Resistant starch

[Novidez]

I can't understand how all "good bacteria" prefer some foods and all "bad bacteria" prefer other foods; probably I don't understand something about it, but really that makes no sense.

Sure, any imbalance is not good.

Can the body balance this 'bad/good bacteria' ratio by itself?

[Kasper]

Can the body balance this 'bad/good bacteria' ratio by itself?

What do you mean by "by itself"? Without medicines?

[Novidez]

What do you mean by "by itself"? Without medicines?

Yes. I mean, people normally take probiotics or feed the 'good' bacteria with some fermented fiber, etc.
I was wondering if the body, for example, notices that the bacteria ratio are imbalanced and, by itself, it does something like "Ok, let's get rid of these 'bad' bacteria by attaching them in feces and preserve the good ones". Well, of course it probably doesn't work like this and it is random xD

[RRM]

people normally take probiotics

No, that is definitely not normal.
It is a trend, making you think that it is normal.
Probiotics are products sold by companies, who want you to think that we need them.

or feed the 'good' bacteria with some fermented fiber, etc.

Fiber may feed good and bad bacteria.
Sugars may feed good and bad bacteria.
Fats may feed good and bad bacteria.
etc etc etc.

I was wondering if the body, for example, notices that the bacteria ratio are imbalanced and, by itself, it does something

Yes, the body fights bad bacteria.
The body maintains healthy ratios.
The most common bad influence on those healthy ratios is the use of antibiotics, or other medications.