The fate of Pseudomonas aeruginosa in Musca domestica: concurrent spatial and temporal examination of bacteria along with expression of house fly antimicrobial responses

The common house fly (Musca domestica L.) lives and breeds in bacteria-rich environments. Due to their synanthropic nature, house flies implicate in the transmission of human pathogens. To demonstrate flies serve as biological vectors, bacteria should persist and/or proliferate and be excreted. However, concurrent physical and physiological defenses in the fly gut may impact pathogen fate. This study analyzed the temporal and spatial location, survivability, and excretion of the opportunistic pathogen Pseudomonas aeruginosa along with concurrent tissue-specific production of antimicrobial peptides (AMPs) and lysozyme in the house fly alimentary canal. Viable, motile P. aeruginosa remained within the gut up to 24 h post-ingestion (PI), and were trapped in the peritrophic matrix. Culture-recovery of P. aeruginosa from flies significantly decreased between 2 and 12 h PI (P<0.04), then significantly increased between 12 and 24 h PI (P<0.02).  In addition, viable P. aeruginosa were recovered from fly excreta throughout 24 h PI. Interestingly, presence of P. aeruginosa induced mRNA expression of all 3 AMPs and lysozyme, but we observed little correlation with bacteria survivability. However, immunofluorescence microscopy showed peak upregulation of AMPs and lysozyme proteins in the gut at 6 h PI, which correlated with a dip in culture-recovery.  Our study determined that although the house fly mounts a robust immune response to P. aeruginosa, it still serves as a biological vector. Bacterial counter-strategies to fly defenses, and their role in bacterial fate, will be discussed.