Which of the following best explains how penicillin-sensitive Streptococcus could grow in the presence of penicillin?

• A. The bacteria acquired a plasmid encoding a beta-lactamase enzyme.
• B. The bacteria increased production of peptidoglycan.
• C. The bacteria switched to anaerobic metabolism.
• D. The bacteria formed endospores to resist penicillin.

Answer: (A)

Verified step by step guidance

1. Understand the mechanism of action of penicillin: Penicillin targets bacterial cell wall synthesis by inhibiting the enzyme transpeptidase, which is essential for cross-linking peptidoglycan layers.
2. Recognize that penicillin-sensitive Streptococcus normally cannot grow in the presence of penicillin because the drug disrupts their cell wall, leading to cell lysis.
3. Consider how bacteria can develop resistance: One common method is acquiring a plasmid that encodes beta-lactamase, an enzyme that breaks down the beta-lactam ring of penicillin, rendering it ineffective.
4. Evaluate the other options: increased peptidoglycan production does not prevent penicillin from inhibiting cross-linking; switching to anaerobic metabolism does not affect penicillin's mechanism; forming endospores is not a typical response of Streptococcus and does not confer penicillin resistance.
5. Conclude that the best explanation for growth in the presence of penicillin is the acquisition of a plasmid encoding beta-lactamase, which enzymatically inactivates penicillin.