My guess is never. I've said it before, and I'll say it again. If you want to bareback, wait until you're in an LTR of at least 6 months. After 6 months, get tested. Even with those precautions, you must be certain that your partner has NOT been with anyone else. And gentlemen, don't forget that condoms can break. Therefore, you need to always be cautious. Get tested on a regular basis unless you haven't had any in a while.
This new bacteria sounds scary. MRSA strains can be deadly. Both my father and grandfather died (in part) because they had MRSA and sudomonis. MRSA is highly infectious. If this strain gets into hospitals, there will be some serious problems. Hospitals are notorious for spreading MRSA, because some of the medical staff fails to use proper hand washing techniques.