Pig Disease: Pathogenic And Non-pathogenic Strains Of The Bacteria Haemophilus Parasuis Differentiated Without Challenging Animals

Haemophilus parasuis is the etiological agent of Glässer’s disease in pigs, and is characterized by fibrinous poliserositis and meningitis. H. parasuis is frequently located in the upper respiratory tract, but not all the strains are capable to induce the disease.

Comparative genomic techniques allow to exactly explore gene contents of bacteria and they have been used in this study.

In a first phase, CReSA researchers sequenced 98% of the genome (2.35 Mbp) of the highly pathogenic Nagasaki strain and deciphered its genetic content. Thirteen genes coding for trimeric autotransporters (AT-2) were identified. These proteins are considered as essential virulence factors.

AT-2 proteins are cell surface exposed on the bacteria and serve for tissue adhesion favouring the dissemination of these micro-organisms in the host. To investigate if the genes coding for AT-2 molecules are present or not in non-pathogenic strains, the technique of DNA hybridization was used. The genes (which are composed of DNA) previously described were spotted onto glass supports and their capacity to bind DNA from other H. parasuis strains was searched. If two DNA strands are similar, they form hybrids and this assay is used to tell if effectively a gene is present or absent in other bacteria.

Results were striking: all pathogenic strains possess AT-2 molecules with diverse degrees of homology with the Nagasaki strain, while the non-pathogenic strains are devoid of these genes. This discovery largely explains why pathogenic strains can colonize diverse tissues in the organism. In a second phase, AT-2 genes from different H. parasuis strains were sequenced revealing their way to evolve.

AT-2 genes not only increase their diversity by mutation and duplication within the same bacteria, in addition they also interchange modules between different bacteria of the same specie. From these basic results, a simple molecular test was developed which distinguish H. parasuis with potential pathogenicity. Moreover, AT-2 molecules could be good candidates for developing new vaccines.