Salmonella is the most prevalent causative agent of food-borne illness in the US. Because cooking is the most cost effective and common means of inactivating bacteria in foods, better understanding of the factors that control the heat tolerance of bacteria is crucial for promoting increased food safety. The Csonka laboratory at Purdue University recently discovered that mutations that increase the synthesis of magnesium (Mg2+) transport proteins greatly elevate the resistance of Salmonella to killing by high temperature. The objective of this research will be to determine whether the increased heat tolerance in these mutants is actually due to the increased accumulation of Mg2+. If results indicate that the mutations do increase the accumulation of Mg2+, then this would imply that the enhanced heat tolerance in the mutants is the consequence of increased concentrations of this nutrient. On the other hand, if results indicate that the mutants do not have increased Mg2+, this would imply that the increased heat tolerance is the consequence of some unknown function of the Mg2+ transport proteins. In either case, these experiments will lay the groundwork for subsequent research to pursue the question of why increased synthesis of Mg2+ transport proteins enhances thermotolerance. This work will contribute to the understanding of the factors that determine the heat tolerance of pathogenic food bacteria, providing important insights for more efficient elimination of pathogens from food products. The project will also provide training opportunities for future scientists from high school students to university undergraduate and graduate students.
Foodborne pathogens continue to pose a serious problem for human health. According to the USDA Economic Research Service, in the US alone there are over 1.3 million cases of foodborne Salmonella infections annually, with an estimated cost of over $2.7 billion (http://webarchives.cdlib.org/sw1rf5mh0k/www.ers.usda.gov/Data/FoodborneIllness/#2010-5-17). Because cooking and other forms of high temperature treatment are most commonly used, effective, and economic means for the inactivation of microbial contaminants in food products, the investigation of factors that determine the ability of bacteria to survive such treatments has important applications for ensuring food safety. The Csonka laboratory discovered that the survival of Salmonella enterica at high temperature can be greatly enhanced by conditions that increase the production of the MgtA or MgtB magnesium (Mg2+) transport proteins. With support from this EAGER (high risk, high reward) grant from the NSF, we demonstrated that: 1) increased production of the MgtA and MgtB proteins results in increased uptake of Mg2+, and 2) site-directed mutations targeted to portions of the MgtA protein that are crucial for Mg2+ transport abolished the increased heat tolerance afforded by the high level production of this protein. The main intellectual merit of this research project is the finding that that Mg2+ accumulation plays a significant role in the regulation of thermotolerance in bacteria, which has not been recognized hitherto. In addition to advances in the basic understanding of the regulation of heat resistance in Salmonella and the practical applications to food microbiology, a broader impact of this project is that the support from the NSF enabled the PI to continue the training of new scientists and professionals in modern techniques and ways of thinking of bacterial physiology, genetics, and bioinformatics. Noteworthy in this effort was the fact that the PI’s laboratory hosted two female high school students, including a Hispanic-American, for Science Fair projects in topics related to adaptation of Salmonella to high temperature and salinity stress. Both of these students are now enrolled at universities, majoring in biology-related sciences. A second important aspect of the broader impact of the project was the involvement of the PI and a Ph. D. student in his lab in the planning and implementation of a novel laboratory course that introduced first year Purdue University undergraduate biology majors to a realistic and challenging research project. In three years, this course was taken by 36 female and 17 male students, including 3 African-Americans and 3 Hispanic-Americans. Results obtained in the first two years of the offering of the course were published in a refereed journal article, with 30 undergraduate students as coauthors (Gasper et al. DNA and Cell Biology 31:956-967, 2012). The writing and review of this paper gave the students valuable training about the process of scientific publication. Our novel approach of introducing students to authentic research projects early in their academic career has been recognized by a "Science Prize for Inquiry-Based Instruction" (Gasper et al. Science 335, 1590-1591, 2012). The manual for this course is available on line to colleagues who would like to implement a similar course based on our syllabus (www.sciencemag.org/cgi/content/full/335/6076//1590/DC1), and the PI is committed to share bacterial strains used in the course upon request to colleagues at other institutions.
