no. you do not need a pressure cooker to kill bacteria. it's temperature that kills bacteria -- and if you've got chopped-up bits of meat you have zero worries. and if you get your meat from a reputable source you'll have far fewer concerns in the future. i buy all my beef from local farmers (the ones whose practices I know are sound) so i know i can trust how they fed and handled their cows.

a pressure cooker is really only necessary if you want to cook something (like a large cut of meat) very quickly. i don't think a typical home cook really needs a pressure cooker for *anything*.