Why are streptococci resistant to penicillin?

Streptococci are resistant to penicillin primarily due to their ability to produce penicillinase, an enzyme that specifically breaks down penicillin and other beta-lactam antibiotics. Penicillin works by targeting the bacterial cell wall synthesis, and the presence of penicillinase allows the streptococci to neutralize the antibiotic's effectiveness before it can act on their cells. This enzymatic action can significantly diminish the drug's efficacy, allowing the bacteria to survive and proliferate even in the presence of penicillin.

Other factors mentioned in the incorrect options do not contribute to this resistance in the same way. For instance, while having a thick capsule can provide some protection against phagocytosis by immune cells, it is not a primary factor in penicillin resistance. Additionally, Gram-negative bacteria are typically more resistant to penicillin due to their outer membrane, but streptococci are Gram-positive, which generally have a thick cell wall rather than a protective outer membrane. Furthermore, the assertion that they lack a cell wall is false; streptococci have a cell wall designed to be targeted by penicillin, making the production of penicillinase a key mechanism for resistance.