TL;DR: This paper describes the finding that there is a specific type of bacterium (Symbiodolus clandestinus) that lives inside of the tissues of several different insects. This bacterium appears to cause no disease, and it is hypothesized that it provides some useful metabolites that the insects are unable to produce themselves. The bacteria can be passed from the mother directly to her offspring. So, this appears to be a widespread symbiotic relation between a bacterium and insects.

The article goes into a lot more depth and describes some other examples of bacteria <-> insect interactions.