I believe toxigenic bacteria in mattresses can be related to SIDS, and this can be a risk with a second hand cot mattress. Can the same be said for a standard second hand mattress protector or can a machine wash be enough to remove the bacteria? I have been given one that looks to be in perfect condition and only 5 years old (held in storage). However I am wondering if it is safest to just purchase new. Thanks in advance.

Red Nose recommends that a second hand mattress can be used as long as you are happy that it remains firm & well-fitting & is clean (& you are ok with how/where it has been stored.)
https://rednose.org.au/article/is-it-safe-to-use-a-second-hand-mattress

I see no reason why this cannot be the same for mattress protector. Would recommend washing in hot water.
Ensure the mattress protector is a tight fit & does not create a soft surface on the mattress.
Always make sure any waterproof mattress protector is strong and a tight fit.
Never put soft bedding (including lambswool) under or on top of the bottom sheet as this makes the sleeping surface too soft.
https://rednose.org.au/article/second-hand-mattresses