Salmonellae are Enterobacteriaceae that cause a spectrum of diseases in humans and animals, including enteric (typhoid) fever and gastroenteritis. Typhoid fever, caused primarily by Salmonella enterica serovar Typhi (S. Typhi), results in a life-threatening systemic disease that is annually responsible for significant morbidity and mortality worldwide. Approximately 5% of individuals infected with S. Typhi become chronic carriers with the gallbladder (GB) as the primary site of persistence. S. Typhi is a human-restricted pathogen, therefore asymptomatic carriers represent a critical reservoir for further spread of disease. We have demonstrated that gallstones (GSs) aid in the development and maintenance of GB carriage in a mouse model (utilizing S. Typhimurium, which causes a typhoid fever-like disease in mice) and in humans, serving as a substrate to which salmonellae attach and form a protective biofilm. Thus, biofilm formation is a key step in the establishment of carriers. Salmonella in biofilms are known to be recalcitrant to antibiotics and host immunity, presenting a challenge for traditional treatment methods. A hallmark of chronic S. Typhi infections is the production of extracellular polymeric substances (EPSs) which are integral to biofilm development on GSs. How the bacteria subvert innate immunity during early stages of biofilm development and establish chronic infections is not known. Immune escape likely involves EPS, but a complete understanding of the EPSs responsible for this function is not known. We hypothesize one or more EPS has a critical role in biofilm development and contributes to the chronic pathogenicity of S. Typhi biofilms via innate immune evasion. Identification of the essential EPS(s) will allow us to determine the mechanism of immune evasion, likely due to a combination of a physical barrier function and regulation of innate host responses. To build on our prelinminary data, we will further investigate which EPSs are responsible for this perturbation and conduct quantitative assays to evaluate innate phagocyte activity in response to WT and EPS mutant biofilms. Assays for soluable factors (e.g. antimicrobial peptides, complement) and polymorphonuclear cell (neutrophil) functions (chemotaxis, neutrophil extracellular trapping, and induction of respiratory burst) have been or will be conducted. Confocal imaging of EPSs from in vitro and in vivo GSs will determine the structural contribution of each EPS during chronic infection and will be correlated to functional assays of immune modulation. Overall, an improved understanding of how biofilms develop in vivo and how EPSs skew innate immunity will be critical for development of new treatment and prevention methods. Disruption of carrier state infections will have a significant impact on endemic S. Typhi persistence and the spread of typhoid fever.
Typhoid fever is a deadly bacterial disease whereby the agent (primarily S. Typhi) establishes long-term residence in the gallbladder and is transmitted to others by these human carriers. The immune system is unable to eliminate the infection in carriers, and understanding why could help develop therapies that aid immune functions. Since typhoid fever requires humans for long-term survival, preventing the development of carriers will avert the spread of disease to others and potentially eradicate typhoid fever.