extensive diversity of O:H serotype combinations within and between pathogenic lineages

The lipopolysaccharide (O) and flagellar (H) surface antigens of Escherichia coli are targets for serotyping that have traditionally been used to identify pathogenic lineages. These surface antigens are important for the survival of E. coli within mammalian hosts. However, traditional serotyping has several limitations, and public health reference laboratories are increasingly moving towards whole genome sequencing (WGS) to characterize bacterial isolates. Here we present a method to rapidly and accurately serotype E. coli isolates from raw, short read WGS data. Our approach bypasses the need for de novo genome assembly by directly screening WGS reads against a curated database of alleles linked to known and novel E. coli O-groups and H-types (the EcOH database) using the software package srst2. We validated the approach by comparing in silico results for 197 enteropathogenic E. coli isolates with those obtained by serological phenotyping in an independent laboratory. We then demonstrated the utility of our method to characterize isolates in public health and clinical settings, and to explore the genetic diversity of >1500 E. coli genomes from multiple sources. Importantly, we showed that transfer of O- and H-antigen loci between E. coli chromosomal backbones is common, with little evidence of constraints by host or pathotype, suggesting that E. coli ‘strain space’ may be virtually unlimited, even within specific pathotypes. Our findings show that serotyping is most useful when used in combination with strain genotyping to characterize microevolution events within an inferred population structure.

Hybrid Shiga Toxin-Producing and Enterotoxigenic Escherichia sp. Cryptic Lineage 1 Strain 7v Harbors a Hybrid Plasmid

Hybrid isolates of Shiga toxin-producing Escherichia coli (STEC) and enterotoxigenic E. coli (ETEC) encoding heat-stable enterotoxin (ST) are being reported with increasing frequency from a variety of sources. However, information regarding the plasmids that these strains harbor is scarce. In this study, we sequence and characterize a plasmid, p7v, from the STEC/ETEC hybrid strain 7v. Whole-genome phylogenetic analyses of STEC/ETEC hybrid strains and prototype E. coli isolates of other pathotypes placed 7v in the Escherichia sp. cryptic lineage 1 (CL1) clade. The complete plasmid, p7v, was determined to be 229,275 bp and encodes putative virulence factors that are typically carried on STEC plasmids as well as those often carried on ETEC plasmids, indicating that the hybrid nature of the strain extends beyond merely encoding the two toxins. Plasmid p7v carries two copies of sta with identical sequences, which were discovered to be divergent from the sta sequences found in the prototype human ETEC strains. Using a nomenclature scheme based on a phylogeny constructed from sta and stb sequences, the sta encoded on p7v is designated STa4. In silico analysis determined that p7v also encodes the K88 fimbria, a colonization factor usually associated with porcine ETEC plasmids. The p7v sequence and the presence of plasmid-encoded virulence factors are compared to those of other STEC/ETEC CL1 hybrid genomes and reveal gene acquisition/loss at the strain level. In addition, the interrogation of 24 STEC/ETEC hybrid genomes for identification of plasmid replicons, colonization factors, Stx and ST subtypes, and other plasmid-encoded virulence genes highlights the diversity of these hybrid strains.

IMPORTANCE

Hybrid Shiga toxin-producing Escherichia coli/enterotoxigenic Escherichia coli (STEC/ETEC) strains, which have been isolated from environmental, animal, and human clinical samples, may represent an emerging threat as food-borne pathogens. Characterization of these strains is important for assessing virulence potential, aiding in the development of pathogen detection methods, and understanding how the hybrid strains evolve to potentially have a greater impact on public health. This study represents, to our knowledge, both the first characterization of a closed plasmid sequence from a STEC/ETEC hybrid strain and the most comprehensive phylogenetic analysis of available STEC/ETEC hybrid genomes to date. The results demonstrate how the mobility of plasmid-associated virulence genes has resulted in the creation of a diverse plasmid repertoire within the STEC/ETEC hybrid strains.

Rapid and Easy In Silico Serotyping of Escherichia coli Isolates by Use of Whole-Genome Sequencing Data

Accurate and rapid typing of pathogens is essential for effective surveillance and outbreak detection. Conventional serotyping of Escherichia coli is a delicate, laborious, time-consuming, and expensive procedure. With whole-genome sequencing (WGS) becoming cheaper, it has vast potential in routine typing and surveillance. The aim of this study was to establish a valid and publicly available tool for WGS-based in silico serotyping of E. coli applicable for routine typing and surveillance. A FASTA database of specific O-antigen processing system genes for O typing and flagellin genes for H typing was created as a component of the publicly available Web tools hosted by the Center for Genomic Epidemiology (CGE) (www.genomicepidemiology.org). All E. coli isolates available with WGS data and conventional serotype information were subjected to WGS-based serotyping employing this specific SerotypeFinder CGE tool. SerotypeFinder was evaluated on 682 E. coli genomes, 108 of which were sequenced for this study, where both the whole genome and the serotype were available. In total, 601 and 509 isolates were included for O and H typing, respectively. The O-antigen genes wzx, wzy, wzm, and wzt and the flagellin genes fliC, flkA, fllA, flmA, and flnA were detected in 569 and 508 genome sequences, respectively. SerotypeFinder for WGS-based O and H typing predicted 560 of 569 O types and 504 of 508 H types, consistent with conventional serotyping. In combination with other available WGS typing tools, E. coli serotyping can be performed solely from WGS data, providing faster and cheaper typing than current routine procedures and making WGS typing a superior alternative to conventional typing strategies.

Identification of new flagellin-encoding fliC genes in Escherichia coli isolated from domestic animals using RFLP-PCR and sequencing methods

Identification of Escherichia coli requires knowledge regarding the prevalent serotypes and virulence factors profiles allows the classification in pathogenic/non-pathogenic. However, some of these bacteria do not express flagellar antigen invitro. In this case the PCR-restriction fragment length polymorphism (RFLP-PCR) and sequencing of the fliC may be suitable for the identification of antigens by replacing the traditional serology. We studied 17 samples of E. coli isolated from animals and presenting antigen H nontypeable (HNT). The H antigens were characterized by PCR-RFLP and sequencing of fliC gene. Three new flagellin genes were identified, for which specific antisera were obtained. The PCR-RFLP was shown to be faster than the serotyping H antigen in E. coli, provided information on some characteristics of these antigens and indicated the presence of new genes fliC.

A novel non-homologous recombination-mediated mechanism for Escherichia coli unilateral flagellar phase variation

Flagella contribute to the virulence of bacteria through chemotaxis, adhesion to and invasion of host surfaces. Flagellar phase variation is believed to facilitate bacterial evasion of the host immune response. In this study, the flnA gene that encodes Escherichia coli H17 flagellin was examined by whole genome sequencing and genetic deletion analysis. Unilateral flagellar phase variation has been reported in E. coli H3, H47 and H17 strains, although the mechanism for phase variation in the H17 strain has not been previously understood. Analysis of phase variants indicated that the flagellar phase variation in the H17 strain was caused by the deletion of an ∼35 kb DNA region containing the flnA gene from diverse excision sites. The presence of covalently closed extrachromosomal circular forms of this excised 35 kb region was confirmed by the two-step polymerase chain reaction. The deletion and complementation test revealed that the Int1157 integrase, a tyrosine recombinase, mediates the excision of this region. Unlike most tyrosine recombinases, Int1157 is suggested to recognize diverse sites and mediate recombination between non-homologous DNA sequences. This is the first report of non-homologous recombination mediating flagellar phase variation.

Alteration of flagellar phenotype of Escherichia coli strain P12b, the standard type strain for flagellar antigen H17, possessing a new non-fliC flagellin gene flnA, and possible loss of original flagellar phenotype and genotype in the course of subculturing through semisolid media

A practically important phenomenon, resulting in the loss of the original flagellar phenotype (genotype) of bacteria, is described in the Escherichia coli H17 type strain P12b possessing two distinct genes for H17 and H4 flagellins, respectively. By PCR, sequencing, and phylogenetic investigation, the H17 gene (originally expressed) was considered a new non-fliC flagellin gene and assigned flnA, while the H4 gene (originally cryptic) was reaffirmed as fliC. H17 and H4 flagella differed morphologically. The phenomenon consisted in the replacement of H17 cells by H4 cells during subculturing through certain semisolid media and resulted from the excision of flnA (H17) entirely or in part. The substitution rate depended on the density and nutrient composition of media and reached 100% even after a single passage through 0.3% LB agar. Such phenomenon can lead to an unexpected loss of original H17 phenotype. Our review of the literature showed that the loss of the original flagellar genotype (phenotype) of P12b has occurred in some laboratories while the authors continued to consider their cultures H17. We showed how to distinguish these alternative flagellin genotypes using popular fliC primers. Attention was also paid to possible discrepancies between serological and molecular results in flagellar typing of E. coli.

A genomic islet mediates flagellar phase variation in Escherichia coli strains carrying the flagellin-specifying locus flk

The occurrence of unilateral flagellar phase variation was previously demonstrated in Escherichia coli strains carrying the non-fliC flagellin-specifying locus flk. In this study, we investigated the mechanism involved in this process. By using sequencing and sequence analysis, the flk region between the chromosomal genes yhaC and rnpB was characterized in all described flk-positive E. coli strains, including the H35 strain identified in this study (the other strains used are H3, H36, H47, and H53 strains), and this region was found to contain a putative integrase gene and flanking direct repeats in addition to the flk flagellin-specifying gene flkA and a fliC repressor gene, flkB, indicating that there is a typical genomic islet (GI), which was designated the flk GI. The horizontal transfer potential of the flk GI was indicated by detection of the excised extrachromosomal circular form of the flk GI. By generating fliC-expressing variants of H3 and H47 strains, unilateral flagellar phase variation in flk-positive strains was shown to be mediated by excision of the flk GI. The function of the proposed integrase gene was confirmed by deletion and a complementation test. The potential integration sites of the flk GI were identified. A general model for flagellar phase variation in flk-positive E. coli strains can be expressed as fliC(off) + flkA(on) --> fliC(on) + flkA(none). This is the first time that a molecular mechanism for flagellar phase variation has been reported for E. coli.

Expressed and cryptic flagellin genes in the H44 and H55 type strains of Escherichia coli

Bacterial H antigens are specified by flagellin molecules, which constitute the flagellar filament. Escherichia coli 781-55 and E2987-73 are the type strains for H44 and H55 antigens, respectively. Unlike E. coli K-12, they possess two flagellin genes, fliC and fllA, on their chromosomes. However, they are monophasic, expressing exclusively the fllA genes, which specify the type antigens. In this study, the flagellin genes were cloned from these strains and their structure and expression were analyzed. It was found that the fliC genes encode apparently intact flagellin subunits but possess inefficient sigma28-dependent promoters, which may result in these genes being silent. The chromosomal locations of the fllA genes are approximately, but not exactly, identical with that of the phase-2 flagellin gene, fljB, of diphasic Salmonella strains. However, unlike the Salmonella fljB gene, the invertible H segment and the fljA gene responsible for the control of flagellar phase variation are both absent from the fllA loci. The fllA genes are highly homologous to the E. coli fliC gene but distantly related to the Salmonella fljB gene. These results suggest a hypothesis that the fllA genes may have emerged by an intra-species lateral transfer of the fliC gene. This hypothesis is further supported by the observation that the fllA genes are flanked by several IS elements and located within cryptic prophage elements.

Fast DNA serotyping of Escherichia coli by use of an oligonucleotide microarray

Classical antibody-based serotyping of Escherichia coli is an important method in diagnostic microbiology for epidemiological purposes, as well as for a rough virulence assessment. However, serotyping is so tedious that its use is restricted to a few reference laboratories. To improve this situation we developed and validated a genetic approach for serotyping based on the microarray technology. The genes encoding the O-antigen flippase (wzx) and the O-antigen polymerase (wzy) were selected as target sequences for the O antigen, whereas fliC and related genes, which code for the flagellar monomer, were chosen as representatives for the H phenotype. Starting with a detailed bioinformatic analysis and oligonucleotide design, an ArrayTube-based assay was established: a fast and robust DNA extraction method was coupled with a site-specific, linear multiplex labeling procedure and hybridization analysis of the biotinylated amplicons. The microarray contained oligonucleotide DNA probes, each in duplicate, representing 24 of the epidemiologically most relevant of the over 180 known O antigens (O antigens 4, 6 to 9, 15, 26, 52, 53, 55, 79, 86, 91, 101, 103, 104, 111, 113, 114, 121, 128, 145, 157, and 172) as well as 47 of the 53 different H antigens (H antigens 1 to 12, 14 to 16, 18 to 21, 23 to 34, 37 to 43, 45, 46, 48, 49, 51 to 54, and 56). Evaluation of the microarray with a set of defined strains representing all O and H serotypes covered revealed that it has a high sensitivity and a high specificity. All of the conventionally typed 24 O groups and all of the 47 H serotypes were correctly identified. Moreover, strains which were nonmotile or nontypeable by previous serotyping assays yielded unequivocal results with the novel ArrayTube assay, which proved to be a valuable alternative to classical serotyping, allowing processing of single colonies within a single working day.

nilRis necessary for co‐ordinate repression ofXenorhabdus nematophilamutualism genes

The bacterial mutualist Xenorhabdus nematophila colonizes a specific region of its nematode host Steinernema carpocapsae. We previously reported the identification of a chromosomal locus encoding three X. nematophila genes of unknown function, nilA, B and C, that are each necessary for colonization. Subsequent work indicated the global regulator Lrp is a repressor of nilC: nilC transcription is elevated in an lrp mutant and Lrp interacts directly with the nilC promoter. In this manuscript, we report the identification of an additional gene, nilR, required for repression of nilC transcription. We show that nilR and lrp mutants also have elevated expression of nilA and nilB, demonstrating that nilA, B and C are co-ordinately regulated. nil gene expression is derepressed most strongly when both nilR and lrp are lacking, suggesting NilR and Lrp synergistically repress nil transcription. NilR contains a helix-turn-helix-type DNA binding domain and likely acts directly at promoters. A comparison of the wild type and nilR proteomes indicates that NilR, unlike Lrp, regulates a small number of genes. Finally, X. nematophila carrying an ectopic copy of nilR colonizes at approximately 60-fold lower levels than the control strain, suggesting that derepression of nil gene expression is necessary for nematode colonization.

Imprecise excision of insertion element IS5 from the fliC gene contributes to flagellar diversity in Escherichia coli

Motile strains of Escherichia coli K12 carrying both a chromosomal fliC-H48 gene and a plasmid encoded fliC-H4 gene express both types of flagellins, which are coassembled into functional flagella. By using flagellar-H48-specific antiserum and a plasmid curing procedure, nonmotile mutants were found that carried an IS5 insertion in the chromosomal fliC-H48 gene. Motile revertants were isolated that showed deletions of the IS5 element together with sections of the fliC-H48 gene resulting in an altered flagellar serotype in these strains. As IS5 elements were found associated with 35 of 53 known H-types in wildtype E. coli strains, this insertion element might play a major role in serotype diversity.

A constitutively expressed, truncated umuDC operon regulates the recA-dependent DNA damage induction of a gene in Acinetobacter baylyi strain ADP1

In response to environmentally caused DNA damage, SOS genes are up-regulated due to RecA-mediated relief of LexA repression. In Escherichia coli, the SOS umuDC operon is required for DNA damage checkpoint functions and for replicating damaged DNA in the error-prone process called SOS mutagenesis. In the model soil bacterium Acinetobacter baylyi strain ADP1, however, the content, regulation, and function of the umuDC operon are unusual. The umuC gene is incomplete, and a remnant of an ISEhe3-like transposase has replaced the middle 57% of the umuC coding region. The umuD open reading frame is intact, but it is 1.5 times the size of other umuD genes and has an extra 5' region that lacks homology to known umuD genes. Analysis of a umuD::lacZ fusion showed that umuD was expressed at very high levels in both the absence and presence of mitomycin C and that this expression was not affected in a recA-deficient background. The umuD mutation did not affect the growth rate or survival after UV-induced DNA damage. However, the UmuD-like protein found in ADP1 (UmuDAb) was required for induction of an adjacent DNA damage-inducible gene, ddrR. The umuD mutation specifically reduced the DNA damage induction of the RecA-dependent DNA damage-inducible ddrR locus by 83% (from 12.9-fold to 2.3-fold induction), but it did not affect the 33.9-fold induction of benA, an unrelated benzoate degradation gene. These data suggest that the response of the ADP1 umuDC operon to DNA damage is unusual and that UmuDAb specifically regulates the expression of at least one DNA damage-inducible gene.

Genetic loci of major antigenic protein genes of Edwardsiella tarda

Seven antigenic proteins of Edwardsiella tarda were identified by using a rabbit polyclonal antiserum. Four of these proteins also reacted with a Japanese flounder antiserum. The amino acid sequences had identity to lipoproteins, periplasmic proteins, and exported and secreted proteins with roles in transport of metabolites across the cell membrane, stress response, and motility. These genes and their products are useful for developing DNA or recombinant subunit vaccines to control edwardsiellosis.

Molecular serotyping of Salmonella: identification of the phase 1 H antigen based on partial sequencing of the fliC gene

The purpose of this study was to develop a simple and non-labour-intensive molecular method to identify the phase 1 H antigens of Salmonella. The variable region of the flagellin gene, fliC, from 96 Salmonella strains representing 51 different phase 1 H antigens was sequenced in one direction. Unique sequences were found for 45 of the 51 different antigens. We were not able to separate either H:z42 from H:d; H:g, q from H:g, m, q; H:l, w from H:Rl, z40 or H:l, (v),z13 from H:l,z,13. Several phase 2 H antigens were found to be encoded by fliC. Polymorphism, at the subspecies level, was observed in fliC of H:b, H:d, H:z10, H:z and especially H:k. By this method we were also able to confirm that one monophasic strain possesses a new antigen, H:z91. This study shows that sequence-based typing of the phase 1 H antigen of Salmonella is a good alternative to serotyping when strains are non-typable by serological methods.