"Feel" is something that requires consciousness and a sense of awareness that the pain being experienced is self-related. Consciousness requires complex systems and a nervous system which insects do not possess. Whilst reactive and reflexive actions are performed, whether they experience pain as a personal subjective phenomenon is something that isn't tested. Reaction isn't a proxy to consciousness.

Would a bacteria "feel" pain because it reacts to the environment?

I say this as someone that has almost a Buddhist approach to respecting living creatures. However, the science of consciousness and a physicalist approach inform my views.