16
µg/mL
2
16
µg/mL
2
The Minimum Bactericidal Concentration (MBC) Calculator determines the MBC from a two-fold serial dilution series and computes the MBC/MIC ratio. The MBC is the lowest concentration of an antibiotic that kills 99.9% of the initial inoculum, determined by subculturing from MIC wells onto antibiotic-free plates. While MIC measures growth inhibition, MBC measures actual killing.
The MBC/MIC ratio is clinically important: a ratio of 4 or less indicates bactericidal activity, while higher ratios suggest the antibiotic is merely bacteriostatic against that organism. This distinction guides treatment of serious infections like endocarditis and meningitis where bactericidal agents are preferred.
In a standard two-fold serial dilution:
Well position 1 is the highest concentration. Each subsequent well is a two-fold dilution. After MIC determination, aliquots from wells with no visible growth are plated on antibiotic-free agar. The lowest concentration showing greater than or equal to 99.9% killing is the MBC.
Inputs
Results
MBC equals MIC (ratio = 1). The antibiotic is bactericidal at the inhibitory concentration, typical of beta-lactams and fluoroquinolones against susceptible organisms.
Inputs
Results
MBC/MIC ratio of 16 indicates the antibiotic is bacteriostatic. It inhibits growth at 4 µg/mL but requires 64 µg/mL to kill. This organism is considered tolerant.
MIC (Minimum Inhibitory Concentration) is the lowest concentration that prevents visible bacterial growth. MBC (Minimum Bactericidal Concentration) is the lowest concentration that kills 99.9% of bacteria. MIC tests only inhibition; bacteria may resume growth if the antibiotic is removed. MBC confirms actual killing. MBC is always equal to or greater than MIC.
An MBC/MIC ratio of 4 or less is generally considered bactericidal. Ratios above 4 suggest bacteriostatic activity or antibiotic tolerance. Some organisms, such as enterococci, are inherently tolerant to certain antibiotics (e.g., vancomycin), showing high MBC/MIC ratios despite susceptible MIC values.
MBC testing is most important for serious infections requiring bactericidal therapy: endocarditis, meningitis, osteomyelitis, and infections in immunocompromised patients. In these cases, bacteriostatic agents may fail because the host immune system cannot clear inhibited but viable bacteria. The choice between bactericidal and bacteriostatic agents can affect patient outcomes.
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