Physical and genetic map of the chromosome of Dichelobacter nodosus strain A198

Novel fimbrial subunit genes of Dichelobacter nodosus: recombination in vivo or in vitro?

Polymerase chain reaction (PCR) was used to amplify the variable region of the fimbrial subunit encoding gene (fimA) of Dichelobacter nodosus from sheep and goats infected with footrot. Two amplimers (designated X and Y) generated single-strand conformation polymorphism (SSCP) patterns different to those of previously identified serogroups and serotypes. DNA sequencing revealed that these two fragments were novel. The upstream of X (nt 1-183) was identical to serotype M1 while its downstream (nt 223-414) was identical to serotype F1; the upstream of Y (nt 1-116) was identical to serotype E1, whereas its downstream (nt 148-423) was identical to serotype F1. A 14-mer sequence consisting of two partially overlapping Chi-like sequences, 5'-GCTGGTGCTGGTGA-3', was also found in these fragments. Two primer sets with the downstream primer specific for serotype F1 and the upstream primer specific for serotype M or E1, generated PCR products of the expected sizes from the footrot samples from which fragments X and Y were isolated, respectively. These primer sets did not appear to amplify artificially mixed genomic DNA from serotypes M and F1 or E1 and F1. However, when the reactions were re-amplified, PCR recombination artifacts were observed, suggesting that PCR recombination does occur, but at a low frequency. It, therefore, seems more likely that fragments X and Y reflect genuine fimA genes of D. nodosus which have resulted from in vivo DNA recombination rather than from a PCR recombination artifact.

Complete nucleotide sequence of the 27-kilobase virulence related locus (vrl) of Dichelobacter nodosus: evidence for extrachromosomal origin

The vrl locus is preferentially associated with virulent isolates of the ovine footrot pathogen, Dichelobacter nodosus. The complete nucleotide sequence of this 27.1-kb region has now been determined. The data reveal that the locus has a G+C content much higher than the rest of the D. nodosus chromosome and contains 22 open reading frames (ORFs) encoding products including a putative adenine-specific methylase, two potential DEAH ATP-dependent helicases, and two products with sequence similarity to a bacteriophage resistance system. These ORFs are all in the same orientation, and most are either overlapping or separated by only a few nucleotides, suggesting that they comprise an operon and are translationally coupled. Expression vector studies have led to the identification of proteins that correspond to many of these ORFs. These data, in combination with evidence of insertion of vrl into the 3' end of an ssrA gene, are consistent with the hypothesis that the vrl locus was derived from the insertion of a bacteriophage or plasmid into the D. nodosus genome.

The diverse and dynamic structure of bacterial genomes

Bacterial genome sizes, which range from 500 to 10,000 kbp, are within the current scope of operation of large-scale nucleotide sequence determination facilities. To date, 8 complete bacterial genomes have been sequenced, and at least 40 more will be completed in the near future. Such projects give wonderfully detailed information concerning the structure of the organism's genes and the overall organization of the sequenced genomes. It will be very important to put this incredible wealth of detail into a larger biological picture: How does this information apply to the genomes of related genera, related species, or even other individuals from the same species? Recent advances in pulsed-field gel electrophoretic technology have facilitated the construction of complete and accurate physical maps of bacterial chromosomes, and the many maps constructed in the past decade have revealed unexpected and substantial differences in genome size and organization even among closely related bacteria. This review focuses on this recently appreciated plasticity in structure of bacterial genomes, and diversity in genome size, replicon geometry, and chromosome number are discussed at inter- and intraspecies levels.

Virulence regions and virulence factors of the ovine footrot pathogen, Dichelobacter nodosus

Ovine footrot is a debilitating and highly infectious disease that is primarily caused by the Gram-negative, anaerobic bacterium Dichelobacter nodosus. The major antigens implicated in virulence are the type IV fimbriae and extracellular proteases. The fimbriae show sequence and structural similarity to other type IV fimbriae, this similarity extends to genes that are involved in fimbrial biogenesis. Several acidic and basic extracellular serine proteases are produced by both virulent and benign isolates of D. nodosus. Subtle functional differences in these proteases appear to be important in virulence. In addition, there are two chromosomal regions that have a genotypic association with virulence. The partially duplicated and rearranged vap regions appear to have arisen from the insertion of a plasmid into a tRNA gene via an integrase-mediated site-specific insertion event. The 27 kb vrl region has several genes often found on bacteriophages and has inserted into an ssrA gene that may have a regulatory role in the cell. The determination of the precise role that each of these genes and gene regions has in virulence awaits the development of methods for the genetic analysis and manipulation of D. nodosus.