text: what is a mesophilic bacteria?   meso- middle philic -loving  a mesophilic bacterium is one that grows best in moderate conditions. usually that means that it grows best at moderate temperatures, between 10 and 50oc. but the exact meaning can also depend on what you are discussing. it might mean neutral ph or isotonic medium or a thousand other things.   the alternative is that the bacterium is an extremophile, meaning that it thrives in extreme conditons: boiling water, ice, battery acid, dehydrated sugar, salt pans and so forth.