Acquistion and selection of virulence traits of Salmonella enterica serovar Typhimurium in the organs of infected mice

Salmonella enterica is a pathogen capable of causing a spectrum of diseases in humans and animals. S. enterica serovar Typhi causes approximately 22 million cases of typhoid fever and over 200,000 deaths annually; serovar Paratyphi causes about 5.5 million annual cases in humans. Other non-typhoidal Salmonella serotypes (NTS) cause gastroenteritis in humans and animals and can spread from animals to humans via contaminated food (e.g. meat and eggs). NTS are a common cause of bacteraemia and sepsis in immuno-compromised individuals (e.g. HIV and malaria patients) and in children, especially in developing countries (e.g. Africa), where they constitute a major cause of death. Current measures in the treatment of S. enterica infections have been revealed as insufficiently effective, and there is a need to develop novel vaccines and therapeutics. The interaction between S. enterica with the host immune system is complex and the outcome of the infection is the result of a fine balance between a continuous escalation of the immune response and the expression of bacterial immunoevasion mechanisms. Consequently, in order to continue growing in the tissues, S. enterica is likely to require the coordinated and sequential regulation of genes. Bacterial gene regulation has so far been investigated largely using exposure to artificial environmental conditions or to in vitro cultured cells and little information is available on how S. enterica adapts in vivo to sustain cell division and survival. Currently, it is impossible to mimic in vitro the many, possibly unknown, inflammatory events that occur during infection. Understanding how gene regulation affects virulence traits of bacterial pathogens in vivo is one of the major challenges of the post-genomic era. Using microbiology, microscopy and sensitive molecular techniques we aim to develop an understanding of the dynamic interactions between host and bacterial mechanisms that determine net growth rates of S. enterica within the host. These experiments will help us to develop an understanding of the influence of passage in a host on the fitness and virulence of S. enterica, which will directly inform the development of novel vaccines and strategies to control and treat a major global health problem.

Bacteria of the species Salmonella enterica are a threat to public health; S. Typhi causes ~22 million cases of typhoid fever and over 200,000 deaths annually. The emergence of multi-drug resistant Salmonella strains and the insufficient efficacy of some of the currently available Salmonella vaccines highlight the urgent need for improved prevention strategies to combat salmonellosis in humans and animals. The design of novel vaccines and antimicrobials would benefit from a more sophisticated evaluation of how S. enterica coordinates the sequential regulation of genes to enable the bacteria to maintain their growth/survival throughout the infection in the face of the continuous escalation of the host immune responses. In vitro approaches have been used to explore the interaction between pathogens and individual host cells, but these strategies lack the anatomical and functional complexity of whole-body systems. The mechanisms underlying the spatiotemporal and genetic basis of in vivo bacterial adaptation and expression of virulent traits remain largely unresolved. The hypothesis behind the proposed research is that the increase in bacterial numbers observed during the early stages of a systemic S. enterica infection is due to a fine and dynamic balance between bacterial adaptation and increasing activation of the host antimicrobial response. To address this hypothesis the work will have a number of objectives, which will collectively 1) determine the spatial and temporal dynamics of S. enterica adaptation to the in vivo environment; and 2) determine the traits that are at the basis of S. enterica adaptation to the in vivo environment. The basic in vivo model for these studies will be the mouse model of invasive S. enterica infection. We will use bacteriological counts from tissue homogenates, fluorescence microscopy and quantitative PCR to determine the spread, localization and distribution of S. enterica in the tissues. In addition, we will use microarrays and proteomics to identifying bacterial gene and protein regulation mechanisms in vivo within the whole animal. The work will develop an understanding of the influence of growth and passage in a host on the fitness and virulence of S. enterica and determine the mechanistic and temporal elements involved in S. enterica in vivo adaptation. This will provide knowledge and technological basis for targeting individual bacterial components in vivo with novel drugs and vaccines and for eliciting immune responses against individual bacterial virulence determinants directed at the sites of infection where these are maximally expressed by the microorganisms.