Characterisation of the Molecular Basis of Systemic Salmonellosis in Calves

Salmonellosis is an important zoonosis and a major cause for concern in both man and domestic animals, resulting in symptoms that range from mild enteritis to severe systemic disease. Annually within the UK there are approximately 17,000 reported cased of Salmonella-induced enteritis associated with the ingestion of contaminated food, and approximately 2,700 reported cases of salmonellosis associated with livestock. Furthermore, in developing countries typhoid fever causes a life threatening disease in humans and accounts for more than 600,000 deaths each year. There are more than 2,400 antigenically distinct serovars within the genus Salmonella and many of these are commonly associated with specific disease outcomes in particular host species. For example, Salmonella enterica serovar Dublin is associated with both systemic and enteric disease in cattle. In contrast, serovar Typhimurium is typically associated with intestinal infections in a wide range of phylogenetically unrelated hosts. The bacterial factors that contribute to systemic salmonellosis in calves at a molecular level remain poorly defined and have received little study. <P> This study aims to use an integrative approach to identify the bacterial factors that enable the development of systemic salmonellosis resulting from infection of calves with serovar Dublin, but not serovar Typhimurium. Initially we aim to assess the kinetics and nature of bacterial translocation using our established bovine oral inoculation model in conjunction with our efferent lymphatic cannulation model. This will confirm the validity of our model systems and enable us to gain insights into factors influencing bacterial translocation at later stages of the infection process. We propose to use these models together with the powerful technique of signature- tagged mutagenesis to simultaneously track the fate of conserved and serovar-specific tagged mutants of serovars Dublin and Typhimurium in calves. Mutants containing transposon insertions in specific genes of interest and in serovar-specific sequences (identified through collaboration with the Sanger Institute) will be selected from banks of approximately 6,000 signature-tagged mutants of each serovar. Defined mutations will be made in any genes required for systemic translocation in either serovar, and their roles in calves will be confirmed by transcomplementation studies. Thus, through the integration of molecular genetic techniques and in vivo analyses of the interaction of different serovars with bovine tissues, we aim to determine why serovar Dublin, but not serovar Typhimurium, is able to translocate effectively and cause severe systemic salmonellosis in calves. Gaining insight into the mechanisms by which salmonellas disseminate from the gut is crucial for the development of the next generation of live attenuated Salmonella vaccines for use in man and domestic animals.