Urine culture from an indwelling catheter shows 50,000 CFU/mL of E. coli with resistance to cephalosporins and sensitivity to ciprofloxacin. This resistance pattern indicates which mechanism?

• A. Extended-spectrum beta-lactamase (ESBL) resistance
• B. Methicillin-resistant resistance
• C. Carbapenemase resistance
• D. AmpC beta-lactamase resistance

Answer: (A)

Extended-spectrum beta-lactamase production. ESBLs are enzymes that inactivate extended-spectrum cephalosporins (such as cefotaxime, ceftriaxone, and ceftazidime), leading to resistance to these drugs in organisms like E. coli. They are often carried on plasmids, so this resistance pattern can spread between bacteria. Importantly, many ESBL producers remain susceptible to fluoroquinolones like ciprofloxacin, depending on the accompanying resistance genes. If a organism were a carbapenemase producer, you’d expect reduced or resistant activity to carbapenems as well, which isn’t described here. AmpC beta-lactamases can also confer cephalosporin resistance but typically show a different pattern (often including resistance to cefoxitin and less predictable fluoroquinolone susceptibility). So the combination of cephalosporin resistance with preserved ciprofloxacin susceptibility most closely fits ESBL-mediated resistance.