Microbial Diversity of Fruit Flies and Fruit Fly Breeding Sites at Multiple Restaurant Sites in Minnesota

Introduction: More than 50% of food handling establishments in the US experience fruit fly (Drosophila spp.) infestations. State restaurant inspectors vary in their assessment of small flies as food safety risks.  No surveys of microbial diversity of small flies and their breeding sites have been carried out to date.

Purpose: In order to assess the microbial diversity of small flies in restaurant environments a survey of restaurants in Minnesota was undertaken. Both flies and breeding sites were sampled and analyzed.

Methods: Fruit flies and swabs of fruit fly breeding sites and from multiple restaurant locations in Minnesota were aseptically gathered and processed on the same day as collection. Flies were exterminated by freezing (-70°C for 3 min), diluted in buffered peptone water, stomached and plated on Tryptone Glucose Extract Agar (TGE). Swabs were also plated on TGE and both sample types incubated at 32°C for 48 h. Isolates were identified using a Vitek II system.

Results: Seventy separate bacteria were isolated from fruit fly and breeding site sample from restaurants visited.  Of the breeding site isolates 80% were Gram negative bacteria, 10% Gram positive and 10% yeast. Flies harbored 53% Gram negative bacteria, 40% Gram positive (mainly Bacillus spp.) and 7% yeast.  Species of biofilm-forming Sphingomonas and Chryseobacterium were the most commonly found bacteria in all samples and were most commonly found on both flies and in corresponding breeding sites. Captured flies were found to carry between 1 x 10³ and 1 x 10⁵ CFU bacteria/fly.

Significance: Little is known of the risk posed by small flies in the food handling environment. This is the first study of the microbial diversity of fruit flies in a food service environment and highlights vector potential between breeding and landing sites of these flies.