This exploration was inspired by the current hot weather and the thought that two other life forms seem to benefit from it.
The blue bottle fly, Calliphora vomitoria (the latin name says a lot about it) is a very common and cosmopolitan insect, with which we share many of our environments. This fly seems to be equally at home feeding on rotting bodies, faeces and 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. In addition, droplets of bacteria rich faeces may be deposited during feeding, about every four to five minutes. Finally, if a blue bottle has recently fed on faeces it may carry as many as six million bacteria on its feet.
I developed a simple process to reveal the way in which flies carry and transmit bacteria. I trapped three blue bottle files in a large square plastic dish filled with solid bacterial growth media and allowed the flies to walk over the surface for just 10 minutes. As the flies travelled over the uninoculated surface they left behind a trail of the bacteria in their footsteps. Because of the invisible nature of bacteria, these tracks were at first invisible. However, after a day or so the bacteria grow into visible points (or colonies) that reveal the activity of the flies and the extent of their contamination. I must admit that even as a well-seasoned microbiologist , these images make me slightly queasy. (Not flies were harmed during the making of these images)