**Question:** Which of the following organisms, when isolated in the blood, requires the synergistic activity of penicillin plus an aminoglycoside for appropriate therapy -

A. Streptococcus pneumoniae
B. Staphylococcus aureus
C. Escherichia coli
D. Klebsiella pneumoniae

**Correct Answer:** **D. Klebsiella pneumoniae**

**Core Concept:** Synergistic Antibiotic Therapy - Combining Penicillins with Aminoglycosides:

When dealing with bacterial infections, understanding the concept of synergistic antibiotic therapy is essential for selecting appropriate treatments. Synergistic therapy occurs when two or more antibiotics work together to provide better antimicrobial activity than expected from their individual effects. This can be achieved by combining a penicillin, such as penicillin G or amoxicillin, with an aminoglycoside, such as gentamicin or tobramycin.

**Why the Correct Answer is D:**

Klebsiella pneumoniae is a Gram-negative bacillus that is commonly isolated from blood stream infections. It is often resistant to beta-lactam antibiotics, including penicillins, due to the production of beta-lactamases enzymes that degrade the beta-lactam ring of penicillins, rendering them ineffective. In such cases, combining a penicillin with an aminoglycoside can achieve synergistic activity, effectively killing the bacteria and improving patient outcomes.

**Why Other Options are Incorrect:**

A. Streptococcus pneumoniae: This organism is commonly susceptible to penicillins, making option A an incorrect choice for a penicillin-resistant Klebsiella pneumoniae infection.

B. Staphylococcus aureus: Staphylococcus aureus is typically susceptible to penicillins, so option B is not a suitable choice for treating Klebsiella pneumoniae infections.

C. Escherichia coli: Similar to A and B, Escherichia coli is generally susceptible to penicillins, making option C an incorrect choice for treating Klebsiella pneumoniae infections.

**Clinical Pearl:**

In clinical practice, understanding the synergistic effect of combining penicillins with aminoglycosides is crucial for effective treatment of antibiotic-resistant Gram-negative bacteria like Klebsiella pneumoniae. This approach can improve antimicrobial efficacy and reduce the risk of antibiotic resistance development. It is essential to consider these combinations when treating infections caused by Gram-negative bacteria, particularly in cases where the pathogen is resistant to beta-lactam antibiotics.