Are flies a significant method of bacterial transmission?

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After analyzing the microbiomes of flies, an international team of researchers have reported that these insects could serve as a significant transmission vessel for bacterial pathogens.

Flies have long been assumed to spread disease, however, the microbiome they harbor and transport has remained largely uncharacterized. In this study, recently published in Scientific Reports, the microbiomes of 116 blowflies and houseflies from 3 different countries were analyzed, revealing these flies were carriers for hundreds of bacterial species, including strains pathogenic to humans.

Author, Donald Bryant from Pennsylvania State University (PA, USA) commented: “We believe that this may show a mechanism for pathogen transmission that has been overlooked by public health officials, and flies may contribute to the rapid transmission of pathogens in outbreak situations.”

The team utilized high-coverage, whole-genome shotgun sequencing, allowing them to analyze the microbiomes at species level. They discovered the two fly species had an overlapping core microbiome, with a proportion either host-specific or influenced by the environment. In addition, they reported that the legs and wings possessed the largest microbial diversity, demonstrating these to be important routes for microbial dispersion.

In one experiment, blowflies were allowed to land on an Escherichia coli lawn, following which the flies landed on the surface of sterile agar plates. When the pattern of E. coli on the second set of plates was observed after incubation the pattern of E. coli growth matched the footprints, demonstrating that flies can disperse bacteria between landing sites and that the inoculum can persist over time.

Lead author, Stephan Schuster (Nanyang Technological University, Singapore) explained: “The legs and wings show the highest microbial diversity in the fly body, suggesting that bacteria use the flies as airborne shuttles. It may be that bacteria survive their journey, growing and spreading on a new surface. In fact, the study shows that each step of hundreds that a fly has taken leaves behind a microbial colony track, if the new surface supports bacterial growth.”

The researchers also discovered 15 instances of Helicobacter pylori, largely in the blowfly samples collected in Brazil, commenting that previously the routes of H. pylori transmission has never accounted for flies as vectors.

First author Ana Carolina Junqueira (Singapore Centre for Environmental Life Sciences Engineering) stated: “This is the first study that depicts the entire microbial DNA content of insect vectors using unbiased methods. Blowflies and houseflies are considered major mechanical vectors worldwide, but their full potential for microbial transmission was never analyzed comprehensively using modern molecular techniques and deep DNA sequencing.”

These flies could also be used for disease surveillance, Schuster concluded: “The environmental sequencing of flies may use the insects as proxies that can inform on the microbial content of any given environment that otherwise would be hard or impossible to sample. In fact, the flies could be intentionally released as autonomous bionic drones into even the smallest spaces and crevices and, upon being recaptured, inform about any biotic material they have encountered.”

Sources: Junquiera ACM, Ratan A, Acerbi E et al. The microbiomes of blowflies and houseflies as bacterial transmission reservoirs. Sci. Rep. 7(16324) doi:10.1038/s41598-017-16353-x (2017); www.eurekalert.org/pub_releases/2017-11/ps-fdp112217.php

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