Actually, that's not really 100% either.
In a single play of a prisoner's dilemma, you still don't know what's best because you don't know what your opponent is going to do; you can only hope that he's going to hold his tongue, but since he won't, you'll both rat each other out no matter what. It's more of a "Sneetches" game, where all the bacteria know that the majority need to survive in order for them to obtain a "stable" outcome and keep living as a whole.
Suppose there are an odd number of bacteria. Then either the individual bacteria wants to spore or not spore, but he doesn't want to do what the majority does. We see nash equilibria at (n-1)/2 bacteria sporing and (n+1)/2 bacteria not sporing if we split them down the middle. FTFA: "Observations have shown that indeed only about 10 percent of the bacteria enter into competence." Therefore instead of having it split down the middle, it's simply a matter of one more than ~10% and one less than ~10%. This makes me wonder whether it's a game at all, or whether there are other reasons they haven't yet seen.
To understand the Sneetches game, read http://books.google.com/books?id=WcvRcTu9RZ8C&lpg=PA142&ots=_SiHtETh_u&dq=Suppose%20there%20are%20k%20sneetches%20born%20with%20stars%20and%20k%20%E2%89%A4%20n&pg=PA119#v=onepage&q=sneetches&f=false