Iron is an essential element for the replication of bacteria inside the host. One of the most important host strategies during bacterial infections is to sequester iron from microorganisms by employing high-affinity iron-binding proteins like transferrin and lactoferrin. To overcome these host defenses, bacteria release small molecules named siderophores, which coordinate iron and are then internalized by dedicated uptake systems expressed by bacteria. A countermeasure of the host is the secretion of the protein lipocalin-2, which binds to a selective group of siderophores and it is effective in reducing bacterial growth. Lipocalin-2 controls bacteremia caused by Escherichia coli, because it chelates the siderophore enterochelin, thereby starving E. coli of iron. This strategy, however, is not successful against Salmonella enterica serotype Typhimurium because this pathogen, in addition to enterochelin, produces a glucosylated enterochelin (salmochelin) which is not sequestered by lipocalin-2. Because of the substantial evidence for the necessity of iron acquisition for the growth of pathogens in the host, one promising therapeutic strategy to limit bacterial colonization is to prevent iron acquisition by these microbes. The primary objective of this application is to develop new methods to limit iron acquisition by S. Typhimurium in the vertebrate host by using siderophore-based immunization. Our central hypothesis is that sequestering the siderophores enterobactin and salmochelin will limit iron uptake by S. Typhimurium and thereby inhibit colonization in the vertebrate host. This hypothesis is based on preliminary data revealing that iron acquisition promotes S. Typhimurium growth and competition with other microbes, and that vaccination of mice with CTB-Ent conjugates results in antibody production. Our long-range goal is to develop new treatments to reduce the growth and dissemination of S. Typhimurium. We plan to test our hypothesis and fulfill the objectives of this application by pursuing the following Specific Aims: (I) The first objective of this initiativ is to determine whether injection of protein- siderophore conjugates results in growth inhibition of S. Typhimurium in the colitis mouse model, which models inflammatory diarrheal disease. (II) The second aim of this investigation is to determine whether injection of protein-siderophore conjugates results in growth inhibition of S. Typhimurium in the typhoid mouse model, which mimics systemic infection.
The rise in antibiotic resistance in human pathogens including Salmonella intensifies the need for new and creative ways to control bacterial infections in livestock and humans. Siderophore-based immunization may lead to new approaches to reduce the growth of Salmonella during infection, thereby limiting its host transmission.