As you probably know, there is a lot of literature on the correlation between Streptococcus bovis and colorectal cancer, but very little on possible mechanisms.
In an editorial (Streptococcus bovis: Causal or incidental involvement in cancer of the colon? in the International Journal of Cancer 119(9):xi-xii), Harald zur Hausen referring to this paper:
Biarc J, Nguyen IS, Pini A, Gosse F, Richert S, Thierse D, van Dorsselaer A, Leize-Wagner E, Raul F, Klein JP, Scholler-Guinard M. (2004) Carcinogenic properties of proteins with proinflammatory activity from Streptococcus infantarius (formerly S. bovis). Carcinogenesis 25: 1477–84.
...used a partially purified S. bovis S300 fraction representing 12 different proteins and triggered the synthesis of proinflammatory proteins (human interleukin-8 and prostaglandin E2, correlated with the in vitro overexpression of cyclooxygenase-2 [Cox-2]) in human colon carcinoma cells (Caco-2) and in rat colonic mucosa. These data could point to a role of oxygen radicals in colon carcinogenesis induced by a chronic infection with S. bovis. The mechanism could be similar to the one suspected for the development of gastric carcinomas after persisting Helicobacter pylori infections. Presently it would still be important to know whether the increased presence of S. bovis in colonic cancers and polyps results from the preferential bacterial colonization of these cancers and their precursors or whether S. bovis represents a carcinogen that is causally involved in colon cancer.
The last sentence summarises the ongoing debate: does the bacterium promote the cancer, or does the cancer promote the bacterial infection?