Final answer:
Mycoplasma is innately resistant to penicillin since it lacks a cell wall, while Staphylococcus aureus has acquired resistance over time through genetic mechanisms, particularly in MRSA strains.
Step-by-step explanation:
Of the bacteria options provided, Mycoplasma inherently lacks a cell wall and therefore has an innate resistance to penicillin, as penicillin targets cell wall synthesis. Meanwhile, Staphylococcus aureus (S. aureus) was initially treatable with penicillin in the 1940s; however, over 90% of current strains are resistant due to the antibiotic resistance genes they have acquired over time. Specifically, Methicillin-Resistant Staphylococcus aureus (MRSA) emerged with resistance to methicillin, a type of penicillin, and subsequently all ß-lactam antibiotics, becoming a major health concern.