Does scaling and deep cleaning of teeth help to remove s.mutans and prevent cavity? when is it required?

I don't think either accomplishes these. I've seen instances of severe swelling and infection following these procedures and caution people with immunosuppression to think carefully about having these treatments and obtain a second opinion to ensure that they are truly advisable. Prophylactic antibiotics would be indicated in these patients.

Scaling and root planning is to make the roots of your teeth tarter free, glass smooth, and harder for bacteria to attach. It is also to curette any damaged gum tissues to reduce inflammation. Make sure to clean between your teeth daily to reduce chance of gum disease damage.