An E-test strip for cefotaxime placed on an agar plate containing an E. coli isolate shows an elliptical inhibition zone intersecting the strip at 0.5 µg/mL. If the EUCAST/CLSI breakpoint for susceptibility of cefotaxime for E. coli is ≤1 µg/mL, and resistant if ≥4 µg/mL, how would this isolate be categorized?
- A Susceptible, increased exposure (I) — intermediate
- B Susceptible (S) ✓
- C Resistant (R)
- D Indeterminate — repeat the test
Correct answer: B. Susceptible (S)
Explanation
The E-test (epsilometer test) directly reads the MIC as the value where the elliptical zone intersects the calibrated strip. An MIC of 0.5 µg/mL is at or below the susceptibility breakpoint of ≤1 µg/mL, so the isolate is classified as Susceptible (S). The intermediate category (now called 'susceptible, increased exposure' by EUCAST) would apply if MIC fell between >1 and <4 µg/mL. MIC ≥4 µg/mL would be resistant. E-test combines the accuracy of broth dilution MIC with the ease of disc diffusion.
Reference: Ananthanarayan & Paniker's Textbook of Microbiology, 11th ed.
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