UC Davis

Bacterial pathogens of the family Enterobacteriaceae must be able to successfully overcome innate host strategies for fending off invading bacteria, including competing with the resident gut microbiota, in order to obtain nutrients, replicate, and ultimately cause disease. However, mechanisms used by bacterial pathogens to outcompete the gut microbiota and establish infection are not well understood. Here, we used the murine pathogen Citrobacter rodentium to show the pathogen can both take advantage of host factors that are available in the gut prior to colonization for growth, as well as alter the host metabolism in a way that benefits the pathogen later on during infection. We also use the model Salmonella enterica serovar (S.) Typhimurium to show the pathogen can affect levels of a neurotransmitter, GABA, to induce an immune response that favors S. Typhimurium growth over competing microbes.

In chapter 2, we show that C. rodentium, via action of the type 3 secretion system (T3SS), can shift host colonic epithelial cell metabolism away from beta-oxidation of short chain fatty acids, a process which consumes a large amount of oxygen, to a more fermentative metabolism, leaving oxygen available for the pathogen. This phenotype was linked to colonic crypt hyperplasia, as blocking hyperplasia was able to restore the competition between a wild type and high affinity cytochrome oxidase CydAB mutant. Therefore, C. rodentium employs its T3SS to cause colonic crypt hyperplasia, which allows for oxygen to become available for the pathogen, thus giving the pathogen a growth advantage over competing gut microbiota members.

We next hypothesized that C. rodentium is respiring hydrogen peroxide prior to causing hyperplasia, to gain a foothold for growth in the gut while intimately attached to the colonic epithelium. Chapter 3 dives deeper into the observation from Chapter 2 that prior to the development of colonic crypt hyperplasia, T3SS-mediated attachment is not necessary to respire oxygen. We found that prior to hyperplasia, the cytochrome c peroxidase Ccp mediates respiration of hydrogen peroxide, which is produced by the epithelial NADPH oxidase NOX1. NOX1 is constitutively expressed, and acts as a habitat filter to help shape spatial organization of the gut microbiota, however, C. rodentium can take advantage of this microbicidal compound for growth. C. rodentium thus can benefit from a presumable host defense mechanism, and in this way the pathogen is able to compete against the already established gut microbiota.

Finally, we used S. Typhimurium to elucidate another mechanism of overcoming colonization resistance, by establishing the hypothesis that by altering the levels of a neurotransmitter, gamma-aminobutyric acid (GABA), the pathogen can compete against other Enterobacteriaceae successfully. While the consumption of GABA does not provide a growth benefit directly to the pathogen, a decrease in GABA levels lead to host expression of IL-22 and downstream genes, such as lipocalin-2, allowing the pathogen to outcompete a related Enterobacteriaceae, Escherichia coli JB2. Lipocalin-2 sequesters iron from the gut lumen, which leads to growth inhibition of JB2, but S. Typhimurium obtains iron via salmochelin, which is resistant to lipocalin-2, thus allowing the pathogen to grow rapidly during infection.

Overall, we provide evidence that enteric pathogens have evolved methods to take advantage of immune responses to the invading pathogen, which helps them overcome habitat filters and colonization resistance.