Antibiotic Susceptibility Calculators

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Antibiotic susceptibility testing (AST) determines whether a bacterial isolate can be inhibited or killed by an antibiotic at concentrations achievable in a patient — guiding appropriate antimicrobial therapy. The two primary methods are broth microdilution (determines minimum inhibitory concentration, MIC) and disk diffusion (Kirby-Bauer test, which gives qualitative S/I/R classification). Results are interpreted against species- and drug-specific breakpoints published annually by CLSI (Clinical and Laboratory Standards Institute) in the US and EUCAST (European Committee on Antimicrobial Susceptibility Testing) in Europe.

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Disk Diffusion (Kirby-Bauer) Test

Protocol: swab Mueller-Hinton agar plate with bacterial suspension (0.5 McFarland turbidity); place antibiotic-impregnated discs; incubate 35°C, 16–18 hours; measure zone of inhibition diameter (mm). Interpret against CLSI or EUCAST zone diameter breakpoints for each drug-organism combination. Result: S (susceptible), SDD (susceptible dose-dependent), I (intermediate), or R (resistant). Automated: VITEK 2, MicroScan — automated card-based AST systems widely used in clinical labs.

MIC vs. Disk Diffusion

MIC (from broth microdilution): quantitative; provides exact drug concentration; used for reporting and PK/PD dosing calculations; slower to set up manually but automated systems are fast. Disk diffusion: qualitative (S/I/R only); simple, cheap; widely available; zone diameter correlates with MIC via regression. For most clinical decisions, disk diffusion is sufficient; MIC values are needed for: serious infections (endocarditis, meningitis, bacteremia); PK/PD optimization (calculating time above MIC); antimicrobial stewardship.

Resistance Mechanisms

  • Beta-lactamases: hydrolyze penicillins, cephalosporins; ESBL, AmpC, KPC (carbapenemase)
  • MRSA: altered PBP2a (mecA gene) with low affinity for all beta-lactams
  • Efflux pumps: pump antibiotics out of cells
  • Impermeability: loss of outer membrane porins (Gram-negative)
  • Target modification: ribosomal methylation; DNA gyrase mutation (fluoroquinolone resistance)

Glossary

Antibiotic Susceptibility Testing (AST)
Laboratory methods determining whether a bacterial isolate can be inhibited by an antibiotic at achievable concentrations; methods: disk diffusion (Kirby-Bauer) and broth microdilution (MIC); classified as S/I/R.
Breakpoint
An MIC threshold (or zone diameter threshold for disk diffusion) published by CLSI or EUCAST; separates susceptible from resistant; specific for each drug-organism combination; updated annually.
MRSA
Methicillin-resistant Staphylococcus aureus; resistance due to mecA gene encoding PBP2a with low beta-lactam affinity; resistant to all beta-lactams; treated with vancomycin, daptomycin, or linezolid.

Frequently Asked Questions

Antibiotic susceptibility testing (AST) determines whether an antibiotic can inhibit bacterial growth at concentrations achievable in a patient, to guide appropriate therapy. Without AST: empirical treatment often fails when resistant organisms are involved; overuse of broad-spectrum antibiotics drives resistance. Two main methods: Disk diffusion (Kirby-Bauer): antibiotic disc placed on seeded agar plate; zone of inhibition measured after incubation; compared to CLSI/EUCAST breakpoints → S/I/R classification. Broth microdilution: serial antibiotic dilutions in broth; lowest clear well = MIC; compared to MIC breakpoints → S/I/R. Clinical use: serious infections (bloodstream infections, meningitis, endocarditis, pneumonia) require AST-guided targeted therapy.

Susceptible (S): the isolate's MIC is at or below the susceptible breakpoint; the infection is likely treatable at the standard dosing regimen for the drug and infection type; high probability of clinical success. Susceptible, dose-dependent (SDD): clinical success likely when maximum approved dosing regimen is used; requires optimized PK/PD target attainment (e.g., extended infusions for time-dependent antibiotics). Intermediate (I): previously meant 'possibly susceptible with higher doses'; in EUCAST 2019+: Intermediate = therapeutic window; consider for treatment when high exposures are achievable (tissue concentration > serum) or when higher doses are used. Resistant (R): MIC above the susceptible breakpoint; treatment likely to fail at any approved dosing; do not use. Interpretation always requires knowing the drug, the organism, the infection site, and the applicable breakpoint standard (CLSI vs. EUCAST breakpoints differ for some drugs).

Resistance mechanisms by category: Enzymatic inactivation: beta-lactamases cleave the beta-lactam ring (penicillinase, ESBL, AmpC, carbapenemase/KPC). Acetyltransferases, phosphotransferases — inactivate aminoglycosides. Chloramphenicol acetyltransferases (CAT) — inactivate chloramphenicol. Target modification: altered penicillin-binding proteins (PBP2a in MRSA encoded by mecA gene) → bind beta-lactams weakly. Ribosomal methylation (erm genes) → resistance to macrolides, lincosamides, streptogramins (MLSb). DNA gyrase/topoisomerase mutations → fluoroquinolone resistance. Efflux pumps: active export of antibiotics from the cell; examples: MexAB-OprM in Pseudomonas (multi-drug efflux); AcrAB-TolC in E. coli. Reduced permeability: loss of outer membrane porins (Gram-negative) → reduced antibiotic entry; OmpF/OmpC loss in Enterobacteriaceae.

The E-test is a strip-based method combining disk diffusion and MIC determination: a plastic strip with an antibiotic concentration gradient (covering several log₂ dilutions, e.g., 0.016–256 μg/mL) is placed on a bacterial lawn on Mueller-Hinton agar. After overnight incubation, an elliptical inhibition zone forms around the strip. The MIC is read where the edge of the inhibition ellipse intersects the concentration scale printed on the strip. Advantages over disk diffusion: provides an actual MIC value rather than just S/I/R; useful for organisms with specialized growth requirements (anaerobes, Streptococcus, Haemophilus, Neisseria) or when precise MIC is needed without broth microdilution. Limitations: more expensive per test than disk diffusion; some drug-organism combinations show trailing endpoints or skipped dilutions that complicate interpretation; E-test MIC may differ slightly from broth microdilution MIC.