Gecko Med
NotesAll Years (1st-Final)All UniversitiesAntibiotic Sensitivity

Antibiotic Sensitivity Chart for MBBS

Antibiotic sensitivity testing (AST) is a core Microbiology practical topic tested through theory, practicals, and viva. This chart walks through the standard disc diffusion method (Kirby-Bauer method), the technique most commonly taught and demonstrated in MBBS Microbiology practicals for determining whether a bacterial isolate is sensitive or resistant to a given antibiotic. This is a study reference for exam preparation, not a clinical treatment guide. Actual antibiotic choice in practice depends on hospital antibiograms, patient factors, and current CLSI or local guidelines.

SME-reviewed. Updated 2026.
Open GeckoMed for Microbiology notes, PYQs, and viva prep in one place.

Antibiotic Sensitivity Testing (Kirby-Bauer Disc Diffusion Method)

Step

1. Inoculum Preparation

What Happens

A standardised bacterial suspension is prepared, typically matched to a 0.5 McFarland turbidity standard

Step

2. Plating

What Happens

The suspension is spread evenly over the surface of Mueller-Hinton agar

Step

3. Disc Placement

What Happens

Antibiotic-impregnated discs are placed on the inoculated agar surface at fixed distances

Step

4. Incubation

What Happens

The plate is incubated, usually at 37°C for 16 to 18 hours

Step

5. Zone Measurement

What Happens

The diameter of the clear zone of inhibition around each disc is measured in millimetres

Step

6. Interpretation

What Happens

The measured zone diameter is compared against standard interpretive charts (CLSI) to classify the isolate as Sensitive (S), Intermediate (I), or Resistant (R)

Exact zone diameter cut-offs differ for each antibiotic-organism combination and are published in CLSI (Clinical and Laboratory Standards Institute) interpretive tables. Always refer to your standard Microbiology textbook or the current CLSI chart for the specific breakpoint values expected in your exam.


How to Use This Chart for Exams

  • Learn the sequence of the Kirby-Bauer method in order, since practical exams often ask you to describe or perform the procedure step by step
  • Understand why Mueller-Hinton agar is the standard medium, rather than just memorising that it is used
  • Be clear on what a larger versus smaller zone of inhibition indicates about antibiotic susceptibility
  • Revise the difference between Sensitive, Intermediate, and Resistant categories conceptually, then cross-check exact numeric breakpoints against your textbook's CLSI table before your exam

How This Topic Appears in University Papers

  • Short notes such as "Describe the disc diffusion method of antibiotic sensitivity testing"
  • Viva questions asking you to interpret a zone of inhibition on a practical specimen or photograph
  • Practical exam stations where you may be asked to identify discs or interpret a plate
  • Applied questions linking sensitivity testing to choice of empirical antibiotic therapy in a clinical case, particularly in later years

Frequently Asked Questions

Is this chart useful for clinical practice?

No. This chart is compiled as an MBBS study reference for understanding the disc diffusion method taught in Microbiology practicals. Actual antibiotic prescribing in practice should follow hospital antibiograms and current clinical guidelines.

What is the standard method taught in MBBS for antibiotic sensitivity testing?

The Kirby-Bauer disc diffusion method is the most commonly taught technique, using Mueller-Hinton agar and standardised antibiotic discs.

Where can I find the exact zone diameter values for Sensitive, Intermediate, and Resistant categories?

Exact breakpoint values are published in CLSI interpretive tables and vary by antibiotic and organism. Refer to your prescribed Microbiology textbook for the specific values expected in your university exam.

Try Gecko Med

Practice smarter with the app behind these resources.

Revise high-yield topics, solve previous year questions, and keep your MBBS prep moving in one place.