Gunn Lab

Our laboratory focuses on Salmonella enterica serovar Typhi (S. Typhi), the causative agent of typhoid fever, a human-specific systemic infection. Approximately 5% of acute infections progress to chronic carriage, with the gallbladder serving as the primary site of persistence. Chronic gallbladder carriage is strongly associated with the presence of gallstones, on which Salmonella spp. form biofilms. These biofilms confer increased resistance to both antibiotic treatment and host immune defenses, enabling bacterial persistence and the development of antimicrobial resistance. Chronic carriers are typically asymptomatic and therefore serve as unrecognized reservoirs for transmission, particularly in regions with inadequate sanitation infrastructure. Consequently, physical removal of the gallbladder is often required to eliminate infection; however, cholecystectomy is an invasive and costly procedure, making it less feasible in endemic settings. Our goal is to improve the identification and treatment of asymptomatic chronic carriers by elucidating the mechanisms that enable persistent infection, using a mouse model of chronic gallbladder carriage.

Previous and ongoing work in the lab has identified novel compounds that can disrupt Salmonella biofilms, which are structured bacterial communities that play a key role in the development and persistence of chronic infection. Within biofilms, bacteria are protected by a dense matrix that limits antibiotic effectiveness and helps them evade the immune system, making infections difficult to fully clear. By breaking apart these biofilms, our compounds can release bacteria from this protective environment, making them more vulnerable to treatment and immune defenses. To study this process, we are using a recently developed collaborative cross mouse model (CC003) that supports infection with Salmonella enterica serovar Typhi, the bacterium that causes typhoid fever. This represents an important advance over previous models, which could not support this human-specific pathogen. By feeding these mice a lithogenic diet to promote gallstone formation, we are able to model chronic infection in the gallbladder, where Salmonella biofilms form on gallstones. Through ongoing laboratory and animal studies, we are testing whether our anti-biofilm compounds can disrupt these structures and help clear chronic infection, with the goal of developing new therapeutic strategies to eliminate the typhoid carrier state.

Our research focuses on chronic typhoid carriage, a condition that primarily localizes to the gallbladder and is strongly associated with the presence of gallstones. We investigate the complex host–pathogen interactions that occur within the gallbladder, with particular emphasis on the local immune environment that permits Salmonella persistence. From the host perspective, we have demonstrated that the establishment of chronic carriage is associated with a shift from a Th1 (bactericidal) to a Th2 (permissive) immune response. This transition coincides with the presence of Salmonella-specific CD4⁺ T cells and macrophages in the gallbladder during disease progression. From the bacterial perspective, we have shown that Salmonella upregulates curli fimbriae in response to human bile, suggesting an adaptive mechanism that promotes persistence within the gallbladder environment. Our current work aims to define the relative contributions of specific immune cells and cytokines that drive the Th1-to-Th2 shift and impair bacterial clearance. In parallel, we are investigating the regulatory mechanisms employed by Salmonella enterica serovar Typhi, which appear to differ from those described in the classical model organism S. Typhimurium.

We have demonstrated that Salmonella enterica serovar Typhi forms biofilms on cholesterol gallstones in humans, a process that facilitates the long-term persistence characteristic of chronic typhoid carriage. The biofilm lifestyle provides protection against the harsh conditions of the gallbladder, enabling the bacteria to survive for extended periods and to be intermittently shed into the environment, thereby sustaining transmission. Most of the current understanding of Salmonella biofilm formation is derived from enteric serovars such as S. Typhimurium, where the transcriptional regulator CsgD plays a central role. However, our work, along with that of others, indicates that CsgD does not play a significant role in S. Typhi biofilm formation. These findings suggest that biofilm formation in S. Typhi is controlled by different regulatory mechanisms. We are currently exploring these alternative pathways using mutant libraries along with both in vitro and in vivo screening approaches.