CAT and infection: To test whether the bacteria inside the larvae could sense the oxidative stress produced during infection, we measured the expression of the P. aeruginosa catalases katA and katB. We observed that during G. mellonella infection, the expression of the constitutive catalase katA was increased at all the time points measured, and the expression of katB, the catalase whose expression is activated only when large amounts of ROS are detected (15), was activated at 16 and 18 h postinfection.