Evolution of Salmonella in a cockroach vector

PROJECT SUMMARY Salmonella enterica serovar Typhimurium is a widespread human enteric pathogen that has a broad range of animal hosts and environmental reservoirs. Meanwhile, the German cockroach, Blattella germanica, is the most common pestiferous cockroach species in human environments. B. germanica frequently harbors Salmonella

spp. in nature, serving as both an environmental reservoir and a vector. Transmission of S. Typhimurium by cockroaches has been previously described as a passive, non-replicative process by which the bacteria are mechanically transferred from one surface to another. However, our laboratory recently demonstrated that S.

Typhimurium actively colonizes the digestive tract of German cockroaches after being ingested. Colonization of the cockroach is characterized in part by an initial population bottleneck during which most ingested bacteria are eliminated. Subsequently, the surviving population of S. Typhimurium undergoes replication and persists in the

cockroach gut, interacting with a highly diverse microbiota. The bacteria that persist are disseminated in the feces into the environment where they can be ingested by other cockroaches via coprophagy and/or acquired by a vertebrate host. The central hypothesis of this project is that cockroaches provide a unique environment for

S. Typhimurium to undergo significant evolution as a result of both selection pressure from the host and interaction with its gut microbiota. Specifically, we hypothesize that evolution in the gut of the cockroach vector may favor the emergence of novel variants of S. Typhimurium with 1) enhanced ability for vector-borne

transmission, 2) altered potential to colonize other animal hosts, and 3) antimicrobial resistance horizontally acquired from diverse constituents of the cockroach gut microbiota. We will pursue two specific aims to test our hypotheses. In specific aim 1, we will use whole genome sequencing to identify the adaptive genotypic changes

that occur in S. Typhimurium after experimental evolution in a laboratory cockroach strain and we will examine the effects of these changes on fitness in the vector. In specific aim 2, we will determine if S. Typhimurium can horizontally acquire antimicrobial resistance from commensal bacteria harbored in the gut of field collected

cockroach strains, and we will explore the mechanisms by which this may occur. Together, these studies will provide novel, naturally relevant insight into how underappreciated insect reservoirs may contribute to evolution of S. Typhimurium and may implicate insect control as a simple intervention to mitigate pathogen evolution.