The type IV fimbrial subunit gene (fimA) of Dichelobacter nodosus is essential for virulence, protease secretion, and natural competence.

Dichelobacter nodosus is the essential causative agent of footrot in sheep. The major D. nodosus-encoded virulence factors that have been implicated in the disease are type IV fimbriae and extracellular proteases. To examine the role of the fimbriae in virulence, allelic exchange was used to insertionally inactivate the fimA gene, which encodes the fimbrial subunit protein, from the virulent type G D. nodosus strain VCS1703A. Detailed analysis of two independently derived fimA mutants revealed that they no longer produced the fimbrial subunit protein or intact fimbriae and did not exhibit twitching motility. In addition, these mutants were no longer capable of undergoing natural transformation and did not secrete wild-type levels of extracellular proteases. These effects were not due to polar effects on the downstream fimB gene because insertionally inactivated fimB mutants were not defective in any of these phenotypic tests. Virulence testing of the mutants in a sheep pen trial conducted under controlled environmental conditions showed that the fimA mutants were avirulent, providing evidence that the fimA gene is an essential D. nodosus virulence gene. These studies represent the first time that molecular genetics has been used to determine the role of virulence genes in this slow growing anaerobic bacterium.

Electroporation-mediated transformation of the ovine footrot pathogen Dichelobacter nodosus.

Studies on the potential virulence genes of the ovine footrot pathogen Dichelobacter nodosus have been hindered by the lack of a genetic system for this organism. In an attempt to accomplish the transformation of D. nodosus cells, we constructed a plasmid that contained part of a native D. nodosus plasmid and carried a tetracycline resistance gene that was located between the D. nodosus rrnA promoter and terminator. This plasmid was used to transform several D. nodosus strains to tetracycline resistance. Analysis of two independent transformants from each parental strain showed that in nearly all of these derivatives, the plasmid was not replicating independently, but that the tetracycline resistance gene had inserted by homologous recombination into one of the three rrn operons located on the chromosome. In most of the transformants, double reciprocal crossover events had occurred. These results are highly significant for genetic studies in D. nodosus and for footrot pathogenesis studies, since by using reverse genetics it will now be possible to examine the role of putative D. nodosus-encoded virulence genes in the disease process.

Glutamate residues located within putative transmembrane helices are essential for TetA(P)-mediated tetracycline efflux.

The tetA(P) gene from Clostridium perfringens encodes a unique membrane protein that is responsible for the active efflux of tetracycline from resistant cells. The novel TetA(P) protein has neither the typical structure nor the conserved motifs that are found in tetracycline efflux proteins from classes A through H or classes K and L. Site-directed mutagenesis of selected residues within TetA(P) was performed to elucidate their role in tetracycline efflux. Glutamate residues 52 and 59, negatively charged residues located within putative transmembrane helix 2, could not be replaced by either glutamine or aspartate and so were essential for tetracycline efflux. Replacement of Glu89, which was located at the end of helix 3, by aspartate but not by glutamine allowed TetA(P) function, indicating the importance of a carboxyl group at this position. After mutation of the Asp67 residue, located within cytoplasmic loop 1, no immunoreactive protein was detected. It is concluded that negatively charged residues that appear to be located within or near the membrane are important for the function of TetA(P).

Confirmation that DNA encoding the major fimbrial subunit of Av24 fimbriae is homologous to DNA encoding the major fimbrial subunit of F107 fimbriae.

Plasmid DNA from porcine enterotoxigenic Escherichia coli strain Av24 (O141:K85ab) was cloned into recipient E. coli strain JM105 using the plasmid vector pUC18. Clones were obtained that produced fimbriae which reacted with antisera specific to the fimbriae produced by strain Av24. Restriction mapping of cloned DNA, PCR with fedA primers and DNA sequencing showed a portion of the cloned DNA to be homologous to that encoding the major fimbrial subunit of F107 fimbriae. This confirms that the fimbriae possessed by strains of E. coli causing both edema disease and post-weaning diarrhoea in piglets are variants of the same fimbriae.

Adhesive fimbriae associated with porcine enterotoxigenic Escherichia coli of the O141 serotype.

Enterotoxigenic Escherichia coli of the O141 serotype, isolated from piglets with postweaning coliform enteritis but producing none of the characterized adhesive fimbriae, was examined for fimbrial production by transmission electron microscopy. Two strains that produced numerous fimbriae were chosen for further characterization. The fimbriae were isolated and purified and had a subunit molecular weight of 17,000 by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Using antiserum raised against this protein, we have shown it to be specific for the 17,000-molecular-weight band by immunoblotting and to be directed against the fimbriae by immunoelectron microscopy. These fimbriae were not produced when the bacteria were grown at 18 degrees C and did not show any mannose-resistant hemagglutination of the erythrocytes tested. We propose that these are a new type of adhesive fimbriae associated with porcine enterotoxigenic E. coli of the O141 serotype.