Which of the following organisms, when isolated in the blood, requires the synergistic activity of penicillin plus an aminoglycoside for appropriate therapy-
**Core Concept**
The synergy between penicillin and an aminoglycoside is crucial in treating certain bacterial infections, particularly those caused by gram-positive bacteria with penicillinase production, such as methicillin-resistant Staphylococcus aureus (MRSA), and also against gram-negative bacteria like Pseudomonas aeruginosa.
**Why the Correct Answer is Right**
The combination of penicillin and an aminoglycoside is necessary to effectively treat these organisms due to the development of resistance. Penicillin inhibits cell wall synthesis by binding to penicillin-binding proteins (PBPs), while aminoglycosides inhibit protein synthesis by binding to the 30S subunit of the bacterial ribosome. This synergistic effect enhances the bactericidal activity against the target bacteria. The synergy is particularly effective against bacterial strains that produce beta-lactamases, which confer resistance to penicillin.
**Why Each Wrong Option is Incorrect**
**Option A:** This organism is typically susceptible to penicillin alone and does not require aminoglycoside synergy for effective therapy.
**Option B:** This organism is often resistant to penicillin and would require alternative antibiotics, but not necessarily the combination with aminoglycoside.
**Option C:** This organism is not typically associated with the need for penicillin-aminoglycoside synergy for effective treatment.
**Clinical Pearl / High-Yield Fact**
When treating suspected MRSA infections, clinicians should consider using a combination of antibiotics that includes a beta-lactam (such as penicillin) and an aminoglycoside to ensure effective synergy and coverage.
**Correct Answer: C. Pseudomonas aeruginosa**