Question 1

Q: A 65-year-old male underwent emergency laparotomy for perforated duodenal ulcer 5 days ago. He now develops fever (38.5°C), productive cough, and increased oxygen requirement. Chest X-ray shows a new right lower lobe infiltrate. Gram stain of endotracheal aspirate shows Gram-negative bacilli, and culture grows lactose-fermenting colonies that are oxidase-negative and produce a metallic sheen on EMB agar. The isolate is resistant to ampicillin and third-generation cephalosporins but sensitive to cefoxitin (a cephamycin). Resistance is inhibited by clavulanic acid in disk synergy testing. The isolate is also resistant to fluoroquinolones, but sensitive to carbapenems and colistin. What is the most likely mechanism of antibiotic resistance in this organism?

Extended-spectrum beta-lactamase (ESBL) production. ***Extended-spectrum beta-lactamase (ESBL) production*** - **E. coli** with resistance to **third-generation cephalosporins** but sensitivity to **cefoxitin** (cephamycin) is classic for **ESBL production**. - The positive **disk synergy testing** with **clavulanic acid inhibition** is pathognomonic for **ESBL enzymes**, which are inhibited by beta-lactamase inhibitors. *AmpC beta-lactamase hyperproduction* - **AmpC producers** are typically resistant to **cefoxitin** (cephamycins), unlike this isolate which is sensitive. - **AmpC enzymes** are **NOT inhibited by clavulanic acid**, contradicting the positive disk synergy test described. *Carbapenemase production* - This mechanism is ruled out as the isolate is explicitly **sensitive to carbapenems** (imipenem, meropenem). - **Carbapenemase-producing Enterobacteriaceae** would show high-level resistance to all **carbapenem antibiotics**. *Porin channel mutation with efflux pump overexpression* - These mechanisms rarely provide the **high-level, specific resistance** to third-generation cephalosporins seen here. - **Efflux pumps** typically cause **low-level, multidrug resistance** patterns rather than the specific beta-lactam resistance profile described.

Question

A 65-year-old male underwent emergency laparotomy for perforated duodenal ulcer 5 days ago. He now develops fever (38.5°C), productive cough, and increased oxygen requirement. Chest X-ray shows a new right lower lobe infiltrate. Gram stain of endotracheal aspirate shows Gram-negative bacilli, and culture grows lactose-fermenting colonies that are oxidase-negative and produce a metallic sheen on EMB agar. The isolate is resistant to ampicillin and third-generation cephalosporins but sensitive to cefoxitin (a cephamycin). Resistance is inhibited by clavulanic acid in disk synergy testing. The isolate is also resistant to fluoroquinolones, but sensitive to carbapenems and colistin. What is the most likely mechanism of antibiotic resistance in this organism?

Accepted Answer

Extended-spectrum beta-lactamase (ESBL) production

Answer

AmpC beta-lactamase hyperproduction

Answer

Carbapenemase production

Answer

Porin channel mutation with efflux pump overexpression

Hospital Acquired Infections — MCQs

Hospital Acquired Infections — MCQs

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