Final answer:
Staphylococcus aureus can develop resistance to penicillin, so it's false that penicillin should always work against this bacterium.
Step-by-step explanation:
False
Although Staphylococcus aureus is Gram-positive, it can develop resistance to penicillin. In the 1940s, penicillin was indeed effective against infections caused by this bacterium. However, over time, populations of Staphylococcus aureus have emerged that are completely resistant to penicillin.