The Food and Drug Administration has approved a blood culture test that can determine if Staphylococcus aureus bacteria infections are methicillin resistant (MRSA) or methicillin susceptible (MSSA).The KeyPath MRSA/MSSA Blood Culture Test can distinguish between the diseases within approximately five hours after any bacterial growth is first detected, the agency said. This should come as good news for nursing homes, where infections among patients with weakened immune systems can be deadly and spread rapidly.