If a child has blood type o and its mother has type a, would a man with type b be the father

He could be, its a possibility. So yes, he could be the father 

If the Mother's blood type is B, and the father's blood type is O, then their child must be either B or O. For instance, if both mother and alleged father have type A blood, then the child can only have A or O blood. ... However, in 1 out of 4 instances, both parents will contribute their recessive O gene to their child.

To kill bacteria from our body especially from stomach