Dichelobacter nodosus: differentiation of virulent and benign strains by gene probe based dot blot hybridisation

Comparative study of the commonly used virulence tests for laboratory diagnosis of ovine footrot caused by Dichelobacter nodosus in Australia

Footrot in sheep and goats is expressed as a spectrum of clinical entities ranging from benign, which is a self limiting interdigital dermatitis to highly virulent, in which severe under running of the horn of the hoof occurs. Interactions between the host, the virulence of the causative strain of Dichelobacter nodosus and environmental conditions determine the severity of the disease. Clinical diagnosis of virulent footrot, which a notifiable disease in some states of Australia, is not always straightforward. Therefore, the gelatin gel and elastase tests for protease activity, and the intA PCR test for an inserted genetic element in D. nodosus are commonly used to support or to confirm a clinical diagnosis. A comparative study of these laboratory tests with a large number of samples collected from 12 flocks of sheep with clinically virulent footrot was conducted. Based on the elastase test, 64% of the isolates tested were classified as virulent compared to 91% on the gelatin gel test and 41% according to the intA test. The agreement between the elastase and the gelatin gel test was low (kappa=0.12) as were the agreements between other tests. Only about 21% of the isolates were virulent in all 3 tests. Therefore these tests on their own may not provide standard and reliable results and are likely to remain as supplementary tests for clinical diagnosis of the disease.

Identification of a gene encoding a putative phosphotransferase system enzyme IIBC in Listeria welshimeri and its application for diagnostic PCR

AIMS

To identify a Listeria welshimeri-specific gene that can be used for identification of this species by PCR.

METHODS AND RESULTS

Through comparative analysis of genomic DNA from Listeria species using dot blot hybridization, an L. welshimeri-specific clone was isolated that contained a gene segment whose translated protein sequence is similar to enzyme IIBC from phosphotransferase systems in other bacteria. Using oligonucleotide primers derived from this L. welshimeri-specific clone, a 608-bp fragment was amplified from L. welshimeri genomic DNA and not from other Listeria species or other Gram-negative and Gram-positive species.

CONCLUSION AND SIGNIFICANCE

The PCR employing L. welshimeri-specific primers shows promise as a useful method for differentiating L. welshimeri from other Listeria species and related bacteria.

Characterization of virulent and avirulent Listeria monocytogenes strains by PCR amplification of putative transcriptional regulator and internalin genes

Listeria monocytogenes is an opportunistic bacterial pathogen that is an important cause of human food-borne illness worldwide. However, L. monocytogenes strains demonstrate considerable variation in pathogenic potential. In this report, virulent and avirulent L. monocytogenes isolates were compared by using a comparative screening strategy. Two clones were identified that contained DNA that was only present in virulent L. monocytogenes strains. PCR primers were designed for three genes from these clones and for five other selected L. monocytogenes genes. All eight primer sets predominantly detected virulent L. monocytogenes isolates, as determined by a mouse virulence assay; one of the putative internalin genes, lmo2821, was detected in all strains that were considered to be virulent. Primers from these eight genes were then tested by PCR against a larger panel of bacterial strains; each of the genes was detected predominantly in clinical or food L. monocytogenes isolates, rather than environmental isolates. The findings from this study suggest that virulent L. monocytogenes strains may possess genes that are not present in avirulent isolates, which could serve as markers for PCR assessment of L. monocytogenes virulence.

Improved laboratory diagnosis of ovine footrot: an update

Ovine footrot is a complex clinical disease syndrome primarily resulting from infection by the anaerobic bacterium Dichelobacter nodosus. In order to aid clinical diagnosis, various laboratory tests based on the detection and measurements of phenotypic properties of D. nodosus have been developed for genus-specific detection as well as virulence determination. However, these tests are generally time-consuming, and tend to be variable with external factors that affect the growth and metabolism of the bacterium. A new generation of diagnostic reagents, such as monoclonal antibodies, gene probes and polymerase chain reaction, has been developed recently. Preliminary assessment of these reagents has shown potential to vastly improve the laboratory identification and determination of the virulence of D. nodosus. It is important that these new reagents are vigorously assessed against existing laboratory tests, such as the elastase test and gelatin gel test, prior to their adoption for the routine diagnosis of footrot.

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.

Comparison of gene probe and conventional methods for the differentiation of ovine footrot isolates of Dichelobacter nodosus

In a collaborative study that involved four Australian veterinary diagnostic laboratories a gene probe test based on the recombinant plasmids pJIR318, pJIR314B, and pJIR313, which contain genomic vap or vrl regions, was compared with conventional tests used for the differential diagnosis of ovine footrot. A total of 771 clinical dichelobacter nodosus isolates were tested and designated as belonging to one of several gene probe categories. The results showed that 87% of the virulent isolates belonged to gene probe category 1, compared to only 6% of the benign isolates. It was concluded that there was good correlation between the gene probe test and the virulence designation of these isolates as well as the results of elastase, gelatin-gel and protease isoenzyme tests. Furthermore, the gene probe test was converted to a polymerase chain reaction (PCR)-based test. It is suggested that diagnostic laboratories consider carrying out both this PCR test and tests based on the extracellular proteases of D. nodosus.

Relationship between clinical manifestations of footrot and specific DNA products of Dichelobacter nodosus amplified through PCR

A total of 141 Dichelobacter nodosus isolates from 46 merino sheep farms with various clinical forms of footrot was examined by the gelatin gel test and the polymerase chain reaction (PCR) using virulent (Vf2 and Vr2) and benign (Bf and Br) specific primers. Isolates from sheep with virulent and high intermediate footrot usually produced relatively thermostable proteases, but a decreasing proportion of the isolates from sheep with medium and low intermediate or benign footrot had thermostable proteases, as determined by the gelatin gel test. The amplification by PCR of a major band of 857 bp by Vf2 and Vr2 was often associated with isolates from the more virulent forms of footrot whereas the presence of a major band of 1300 bp by Vf2 and Vr2 and/or a band of 609 bp by Bf and Br was associated with isolates from less virulent forms of footrot. Nevertheless, the virulent and benign gene regions represented by Vf2 and Vr2 and Bf and Br are only two of the many factors involved in determining the virulence of D nodosus. As a result the relationship observed between the clinical manifestations of footrot and specific DNA products amplified by PCR was not complete.

A retrospective study of clinical and laboratory characteristics of ovine footrot

The infection of the feet of sheep by the anaerobic bacterium Dichelobacter nodosus results in a spectrum of diseases ranging from virulent through intermediate to benign footrot. By examining the clinical characteristics of various forms of footrot from 22 properties in Victoria, Australia in association with the results of several laboratory tests including the gene probe-based dot blot hybridisation, it was shown that virulent footrot could not be differentiated from intermediate footrot using the clinical characteristics alone. In addition, D. nodosus isolates from sheep with benign footrot as diagnosed by the clinical manifestations might in fact show characteristics associated with those causing virulent, intermediate or benign footrot as determined by the laboratory tests. Although the elastase test, gelatin gel test and dot blot were in agreement with isolates from 9 of the 22 properties under investigation, they displayed differences from one another in many other cases, especially those with clinical benign footrot. The results of the elastase test appeared to be slightly closer to those of the dot blot compared with the gelatin gel test. Like any other tests based on the measurements of phenotypic characteristics of D. nodosus bacteria, the elastase test and gelatin gel test are subject to changes with factors that affect bacterial growth and metabolism. The use of DNA based detection methods would eliminate many of the problems associated with the conventional tests for differentiating strains causing virulent, intermediate and benign footrot.

Development of gene probes of Dichelobacter nodosus for differentiating strains causing virulent, intermediate or benign ovine footrot

Seven Dichelobacter nodosus genomic DNA clones including six specific for virulent and one for benign strains were identified. A collection of 96 footrot isolates, which in turn comprised 27 virulent isolates showing elastase activity at 7 days, 25 high intermediate isolates with elastase activity at 14 days, 24 low intermediate isolates with elastase activity at 21-28 days and 20 benign isolates with no elastase activity at up to 28 days, were used to assess these clones. Of the six virulent specific clones, five (pV238-83, pV405-239, pV470-65, pV470-145 and pV470-178) reacted with 27 virulent isolates, and 12 of 25 high intermediate isolates, but none of 24 low intermediate isolates and 20 benign isolates in dot blot hybridization. The other virulent-specific clone (pV470-13) recognized all 27 virulent and 25 intermediate isolates, and 22 of 24 low intermediate isolates and three of 20 benign isolates in dot blot hybridization. By contrast, the benign-specific clone (pB645-335) detected all 20 benign isolates and 24 low intermediate isolates, and also 13 of 25 high intermediate isolates, but none of 27 virulent isolates in dot blot hybridization. Southern hybridization analysis indicated that whereas clones pV238-83, pV405-239 and pV470-178 bound a Sau3A band of 0.5 kb, clones pV470-65 and pV470-145 recognized two Sau3A bands of 0.7 and 0.5 kb in virulent strains of serogroups A to I. However, clone pV470-13 detected a Sau3A band pattern in virulent strains different from those recognized by the other five virulent specific-clones. Besides showing a distinct Sau3A band pattern in intermediate strains, pV470-13 also reacted with three benign strains that showed binding with it in dot blot hybridization. The benign-specific clone pV645-335 detected a Sau3A band of 0.5 kb in both intermediate and benign strains of serogroups A to I. Thus the combination of pV470-13 and pB645-335, or any other virulent-specific clone, would clearly differentiate among D. nodosus strains causing virulent, intermediate or benign footrot.

Use of elastase test, gelatin gel test and electrophoretic zymogram to determine virulence of Dichelobacter nodosus isolated from ovine foot rot

A comparative study using the elastase test, gelatin gel test and electrophoretic zymogram was conducted for virulence determination of 96 Dichelobacter nodosus isolates from clinical cases of ovine foot rot. Despite being time consuming, the elastase test provided a classification of D nodosus bacteria in general agreement with the severity of clinical characteristics. The gelatin gel test showed some correlation with the elastase test. While isolates showing elastase activity at 14 days or less were likely to produce more proteases which were of higher thermostability, those showing elastase activity at 21 to 28 days, or elastase negative up to 28 days tended to produce less proteases which were also of lower thermostability. The electrophoretic zymogram based on the virulent pattern 1 (V1), virulent pattern 2 (V2) and benign pattern (B) did not cover all isolates from the field as nine (9.4 per cent) of the 96 isolates formed specific isoenzyme patterns that were different from the V1, V2 and B patterns of classification. However, the patterns formed by these nine isolates could be fitted into another previously reported isoenzyme classification. Nevertheless, the electrophoretic zymogram by itself did not provide adequate information in regard to intermediate foot rot. It appeared that a combination of two tests such as elastase test and zymogram or gelatin gel test and zymogram would offer a useful complementary definition of the virulence of D nodosus bacteria.