There's never an equal number of each bacteria though.

So there's an odd number of each kind of bacteria in the gut?

Also, one bacteria isn't equal to another of a totally different kind, there's some that can survive better than others, there's good and bad bacteria that has a high survivability against the invasions of others.

There's probably trillions of different kinds of bacteria and each one has its own capabilities....and let's not forget, they share genetic information with each other all the time.

So after the snap, there's probably a lot of people who have fucked up gut biomes and other people with extremely healthy ones. Because there's a different quantity of each kind of bacteria inside of each living thing.

That would probably cause some insane mutations in the next coming months after the snap too. Half the bacteria being gone means there's 50% less genetic material for them to share.

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.

I responded to a different comment explaining more.

Micro biology changes a lot faster than what you see in complex multi-celluar organisms.