Experiment 06Antimicrobial Pharmacology • B.Sc. / M.Sc.
Antimicrobial Susceptibility Testing by the Kirby-Bauer Disk Diffusion Assay
🎯 Aim: To perform standardized antibiotic susceptibility testing on bacterial pathogens (E. coli and S. aureus) using the Kirby-Bauer disk diffusion protocol on Mueller-Hinton agar, measure clear zones of inhibition with calipers, and interpret Susceptible (S), Intermediate (I), and Resistant (R) clinical categories according to CLSI guidelines.
🔬 Theoretical Principle & Fickian Diffusion Physics
The Kirby-Bauer agar disk diffusion assay (Bauer et al., 1966) evaluates the susceptibility of a bacterial isolate to specific therapeutic concentrations of antimicrobial agents:
- Radial Molecular Diffusion: When a filter paper disk impregnated with a defined quantity of antibiotic is placed onto Mueller-Hinton agar inoculated with a standardized bacterial lawn, the drug immediately absorbs water from the agar and diffuses radially outward through the gel matrix down a concentration gradient governed by Fick’s laws of diffusion.
- Critical Concentration & Zone of Inhibition: Close to the disk, antibiotic concentration exceeds the Minimum Inhibitory Concentration (MIC) of the organism, completely suppressing bacterial proliferation and creating a circular, clear zone of no growth. As diffusion proceeds outward, concentration drops below the MIC, allowing normal confluent lawn growth. The diameter of the clear zone is inversely proportional to the logarithm of the MIC.
- CLSI Standardization Rules (M02 Standards):
- Inoculum Density: Standardized to $0.5 text{McFarland turbidity}$ ($1.5 times 10^8,text{CFU/mL}$).
- Medium: Mueller-Hinton Agar (MHA), which contains low thymine/thymidine (preventing sulfonamide inhibition) and low concentrations of divalent cations ($text{Ca}^{2+}, text{Mg}^{2+}$).
- Agar Depth: Precisely $4.0,text{mm} (pm 0.5,text{mm})$. Shallower agar yields falsely enlarged zones; deeper agar produces falsely small zones due to vertical diffusion dissipation.
Figure 6.1: Kirby-Bauer Antibiotic Disk Diffusion Plate & Zone of Inhibition Sizing
💬 Viva-Voce Questions & Model Answers
Q1: Can you assume that an antibiotic with a 30 mm zone of inhibition is clinically more effective than one with a 20 mm zone?
No! Zone size depends not only on antimicrobial potency but also on the drug’s physical molecular weight, solubility, and diffusion coefficient through agar. Polymyxin (colistin) is a bulky polypeptide that diffuses slowly, producing small zones even against highly sensitive strains, whereas small molecules like ciprofloxacin diffuse rapidly. Results must be interpreted strictly against CLSI reference tables.
Q2: Why does high agar depth (>4 mm) cause false-resistant results?
If the agar layer is deeper than 4 mm, the antibiotic diffuses downwards into the depth of the gel rather than laterally across the surface, reducing surface antibiotic concentration and resulting in artificially smaller zones of inhibition (false resistance).
Q3: What causes isolated colonies to grow inside the clear zone of inhibition?
Colonies growing inside the zone represent spontaneous resistant mutant sub-populations or a mixed bacterial culture containing a resistant contaminant strain. In clinical reporting, the isolate is classified as Resistant regardless of overall zone diameter.
