Interaction of filth flies, plants and human pathogens

Filth flies have been implicated in the mechanical transmission of food borne pathogens such as Escherichia coli O157:H7 and Salmonella enterica. When at rest, filth flies regurgitate and defecate on solid surfaces, leaving visible spots. Fly specks were utilized to assess blow fly preference. They preferred sunflowers and wheat over lettuce in choice tests. The strong attraction to wheat and sunflowers suggests involvement of visual or volatile cues for orientation to plants. This data could be used to create a novel control method to prevent fly entry of lettuce fields and avoid contamination of produce. Although filth flies disperse human disease pathogens, fly transmission parameters of human pathogens to plants are largely undescribed. The ability of the black blow fly, Phormia regina to acquire and subsequently release bacteria onto baby lettuce leaves was compared to that of the house fly, Musca domestica. Blow flies acquired significantly more E. coli O157:H7 cells than did house flies; however, there was not a significant difference in the number of cells deposited by these two species onto lettuce. To more accurately assess attachment and release of bacteria, flies were given precisely timed exposure and inoculation periods. Blow flies deposited about the same amount of E. coli O157:H7 as S. enterica. House flies and blow flies may be about even in their vector competence for S. enterica. The data presented suggests blow flies are more efficient vectors of E. coli O157:H7 to leafy greens than house flies. Blow adults were exposed to five different treatments; one negative control, an absolute positive control and three concentrations of E. coli O157:H7 mixed with manure. The flies were then prepared for Scanning Electron Microscopy (SEM) and descriptive results obtained. Many bacteria-like organisms that are likely E. coli O157:H7 were observed on the surface of the flies. Bacteria are able to attach and form groups possibly resulting in biofilms on the surface of P. regina. These data more precisely describe the interaction between blow flies and pathogens E. coli O157:H7 and S. enterica, supporting the hypothesis that the transmission mode is more than simple mechanical transmission.