Can some insects see bacteria

While it is frequently quite acute, the resolving power of insects' compound eyes is insufficient to resolve an individual bacterium, which would be microscopic; there is little if any selective pressure for "microscopic vision" in the animal kingdom.

It's easily conceivable that a bug would be able to see a large bacterial colony which had multiplied sufficiently and grown in size to a macrocopic scale; indeed some arthropods actually feed on biofilms containing bacteria.