It depends on the bacteria, when in it’s lifecycle half of it is killed, and what half is killed. To keep things short, the odds are in the bacteria’s favor. Suppose if half the bacteria in your gut died right now how long do you think it would take for the bacteria population in your gut to return to pre-snap levels? A month? A year? Decades? How about less than an hour. Bacteria reproduce exponentially and on average, a bacterial generation lasts 20 minutes. Meaning that every 20 minutes the population doubles, assuming there are no deaths in the population during this time. If there is space for bacteria to grow, they will.