Safety And Hygiene Standards For Baby Cot Mattress?

Question:

I have heard that one should not buy a second hand cot mattress because of possible bacteria/mold/etc. that may grown in the mattress...  Logically, a mattress from the first born child that has not been stored  or cleaned properly may also pose the same problem as a second hand  mattresses.  My question is how to properly treat, clean or store a cot mattresses to make sure it is safe for a baby to sleep on?  Is there  a rule of thumb about how long a mattress should be kept before we need to change it (like for pillows)?  Any other comments and tips would be  greatly appreciated.