Spatial and temporal persistence of Bacillus cereus and Pseudomonas aeruginosa in the house fly and effects on antimicrobial peptide and lysozyme expression

Due to its indiscriminate association with garbage, excrement, and human food items, the house fly (Musca domestica) has been implicated as a vector of many infectious diseases, especially in developing countries. Although house flies both feed and oviposit on septic substrates, they apparently do not become diseased from the bacteria that they carry and ingest. This resistance may be mediated by secreted factors in the gut, including antimicrobial peptides or digestive enzymes such as lysozyme. To elucidate the interaction between fly and bacteria, flies were fed known doses of GFP-tagged Pseudomonas aeruginosa and Bacillus cereus, both of which are human pathogens. Western blot and RT-qPCR analyses were performed on samples taken over a 6 hour interval to determine the effects of bacterial ingestion on both systemic and local antimicrobial gene and peptide levels. Also, viability of the bacteria within the fly was examined both spatially and quantitatively, by microscopy and culture from fly gut and recovered excreta. Both bacteria were recovered at infectious levels in dissected hindgut and collected excreta, demonstrating transmissibility of these pathogens and further implicating the house fly as a true biological vector. Understanding the interactions between flies and bacteria will lead to a better understanding of both the mediators of disease resistance and the vector potential of these insects for human pathogens.