Evaluation of antibacterial immune response in Drosophila melanogaster and Drosophila ananassae

Insects are excellent model organisms to examine the phenotypic variation of innate immunity. The innate immunity is the immediate response of invertebrates for protection against pathogens. In this study, we assessed the immune responses of Drosophila melanogaster and Drosophila ananassae against Staphylococcus aureus and Escherichia coli. It includes survival ability against infection, systemic pathogen load, and estimation of phenoloxidase activity, melanization reaction, the total number of hemocytes, quantification of fat, and the nitrite levels in adult flies and also the bacterial inhibitory effect of hemolymph. The results showed increased bacterial growth in D. ananassae compared to D. melanogaster. Due to bacterial infection, reduced fat accumulation was observed in both D. ananassae and D. melanogaster, which suggests a possible decline in physiological condition. Further, a significant elevation in phenoloxidase, nitrite levels and hemolymph reveals that bacterial cell growth was found to be inhibited due to the immune response of both Drosophila species. D. ananassae showed high sensitivity to bacterial infection, while D. melanogaster showed moderate sensitivity. The results have demonstrated the level of immunocompetence of laboratory stocks of D. melanogaster and D. ananassae against Gram-positive and Gram-negative bacteria