Answer:
No, not all are harmful.
Step-by-step explanation:
There are bacterias that are harmful, yes, but millions, maybe even billions are helpful. Probiotics, healthy bacteria, as some like Lactobacilli and Bifidobacterium. There are a lot more, but just these two examples are helpful. Lactobacili is helpful with your digestive system. Bifidobacterium is geared towards your mouth and the vaginal area of women in mammals, like humans.
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I hope this helps!
-No one