The role that bacteria play will depend very much on which termite species you're talking about. Higher termites don't have them, but in lower termites, the biggest players in cellulose digestion are the flagellates. Those flagellates, in turn, have both intra- and extracellular bacterial symbionts, notably members of the Elusimicrobia. There are also bacteria in the gut that do not have an obligate association with flagellates.
In Reticulitermes flavipes, spirochaetes are particularly dominant in the gut (backed up by studies that showed that the dominant bacterial taxon is Treponema: https://doi.org/10.3389%2Ffmicb.2016.00171). That study also found caste-specific differences in which microbes were present.
A lot of these bacterial species will be playing some role in digestion, even if it is not a direct role (e.g. as essential symbionts of flagellates).
Hi there! What do exactly want to know about it? As said before, the symbiotic relation between bacteria and termites is species-specific and implies several diferente roles. It is a world on itself, and picking up articles to suggest to you will very much depend on what you are looking for. If you want further help, you need to be more specific.