Cloning and characterization of the capsule transport gene region from Haemophilus paragallinarum

[摘要] English: Haemophilus paragallinarum causes an acute respiratory disease of chickens known as infectious coryza (IC), a disease first recognized as a distinct entity in the late 1920's. Since the disease proved to be infectious and primarily affected nasal passages, the name infectious coryza was adopted (Blackall, 1989). Infectious coryza may occur in both growing chickens and layers. The major economic effect of the disease is an increased culling rate in meat chickens and a reduction in egg production (10-40%) in laying and breeding hens. The disease is limited primarily to chickens and has no public health significance (Yamamoto, 1991). The most common clinical signs are a nasal discharge, conjunctivitis, and swelling of the sinuses and face. Various sulfonamides and antibiotics are useful in alleviating the severity and course of infectious coryza; however, none of the therapeutic agents has been found to be bactericidal. Relapse often occurs after treatment is discontinued, and the carrier state is not eliminated (Yamamoto, 1991). All the commercially available bacterins against IC, consist of inactivated broth cultures of a combination of two or three different serotypes. Although vaccines against IC have been used in South Afr ica since 1975, it became apparent in the 1980s that the vaccines were becoming less effective in controlling the disease (Bragg et al., 1996). This could have been due to the emergence of a previously unknown serovar, or even serogroup and the possibility of changes in the population dynamics. Vaccine efficiency is therefore a problem and an alternative to available vaccines is needed. Capsules have long been associated with virulence properties of bacteria. The role that the capsule play in the virulence of bacterial species related to H. paragallinarum has been investigated by several workers (Kroll et al., 1988; Inzana et al., 1993; Boyce and Adler, 2000). Mutation, deletion or allelic exchange of gene/s involved in the transport of capsule polysaccharides in related species like Haemophilus influenza, Actinobacillus pleuropneumoniae and Pasteurella multocida, resulted in organisms with reduced virulence. The noncapsulated mutants of Actinobacillus pleuropneumoniae reported by Inzana et al. (1993) showed extreme stability and induceda protective immune response without any symptoms of disease. This not only proves the capsule�s involvement in virulence of bacteria but also offers the opportunity to investigate the possibility of producing live vaccines. The aim of this study was an attempt to understand the genetic organization of the capsular genes of H. paragallinarum in comparison to related HAP organisms and the possibility of producing a mutant lacking the capsule. The goals were: 1. Isolation and cloning of the capsule transport gene locus. 2. Sequencing and characterization of the locus 3. Transplacement of a gene/s to produce a noncapsulated mutant of H. paragallinarum.

[发布机构] University of the Free State