Up close photograph of a red-eyed fruit fly standing on white sugar grains.
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A Sweeter Living Space Promoted Bacterial Survival in Flies

A high-sugar diet rendered flies susceptible to infections through distinct mechanisms.

Shelby Bradford, PhD
| 2 min read
Image credit© istock.com, jamesbenet

Patients with increased blood sugar concentration often experience increased disease severity following infection.1 In 2015, Brian Lazzaro, a geneticist studying host-pathogen interactions at Cornell University, and his group demonstrated that lifetime high-sugar diets made flies more susceptible to Providencia rettgeri .2

“I was interested in looking into what were some of the physiological mechanisms that led to diet having an effect on infection outcome from that work,” said Andrea Darby, a postdoctoral fellow at the Carnegie Institute of Science who was recently a graduate student in Lazzaro’s lab. In a study published in PLoS Pathogens, Darby, along with Lazzaro and his team, demonstrated that a high-sugar diet in flies caused higher infection-driven mortality by enhancing nutrient availability for infecting bacteria and reducing the flies’ immune response.3

The team fed flies different concentrations of sugar three to five days before infecting them with P. rettgeri or Serratia marcescens to determine if their previous findings were pathogen specific. More dietary sugar increased the bacterial burden and mortality in flies. Diet composition affects metabolic state, so the team assessed changes in circulating sugar in infected flies. P. rettgeri-infected flies fed high-sugar diets showed higher amounts of free sugar compared to those fed a low-sugar diet. However, the researchers observed less circulating sugar after S. marcescens infection even on high-sugar diets, suggesting S. marcescens used these sugars for growth.

The team then used a fly model that lacked 10 antimicrobial peptides (AMPs) to determine if the increased bacteria survival was caused by more available nutrients for the bacteria or reduced immune responses. They observed that P. rettgeri grew comparably in these AMP-deficient flies regardless of the concentration of dietary sugar, suggesting that their growth advantage came from impaired immune responses. However, the sugar concentration continued to influence the growth of S. marcescens in AMP-deficient flies, indicating an effect of nutritional availability. “We're seeing at the organismal level that high-sugar diets make the flies more sensitive to infection in both cases, but the underlying reasons may be slightly different,” Lazzaro said.

“This could have implications for how clinicians treat somebody presenting with an infection,” said Laura Musselman, a fly biologist at Binghamton University who was not involved with the study.