Effects of bacterial dose and fly sex on persistence and excretion of Salmonella enterica serovar Typhimurium from adult house flies (Diptera: Muscidae)

Authors: Nayduch, Dana; ZUREK, KLARA - Kansas State University; THOMSON, JESSICA - Kansas State University; Yeater, Kathleen

Submitted to: Journal of Medical Entomology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/20/2018
Publication Date: 4/11/2018
Citation: Nayduch, D., Zurek, K., Thomson, J., Yeater, K.M. 2018. Effects of bacterial dose and fly sex on persistence and excretion of Salmonella enterica serovar Typhimurium from adult house flies (Diptera: Muscidae). Journal of Medical Entomology. 55:1264-1270. https://doi.org/10.1093/jme/tjy055.
DOI: https://doi.org/10.1093/jme/tjy055

Interpretive Summary: Salmonella Typhimurium is a pathogen harbored by livestock and shed in their dung that can cause gastrointestinal disease in humans. Because house flies associate with dung and other filth, this pathogen and can be ingested and carried in their gut to be shed in their excreta (vomit, feces droplets). Previous studies in our group have shown that Salmonella survives in the house fly digestive tract. Because transmission involves excretion, we were interested in identifying factors that influence the excretion of S. Typhimurium from infected flies. In this study, we determined the persistence and excretion of two different doses of S. Typhimurium from male and female house flies. Individual male and female flies were fed either sterile culture media (Luria-Bertani broth, LB; controls) or LB cultures of “high” (~100,000 cells) or “low” (~10,000 cells) doses of bacteria (treatments). Bacterial persistence within the fly gut was determined over 16 h by culture of homogenized flies. Both fly sex and bacterial dose affected persistence of Salmonella in flies between 6 and 12 h post-ingestion. In a separate experiment, flies were again fed the control broth and the two doses of bacteria and their excretion events were monitored during this time interval (6-12 h). Each of the excreta droplets were individually-cultured for bacteria. Female flies had more excretion events than males across all treatments. There was a statistically-significant interaction between fly sex and bacterial abundance (dose), both on the proportion of Salmonella-positive droplets and the number of bacteria shed per droplet. In flies that ingested the low dose of bacteria, males excreted a greater proportion of positive droplets than females. In the high dose treatment, males excreted more bacteria per droplet than females. High dose male flies excreted more bacteria per droplet than low dose males, but low dose females excreted more bacteria per droplet than high dose females. Irrespective of sex, flies that ingested the low dose flies excreted a greater portion of what they ingested than high dose flies. This study demonstrates that both bacterial abundance and fly sex may influence excretion of bacteria from flies, and both factors should be considered when assessing the risk of house fly transmission of pathogens.

Technical Abstract: Salmonella Typhimurium is a pathogen that causes gastroenteritis in humans and can be harbored by house flies. Factors influencing excretion of S. Typhimurium from infected flies have not been elucidated, but are essential for assessing transmission potential. We determined the persistence and excretion of GFP S. Typhimurium from house flies. Individual male and female flies were fed either sterile Luria-Bertani broth (LB; controls) or cultures of “high” (~105 CFU) or “low” (~104 CFU) doses of bacteria (treatments). Bacterial persistence was determined over 16 h by culturing whole-fly homogenate. Both sex and dose affected persistence between 6 and 12 h post-ingestion. In a separate experiment, fly excretion events were monitored during this time interval and excreta droplets were individually-cultured for bacteria. Female flies had more excretion events than males across treatments. We observed interactions of fly sex and bacterial abundance (dose), both on the proportion of Salmonella-positive droplets and the CFU shed per droplet (CFU/droplet). In the low dose, males excreted a greater proportion of positive droplets than females. In the high dose treatment, males excreted more CFU/droplet than females. High dose male flies excreted more CFU/droplet than low dose males, but low dose females excreted more CFU/droplet than high dose females. Irrespective of sex, low dose flies excreted a greater dose-adjusted CFU (CFU droplet/CFU fed) than high dose flies. This study demonstrates that both bacterial abundance and fly sex may influence excretion of bacteria from flies, and should be considered when assessing the risk of house fly transmission of pathogens.