Minimum Inhibitory Concentration Calculators

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The minimum inhibitory concentration (MIC) is the lowest concentration of an antimicrobial agent that visibly inhibits the growth of a microorganism after overnight incubation under defined conditions. MIC is the gold-standard measure of antimicrobial susceptibility and is used to guide treatment decisions, set susceptibility breakpoints, and compare antimicrobial potency. The standard method is broth microdilution in 96-well plates using a twofold dilution series (e.g., 128, 64, 32, 16, 8, 4, 2, 1, 0.5, 0.25 μg/mL) defined by CLSI M07 and EUCAST standards.

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MIC Determination: Broth Microdilution

Protocol: prepare twofold serial dilutions of antibiotic in Mueller-Hinton broth (e.g., 0.25–128 μg/mL); add standardized bacterial inoculum (5 × 10⁵ CFU/mL, per 0.5 McFarland); incubate 35°C ± 2°C for 16–20 hours; read MIC as the lowest clear (no visible turbidity) well. Must include: growth control (no antibiotic — should be turbid), sterility control (no bacteria — should be clear).

MIC Breakpoints

MIC values are compared to breakpoints to classify as S/I/R: Susceptible (S): MIC ≤ susceptible breakpoint — infection likely treatable at standard dosing. Susceptible, dose-dependent (SDD): may be treated with higher/more frequent dosing. Intermediate (I): narrow safety margin; higher dosing needed. Resistant (R): MIC above breakpoint — treatment likely to fail. CLSI and EUCAST publish breakpoints by species and drug combination; updated annually. Example: E. coli to ciprofloxacin (EUCAST 2024): S ≤ 0.25 mg/L; R > 0.5 mg/L.

MBC (Minimum Bactericidal Concentration)

MBC = lowest concentration killing ≥ 99.9% of the original inoculum (≥ 3-log reduction). MBC is determined by subculturing MIC wells onto drug-free agar. MBC:MIC ratio: ≤ 4 = bactericidal; > 4 = bacteriostatic (drug inhibits but does not kill at 4× MIC). Bactericidal activity important for endocarditis, meningitis, immunocompromised patients.

Glossary

MIC (Minimum Inhibitory Concentration)
The lowest antimicrobial concentration that visibly inhibits bacterial growth after overnight incubation; measured in μg/mL; the gold standard for antimicrobial susceptibility testing.
MBC (Minimum Bactericidal Concentration)
The lowest concentration killing ≥ 99.9% of the original inoculum; MBC:MIC ≤ 4 = bactericidal; > 4 = bacteriostatic; clinically important for endocarditis and meningitis treatment.
Susceptibility Breakpoint
MIC threshold published by CLSI or EUCAST for each drug-organism combination; below breakpoint = Susceptible (S); above = Resistant (R); guides antibiotic treatment decisions.

Frequently Asked Questions

MIC (minimum inhibitory concentration) is the lowest antibiotic concentration that prevents visible bacterial growth after overnight incubation. Measured by broth microdilution: prepare twofold serial dilutions (128, 64, 32 ... 0.25 μg/mL) in Mueller-Hinton broth in 96-well plates; inoculate with 5 × 10⁵ CFU/mL; incubate 35°C, 16–20 hours; MIC = lowest clear well. Always include growth control (no drug → should be turbid) and sterility control (no bacteria → should be clear). MIC is expressed in μg/mL (= mg/L) and interpreted against CLSI or EUCAST species-specific breakpoints.

MIC: the lowest concentration inhibiting visible growth — bacteriostatic endpoint. Growth resumes if antibiotic is removed. MBC: the lowest concentration killing ≥ 99.9% of the original inoculum — bactericidal endpoint. To determine MBC: after reading the MIC, subculture the MIC and all higher-concentration wells onto drug-free agar; incubate 24h; lowest concentration with ≤ 0.1% survival = MBC. MBC:MIC ratio ≤ 4 = bactericidal agent; > 4 = bacteriostatic. Bactericidal activity is clinically important for: endocarditis (vegetations impair immune access); meningitis; neutropenic patients (no immune backup). Beta-lactams and fluoroquinolones tend to be bactericidal; tetracyclines, macrolides bacteriostatic.

Breakpoints are concentration thresholds classifying bacteria as Susceptible (S), Intermediate (I), or Resistant (R) to a specific antibiotic. They integrate: pharmacokinetic/pharmacodynamic (PK/PD) targets (e.g., time above MIC for beta-lactams); MIC distributions in wild-type and resistant populations; clinical outcome data. CLSI (US) and EUCAST (European) publish breakpoints annually in M100 and the EUCAST website, respectively. Example EUCAST breakpoints for Staphylococcus aureus to oxacillin: S ≤ 2 mg/L; R > 2 mg/L. Breakpoints differ between organisms for the same drug — always use the correct species-drug combination.

The E-test is a strip-based MIC method combining features of disc diffusion and MIC determination. An antibiotic gradient strip (covering 0.016–256 μg/mL in twofold steps) is placed on a Mueller-Hinton agar plate inoculated to 0.5 McFarland turbidity; after overnight incubation, an ellipse of inhibition forms; the MIC is read where the edge of the inhibition ellipse intersects the scale on the strip. E-test is faster and simpler than broth microdilution; useful for slow-growing organisms; results correlate well with broth microdilution. Limitations: higher cost per test; gradient may be affected by agar composition; less precise for organisms with trailing endpoints.