Background: Excretory and secretary products (ES) from blow fly larvae are alternative sources for the discovery of new
antimicrobial agents and may be a solution to an increasing global drug resistance problem. Chrysomya megacephala
(Diptera: Calliphoridae) is recognized as a medically important blow fly species and it is the most prevalent blow fly species
of Thailand. However, study of antimicrobial activity of this species is limited.
Objective: To determine the antimicrobial activity of ES products from the third stage larvae of Chrysomya megacephala and morphological changes of pathogens after treatment with ES products.
Material and Method: Five hundred 3-day-old larvae of Chrysomya megacephala were used to collect the ES products. The disc diffusion method and minimum inhibitory concentration (MIC) assay were used for the determination of the antimicrobial
activity of the ES products against pathogenic bacteria (Staphylococcus aureus ATCC 25923, Bacillus cereus ATCC 1178, Escherichia coli ATCC 25922, and Pseudomonas aeruginosa ATCC 25983), and fungi (Candida albicans ATCC 90028 and Malassezia furfur CBS 1878T). The morphological changes of the pathogens after treatment with the ES products were observed under scanning electron microscope (SEM).
Results: The results of the disc diffusion method showed that the ES products can inhibit the growth of Pseudomonas
aeruginosa. The MIC of the ES products against Pseudomonas aeruginosa was 150-200 μg/ml. Pseudomonas aeruginosa
cells treated with the ES products displayed longer and larger sizes than non-treated cells and some of them had atrophy
forms with irregular shapes. No fungicidal activity was found in this study.
Conclusion: The ES products of this fly species are good sources for the discovery of anti-P. aeruginosa agents in the future.

Keywords: Antimicrobial activity, Excretory and secretory products, Larvae, Chrysomya megacephala