Antimicrobial susceptibility testing — clinical microbiology: Which laboratory methods are used to determine how susceptible a bacterial isolate is to antibiotics and other chemotherapeutic agents in routine practice?

Introduction / Context:
Choosing an effective antibiotic depends on demonstrating that a patient’s pathogen is inhibited or killed by achievable drug concentrations. Microbiology laboratories therefore perform antimicrobial susceptibility testing (AST) to translate pharmacology into actionable reports for clinicians.

Given Data / Assumptions:

• We want standardized, reproducible methods used worldwide.
• Results are typically interpreted as susceptible, intermediate, or resistant using clinical breakpoints.
• Two classic approaches are listed: dilution and disk diffusion.

Concept / Approach:
Two complementary gold-standard methods dominate AST. Dilution methods (broth or agar) quantify the minimum inhibitory concentration (MIC), the lowest drug concentration that prevents visible growth. Disk diffusion (Kirby–Bauer) provides qualitative zone diameters around antibiotic-impregnated disks; zone sizes are correlated to breakpoints. Both are standardized by bodies such as CLSI or EUCAST and are appropriate for routine diagnostics, quality control, and surveillance.

Step-by-Step Solution:
Identify the purpose of AST: predict clinical efficacy by measuring growth inhibition in vitro.Match each listed method to standard AST: dilution → MIC; disk diffusion → zone diameters.Recognize both are validated and widely used; therefore the best choice includes both.Select option ‘‘both (a) and (b)’’ as correct.

Verification / Alternative check:
Automated systems (e.g., Vitek, Microscan) are essentially MIC dilution platforms. E-tests provide a gradient diffusion hybrid that also yields an MIC, reinforcing that dilution principles remain central.

Why Other Options Are Wrong:

• Tube dilution alone: Correct method but incomplete; disk diffusion is also used.
• Paper disk plate alone: Also valid but incomplete without acknowledging dilution.
• None of these / Gram stain only: Gram stain does not measure susceptibility; ‘‘none’’ ignores standard practice.

Common Pitfalls:
Assuming zone sizes equal MIC numerically; they correlate but are not the same. Misinterpreting ‘‘intermediate’’ as treatment failure instead of considering site/pharmacokinetics.

Final Answer:
both (a) and (b).