Kill Curve / Time-Kill Calculator

Calculate log10 reduction in bacterial counts from time-kill studies. Determine bactericidal activity (>3 log10 reduction = 99.9% killing) and killing rate constant k from time-kill data.

🦠 Microbiology📐 log10 reduction = log10(N0) - log10(N)🧬 Biology
Initial count N0 (CFU/mL)
Final count N (CFU/mL)
Please enter valid values.

Formula & Reference

VariableSymbolFormulaUnits
Kill Curve / Time-Kill Calculator—log10 reduction = log10(N0) - log10(N)log10 CFU/mL

Step-by-Step Examples

Example 1
Penicillin Kill Curve

N0=1x10^6 CFU/mL E. coli. After 4hr with 10x MIC: N=500 CFU/mL.

  • log10 reduction = log10(1e6) - log10(500)
  • = 6.0 - 2.70 = 3.30 log10 reduction
  • >=3 log10 = bactericidal (99.95% killed)
  • Penicillin is bactericidal for gram-negatives
✓ 3.3 log10 reduction — BACTERICIDAL
Example 2
Bacteriostatic Antibiotic

N0=1x10^6, after 24hr: N=1.2x10^6 (slight growth).

  • log10 reduction = 6.0 - 6.08 = -0.08
  • Negative: bacteria grew slightly despite drug
  • Confirms bacteriostatic mechanism
  • Check concentration vs MIC
✓ Growth occurred — below MIC or bacteriostatic
Example 3
Disinfectant Efficacy

Hand sanitizer: N0=1x10^8, N=10 after 30 seconds.

  • log10 reduction = 8.0 - 1.0 = 7.0
  • 7-log reduction = kills 99.99999% of bacteria
  • FDA requires >5 log10 for surgical antiseptics
✓ 7 log10 reduction — effective disinfectant

Real-World Applications

🦠
Antibiotic Research
Time-kill curves characterize bactericidal vs bacteriostatic antibiotics.
🏭
Disinfectant Validation
Required by FDA and EPA for sanitizer and disinfectant approval.
🏥
Clinical Microbiology
Combination kill curves identify synergistic antibiotic pairs for resistant infections.
🧪
Sterilization Validation
Log reduction targets validate sterilization cycles in pharmaceutical manufacturing.

Common Mistakes to Avoid

⚠️
Confusing log10 reduction with percent reduction

3 log10 = 99.9% reduction (not 3%). 4 log10 = 99.99% = 1 in 10,000 survive. 6 log10 = 99.9999% = 1 in million. Log scale is critical for interpretation.

⚠️
Sampling only at one time point

Time-kill curves require multiple time points. Some antibiotics kill quickly (beta-lactams); others slowly (aminoglycosides). Single endpoint misses kinetics.

⚠️
Ignoring regrowth

After initial killing, surviving cells may regrow. Examine late time points (24h) for regrowth, which indicates resistance emergence or persister cells.

Frequently Asked Questions

What is a 3-log reduction? ▾
10^3 = 1000-fold reduction. 99.9% of bacteria killed. Standard definition of bactericidal activity in time-kill studies. MBC achieves >=3 log10 reduction at 18-24 hours.
What are persister cells? ▾
A subpopulation (~0.001%) of bacteria in metabolically dormant state. Antibiotic-tolerant but not resistant. Cause antibiotic treatment failure and infection relapse.
What is antibiotic synergy? ▾
Two antibiotics together achieve >2 log10 more killing than the most active single agent. Common synergistic pairs: beta-lactam + aminoglycoside (endocarditis), colistin + rifampin (XDR bacteria).
What is a 5-log reduction standard? ▾
FDA requires 5 log10 reduction for surgical antiseptics, 3 log10 for non-surgical antiseptics, 6 log10 for sterilization of medical devices.
What are CFU/mL? ▾
Colony Forming Units per milliliter. Each colony on an agar plate represents one viable cell (or clump). CFU/mL measures live bacterial density.

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