Small Flies as Vectors of Foodborne Illness: Cross-contamination of Food with Escherichia coli by Indoor Breeding Fruit Flies

Introduction: More than 50% of food handling establishments in the US experience fruit fly (Drosophila spp.) infestations. Little is known about their potential to transfer foodborne and other pathogen from their breeding site to food and food handling surfaces. 

Purpose: This research presents results from comprehensive studies of the capability of fruit flies to transfer bacteria in the laboratory. The aim of this study was to provide evidence of the ability of small flies to act as vectors for cross contamination in foods.

Methods: Up to 20 fruit flies were introduced to a 1 ft³ enclosure containing the following: 10g of sanitized chopped lettuce and two 10g samples of yeast-risen donut.  One of the donut samples was previously inoculated with >1 x 10⁶ CFU/g Nalidixic acid-resistant E. coli. A control box contained similar samples without E. coli. Following a two day exposure of flies to food samples the flies were exterminated and samples were enumerated for E. coli using MacConkey agar supplemented with Nalidixic Acid. Scanning electron microscopy was used to visualize contaminated flies.

Results: Over the course of three replicate experiments cross contamination of food with E. coli occurred in both foods during the first replicate and in either lettuce or donut during the other two. E. coli was found at levels of 1.2 to 1.9 x 10³ CFU/g in the non-inoculated donut samples and 1.1 to 4.20 x 10³ CFU/g in the non-inoculated lettuce sample. Bacteria can be seen on the appendages in SEMs of contaminated flies. E. coli was not detected in control samples.

Significance: Recovery of E. coli from non-inoculated food samples that have been exposed to fruit flies with access to contaminated food proves that fruit flies can be effective vectors of foodborne illness and should be considered as a risk to food safety in kitchen and food manufacture environments.