My naïve thought, assuming it was effective, could outcompete acid spewing species, and you had dosed a handful of the population: why would it not have been able to spread through the population?

Inoculated person X kisses two people, they go on to kiss two people, etc. Probably too simple a model, but I assume that kissing spreads all manner of microorganisms. How much do you need for the bacteria to take hold?