Which AST method is generally less reliable for fastidious organisms unless specialized media or conditions are used?

Which AST method is generally less reliable for fastidious organisms unless specialized media or conditions are used?

• A. Disk diffusion
• B. Broth microdilution
• C. E-test
• D. Agar dilution

Answer: (A)

Disk diffusion relies on a clear, uniform diffusion of the antibiotic through a standard agar and on robust growth of the organism on that same medium. Fastidious organisms often require enriched media and specific growth conditions (nutrients, atmosphere, incubation time) to grow well. When these needs aren’t met, growth around the antibiotic disk can be irregular or weak, and the resulting inhibition zones can be difficult to interpret or not correlate with true susceptibility. Providing the right media and conditions improves zone clarity and reproducibility, but under standard setups this method is generally less reliable for these organisms. Other methods, like broth microdilution or E-test, can be adapted to use the appropriate nutrients and conditions and thus tend to give more dependable results for fastidious bacteria when correctly set up.

Disk diffusion relies on a clear, uniform diffusion of the antibiotic through a standard agar and on robust growth of the organism on that same medium. Fastidious organisms often require enriched media and specific growth conditions (nutrients, atmosphere, incubation time) to grow well. When these needs aren’t met, growth around the antibiotic disk can be irregular or weak, and the resulting inhibition zones can be difficult to interpret or not correlate with true susceptibility. Providing the right media and conditions improves zone clarity and reproducibility, but under standard setups this method is generally less reliable for these organisms. Other methods, like broth microdilution or E-test, can be adapted to use the appropriate nutrients and conditions and thus tend to give more dependable results for fastidious bacteria when correctly set up.