Interaction of filth flies, plants and human pathogens: a dangerous synergy

Filth flies have been implicated in dispersal of human disease pathogens; however, fly transmission parameters of human pathogens to plants are largely unknown.    The ability of Phormia regina (black blow flies) and Musca domestica (house flies) to acquire and subsequently release bacteria onto lettuce leaves was compared.  Adult flies were exposed to GFP-tagged E. coli O157:H7 or S. enterica inoculated manure for one hour then allowed overnight access to lettuce.  Plants and flies were then tested for presence of bacteria by standard microbiological methods.  Blow flies acquired more E. coli O157:H7 than house flies but no difference was detected in deposition.  The two species of flies acquired similar numbers of S. enterica cells, but blow flies deposited more cells than house flies. In further experiments, flies were given precisely timed exposure and inoculation periods.  Blow flies acquired more E. coli O157:H7 than S. enterica.  Later experiments exposed blow flies to manure, then tested deposition by forcing flies to walk on lettuce tissue.  Blow flies deposited about the same amount of E. coli O157:H7 as S. enterica.  House flies acquired more S. enterica than blow flies.  There was little difference in acquisition of E. coli O157:H7 between house flies and blow flies.  These data are consistent with the hypothesis that blow flies and house flies are bioenhanced mechanical vectors of E. coli O157:H7 and S. enterica.