Molecular characterisation of Salmonella enterica Serovar Sofia in Australia

Gan, T 2008, Molecular characterisation of Salmonella enterica Serovar Sofia in Australia, Doctor of Philosophy (PhD), Applied Sciences, RMIT University.


Document type: Thesis
Collection: Theses

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Title Molecular characterisation of Salmonella enterica Serovar Sofia in Australia
Author(s) Gan, T
Year 2008
Abstract Despite its high isolation frequency in Australian chickens, S. II Sofia is rarely associated with animals or human salmonellosis as this serovar is avirulent in nature. The reason for its persistence and avirulence is unknown as very few studies have been conducted on the epidemiology and pathogenicity of this strain. This study details the various experimental methods utilised to investigate the genetic relatedness and molecular mechanisms involved in S. II Sofia pathogenesis. Using PFGE and Rep-PCR, the Australian S. II Sofia isolates were found to show limited genetic diversity and probably share a clonal relationship. A majority of the S. II Sofia isolates were not geographically restricted with the predominant pattern subtype spread out among the isolates from various states.

Distribution and variation of the SPI-associated virulence genes within S. II Sofia was also examined. Based on RFLP and sequence analysis, most of the differences observed in SPI1 to SPI5 of S. II Sofia could be attributed to a loss or gain of restriction cleavage sites within these regions. However, a number of genes in SPI1, SPI2, SPI3 and SPI5 were found to have accumulated changes (mutations, insertions and deletions) that could have affected gene transcription and/or protein translation - these genes have been shown to be involved in different aspects of the virulence process. The avirulence of S. II Sofia is probably not the result of a single genetic change but rather a series of alterations to a large number of its virulence-associated genes.

Plasmid-mediated virulence was also assessed in S. II Sofia isolates. Southern hybridisation with probes derived from the virulence plasmid of S. Typhimurium indicated either the total absence of the virulence plasmid or possible presence of a virulence plasmid containing major deletions. Clones were constructed with the missing spv operon using high-copy pCR®2.1 and low-copy pWSK29 plasmids and the adherence, invasion and intracellular survival of the mutant strain was evaluated in vitro. The presence of spvRABCD was shown to have no effect on intracellular survival and replication. Although the cloning of spv with pCR®2.1 was observed to significantly increase invasiveness of S. II Sofia, it was not capable of restoring the invasive ability of S. II Sofia to the level of pathogenic S. Typhimurium 82/6915. On the other hand, the uneven adherence and invasion ability of the other mutant strains appeared to be linked to the presence of pWSK29 and this observation is further supported by RT-PCR analysis of the clones - indicating that perhaps pWSK29 is not a suitable vector for this study.

Wild-type S. II Sofia isolates are unlikely to regain full pathogenicity because of the numerous mutations in many important virulence genes: even the chance acquisition of a virulence factor (e.g. spvRABCD) is not sufficient to completely restore S. II Sofia virulence. Therefore, S. II Sofia should not be considered similar to other Salmonella spp. when monitoring Salmonellae in food samples.
Degree Doctor of Philosophy (PhD)
Institution RMIT University
School, Department or Centre Applied Sciences
Keyword(s) Salmonella -- Australia
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