**Question:** All of the following are true regarding Haemophilus influenzae except

A. It is a Gram-negative diplococcus;
B. It is a facultatively anaerobic bacterium;
C. It is a causative agent of meningitis;
D. It is resistant to penicillin.

**Correct Answer: D. Haemophilus influenzae is resistant to penicillin.**

**Core Concept:** Haemophilus influenzae is a Gram-negative diplococcus and a facultatively anaerobic bacterium that can cause various infections, including meningitis. The bacterium belongs to the family Pasteurellaceae and is typically found in the respiratory tract of humans.

**Why the Correct Answer is Right:** Haemophilus influenzae is not resistant to penicillin, which is a common antibiotic used to treat bacterial infections. Penicillin works by inhibiting bacterial cell wall synthesis, preventing the formation of peptidoglycan, and ultimately leading to bacterial cell lysis. Resistance to penicillin would mean that the bacterium has acquired mutations or other mechanisms that allow it to survive penicillin's inhibitory effect. Since H. influenzae is susceptible to penicillin, it is incorrect to label it as penicillin-resistant.

**Why Each Wrong Option is Incorrect:**

A. Haemophilus influenzae is indeed a Gram-negative diplococcus, which means it has a thin peptidoglycan layer and is negatively charged due to its outer membrane. This characteristic is consistent with this bacterium.

B. Although Haemophilus influenzae is facultatively anaerobic, meaning it can grow in both aerobic and anaerobic conditions, it does not make it resistant to penicillin. Anaerobic bacteria are typically resistant to oxygen, but penicillin works by targeting the bacterial cell wall, regardless of its oxygen requirements.

C. Haemophilus influenzae can cause meningitis, which is a severe complication of bacterial infections affecting the brain and spinal cord. Meningitis is a serious condition that requires prompt antibiotic treatment, including penicillin, if H. influenzae is the causative agent. Penicillin is a suitable antibiotic for treating meningitis due to H. influenzae.

**Clinical Pearl:** Penicillin remains a crucial antibiotic in treating bacterial infections, particularly in the context of meningitis caused by H. influenzae. However, penicillin resistance is a growing concern globally. In certain regions, H. influenzae has developed resistance to penicillin through the production of beta-lactamases, which are enzymes that degrade the beta-lactam ring of penicillin, rendering it ineffective. Penicillin-resistant H. influenzae strains must be appropriately treated with alternative antibiotics, such as amoxicillin, amoxicillin-clavulanic acid, or cephalosporins.