MIC (Minimum Inhibitory Concentration) Calculators

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The minimum inhibitory concentration (MIC) is the lowest concentration of an antimicrobial agent that prevents visible bacterial growth after an 18–24-hour incubation. MIC is the primary quantitative measure of antibiotic potency and bacterial susceptibility. It is measured by broth microdilution: two-fold serial dilutions of the antibiotic in liquid medium are inoculated with a standard bacterial inoculum (~5 × 10⁵ CFU/mL), incubated at 37°C for 18–24 hours, and the lowest clear (non-turbid) well is the MIC. MIC breakpoints (defined by CLSI or EUCAST) classify organisms as Susceptible (S), Intermediate (I), or Resistant (R) to guide clinical treatment.

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MIC Measurement: Broth Microdilution

CLSI and EUCAST standard method: Prepare 2-fold serial dilutions of antibiotic in cation-adjusted Mueller-Hinton broth (CAMHB): e.g., 128, 64, 32, 16, 8, 4, 2, 1, 0.5 μg/mL + growth control (no antibiotic) + sterility control. Inoculate each well with ~5 × 10⁵ CFU/mL (0.5 McFarland standard). Incubate at 35 ± 2°C for 18–24 h. Read: turbid = growth; clear = no growth. MIC = lowest concentration in clear well.

Breakpoints and S/I/R Classification

S (Susceptible): MIC ≤ susceptibility breakpoint → standard dosing expected to achieve therapeutic success. I (Intermediate): MIC between S and R breakpoints → may succeed with higher dose or at infection site with drug concentration. R (Resistant): MIC ≥ resistance breakpoint → standard treatment likely to fail. Example: E. coli MIC for ampicillin: ≤ 8 μg/mL = S; 16 μg/mL = I; ≥ 32 μg/mL = R.

MIC₅₀ and MIC₉₀

Population pharmacodynamics: MIC₅₀ = MIC inhibiting 50% of isolates; MIC₉₀ = MIC inhibiting 90% of isolates. Used to characterize drug potency against a pathogen species.

Glossary

MIC (Minimum Inhibitory Concentration)
Lowest antibiotic concentration preventing visible bacterial growth in vitro (18–24 h); measured by broth microdilution; classifies pathogens as Susceptible (S), Intermediate (I), or Resistant (R).
S/I/R Breakpoints
CLSI/EUCAST-defined MIC thresholds classifying organisms as Susceptible, Intermediate, or Resistant; drug-species-infection site specific; guide clinical treatment decisions.
PK/PD Index
The pharmacokinetic/pharmacodynamic parameter predicting antibiotic efficacy; T > MIC for time-dependent drugs (β-lactams); AUC/MIC or Cmax/MIC for concentration-dependent drugs (aminoglycosides, fluoroquinolones).

Frequently Asked Questions

MIC (minimum inhibitory concentration) = the lowest concentration of an antibiotic that completely inhibits visible bacterial growth in vitro after a standardized incubation period (18–24 hours at 37°C). Units: μg/mL (equivalent to mg/L). Why MIC matters: predicts whether a standard antibiotic dose will achieve drug concentrations at the infection site that exceed the MIC. If drug concentration at the site > MIC: likely therapeutic success. If drug concentration < MIC: treatment failure → risk of selecting resistant mutants. MIC is the central parameter linking in vitro susceptibility testing to clinical treatment decisions.

Broth microdilution (CLSI/EUCAST standard): (1) Prepare 2-fold dilution series of antibiotic in Mueller-Hinton broth (e.g., 128→64→32→16→8→4→2→1→0.5→0.25 μg/mL) in a 96-well plate. (2) Inoculate each well with standard bacterial suspension: 5 × 10⁵ CFU/mL (0.5 McFarland turbidity = ~1.5 × 10⁸ CFU/mL; dilute 1:300). Include: growth control (no antibiotic); sterility control (no bacteria). (3) Incubate 35–37°C for 18–24 h (aerobic; or 24–48 h for slow growers). (4) Read visually (or spectrophotometrically at 600 nm): turbid well = growth; clear well = no growth. MIC = lowest concentration where well is clear (no visible growth).

MIC breakpoints are defined by clinical laboratory standards organizations: CLSI (Clinical and Laboratory Standards Institute, USA); EUCAST (European Committee on Antimicrobial Susceptibility Testing, Europe). Breakpoints divide MIC distributions into: Susceptible (S): MIC ≤ susceptible breakpoint. Infection caused by this organism should respond to standard doses of the agent. Intermediate (I): MIC between S and R breakpoints. May respond to higher doses or at sites with high drug concentration. Some classify as 'susceptible — increased exposure.' Resistant (R): MIC ≥ resistant breakpoint. Standard treatment unlikely to be effective. Breakpoints are drug-species-infection site specific — the ampicillin breakpoint for urinary E. coli differs from that for E. coli meningitis.

MIC alone does not predict efficacy — what matters is the relationship between drug concentration vs. MIC over time: PK/PD parameters (link pharmacokinetics to pharmacodynamics): Time-dependent killing (β-lactams, macrolides): efficacy ∝ time above MIC (T > MIC). For penicillins: T > MIC > 40–50% of dosing interval needed for bacteriostasis; > 70% for bactericidal effect. Concentration-dependent killing (aminoglycosides, fluoroquinolones): efficacy ∝ Cmax/MIC ratio or AUC/MIC (AUIC). Once-daily aminoglycoside dosing exploits Cmax/MIC > 10 for killing + post-antibiotic effect. Exposure-response breakpoints: set so that a defined dose achieves PK/PD target vs. pathogens at or below the breakpoint. Mutant prevention concentration (MPC): the concentration above which resistant mutants cannot be selected; MIC < drug concentration < MPC = 'mutant selection window.'