Infection of Drosophila melanogaster adults with Pseudomonas aeruginosa (PA14) can kill the flies within 48h. We found that the virulence of PA14 was significantly attenuated when treated with a reducing agent, dithiothreitol (DTT), prior to infection. Infection with DTT-treated PA14 elevated Metchnikowin expression at 22 h post-infection and the virulence of DTT-treated PA14 was not attenuated in Dif and Relish mutants. These results suggest that DTT pre-treatment of PA14 can aggravate certain virulence factors that may be required to paralyze fly immune responses, triggering Metchnikowin expression via Dif and Relish activations.