[otakucode]

That would be a really weird reason. The natural bacteria has the exact same problem. The engineered one is uncertain to have the problem. Why would you opt for the one that definitely carries the risk and ALSO causes expensive tooth decay?

[John_KZ]

Because the engineered caries might behave differently and unexpectedly. Regular caries has a pretty hard time getting into the bloodstream and won't infect other regions. Can the immune recognize the new strain? Can the new strain infect other regions of the body? Does it permeate into the bloodstream more often? There are more questions to be answered, experimentally.