Published: 07 August 2013

Summer’s most unsavoury picnic guests

Dr Simon Park reveals why flies and eating al fresco really don’t mix.

They’re an unwelcome accompaniment to most outdoor occasions, but, aside from being irritating, what impact do flies really have when they buzz about the barbecue?

Dr Park, Senior Lecturer in Molecular Biology, temporarily trapped three blue bottle flies in a plastic Petri dish filled with solid bacterial growth media and allowed them to walk over the surface for 10 minutes.

After a day or so, the bacteria tracks left by the flies grew into visible points or colonies - showing the grim effect tiny fly footprints could have on picnic or barbecue food.

If a blue bottle has recently fed on faeces it may carry as many as six million bacteria on its feet.

Dr Park said: “The blue bottle fly seems to be equally at home feeding on rotting bodies, faeces or our carefully prepared food, and this and other habits make it an unparalleled vector for transmitting disease.

“It prefers to swallow liquid food, and usually regurgitates ingested material in order to liquefy its meal and to facilitate digestion. In this manner, flies can contaminate clean surfaces with approximately 0.1mg of ‘food’ per landing.  Droplets of bacteria-rich faeces may also be deposited during feeding.

“I must admit that even as a well-seasoned microbiologist, the images from this experiment make me slightly queasy.”

So what’s the best way to prevent flies from adding their own accompaniment to your summer meals? Dr Park advises keeping food covered and in the fridge, where possible, to prevent the growth of any harmful bacteria.

Read the full press release here and listen to Dr Park discuss his research on BBC Radio Surrey (1 hour 55 minutes into the programme).

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