Bacterial histamine production as a novel mechanism of host immune evasion - PROJECT SUMMARY Antimicrobial resistance (AMR) is one of the most pressing global health threats of the 21st century, endangering decades of medical progress. The National Institute of Health (NIH) and the Center for Disease Control (CDC) have declared AMR a global threat, warning that without urgent action, AMR infections could cause upwards of 10 million deaths annually by 2050, surpassing combined deaths due to cancer and diabetes. Addressing this crisis requires innovative solutions including the development of novel therapeutics. Our long- term goal is to contribute to that effort by seeking to clearly define novel virulence mechanisms associated with multidrug-resistant (MDR) pathogens which may be targeted by alternative therapeutics. Aeromonas is a gram- negative, opportunistic pathogen with a high propensity to acquire and spread AMR genes to surrounding bacteria, thus contributing to global spread. It is a leading etiological agent of pediatric diarrhea, and can cause fatal, systemic diseases including septicemia and necrotizing fasciitis. Given Aeromonas’ biochemical and genetic similarities to other MDR pathogens, its prototypical global AMR patterns, and our laboratory’s experience, we propose using Aeromonas species, A. dhakensis, as a model organism for studying novel virulence mechanisms. To this end, two MDR A. dhakensis isolates were recently cultured from the blood of patients who succumbed to monomicrobial bacteremia. Triplicate cultures of each were grown either in vitro (growth medium) or in vivo (peritoneal cavity of a mouse) followed by bacterial RNA isolation and sequencing. This provided us with parallel transcriptomic profiles, allowing us to easily identify which genes were upregulated in vivo. Emerging from this data was hdc, a gene encoding the biogenic enzyme histidine decarboxylase, responsible for the production of histamine. While bacteria producing histamine is not an unknown phenomenon, its relationship to virulence especially during acute infections is almost entirely unexplored. Since histamine is a well-characterized mammalian signaling molecule, its production represents an important and understudied host-pathogen interface. We hypothesize that bacterial pathogens produce histamine during infection which dysregulates host immune responses and increases bacterial virulence. To test this hypothesis, our first aim is to assess the effect of bacterial histamine on the host immune response during A. dhakensis infection. Macrophage and neutrophil responses to an hdc mutant, Δhdc, its complement Δhdc::hdc, and wild type (WT) A. dhakensis will be measured. We will then perform in vivo infections using all three strains in both WT and histamine-deficient mice followed by a comprehensive analysis of differences in the host immune response. Second, we aim to assess if histamine increases the virulence of A. dhakensis. This will be achieved by measuring expression of selected virulence traits and comparing the results between Δhdc, Δhdc::hdc, and the parental strain. These aims will contribute to our understanding of the role of pathogen-derived histamine during infection, with a long-term goal of developing novel treatments targeting this host-pathogen interface.
