E. coli can take flight.

Unpasteurized apple juice and cider tainted with an especially virulent strain of Escherichia coli triggered a series of lethal food-poisoning outbreaks that made headlines--first in 1991 and again 5 years later. While the source of the fecal bacteria might have been poor hygiene by juice processors, a new study opens the possibility that the culprit might also have been a factor largely out of the industry's hands: contamination of apples still on the tree by fruit flies.

Wojciech J. Janisiewicz stumbled onto the potential role of flies while developing natural biocontrol agents--bacteria and yeasts--to stem the growth of rot-triggering pathogens in stored apples and pears (SN: 6/4/94, p. 359). A plant pathologist at the Department of Agriculture's Appalachian Fruit Research Station in Kearneysville, W. Va., he observed that even sterilized apples could develop microbial contamination when fruit flies dropped by. Now, in the January Applied and Environmental Microbiology, his team describes laboratory experiments demonstrating that fruit flies can transmit the deadly E. coli

O157:H7 from a contaminated apple to a clean one.

To be vulnerable, apples must have an open wound, Janisiewicz says. When exposed inner tissue becomes infected, even small populations of the food-poisoning agent can grow exponentially, his data show.

Though pasteurizing kills E. coil, this heat treatment is not an option for those who sell sliced, fresh fruit. The growing commercial market for such produce--in salad bars, for instance--makes increasingly important an understanding of what may even be rare modes of orchard contamination.

The good news, Janisiewicz says, is that his preliminary studies indicate that some of the rot-inhibiting microbes he works with appear effective in deterring E. coli. Although these natural biocontrols won't kill it, "they do prevent E. coli from growing in apple wounds," he notes--provided that they get into these sites before pathogen-ferrying flies stop by.