Colony Forming Units Calculators

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Colony-forming units (CFU) measure the number of viable, culturable microorganisms in a sample by counting the discrete colonies that form on agar plates after incubation. Each colony is assumed to originate from a single cell (or clump). CFU/mL is calculated from the colony count, dilution factor, and volume plated: CFU/mL = colonies / (dilution × volume plated in mL). CFU is the standard measure of bacterial concentration in clinical microbiology, food safety, pharmaceutical QC, and research. Plates with 30–300 colonies give statistically reliable counts.

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CFU/mL Calculation

CFU/mL = colonies counted / (dilution factor × volume plated in mL)

Example: 85 colonies on a 10⁻⁵ dilution plate, 0.1 mL plated:
CFU/mL = 85 / (10⁻⁵ × 0.1) = 85 / 10⁻⁶ = 8.5 × 10⁷ CFU/mL.

Serial Dilution Scheme

Transfer 1 mL into 9 mL diluent for 1:10 dilutions. Series: 10⁻¹, 10⁻², 10⁻³, 10⁻⁴, 10⁻⁵. Plate 0.1 mL from each dilution. Use the plate with 30–300 colonies for the calculation.

30–300 Colony Rule

Below 30 colonies: statistically unreliable (high relative error); report as TFTC (Too Few To Count). Above 300: colonies crowd and overlap → undercounting; report as TNTC (Too Numerous To Count). Use only plates within 30–300 for CFU calculation.

Spread Plate vs. Pour Plate

Spread plate: 0.1 mL on pre-hardened agar surface; colonies on surface only; 30–300 target. Pour plate: 1.0 mL mixed with molten agar; subsurface colonies smaller; can use larger volume. Both methods give CFU/mL by the same formula — adjust volume plated accordingly.

Viability vs. Total Count

CFU measures only viable, culturable cells. Total count (DAPI staining, flow cytometry) measures all cells including dead and viable but non-culturable (VBNC) cells. CFU typically undercounts total cells.

Glossary

Colony-Forming Unit (CFU)
A single viable microorganism capable of forming a visible colony; CFU/mL = colonies / (dilution × volume); measures only viable, culturable cells; 30–300 colonies per plate required for reliable counts.
TNTC / TFTC
Too Numerous To Count (>300 colonies) / Too Few To Count (<30 colonies); plates outside the 30–300 range give unreliable CFU/mL estimates; use nearest valid dilution.
Serial Dilution
Sequential 1:10 dilution series (1 mL into 9 mL diluent) to reduce microbial concentration to a countable range; each step = 10-fold reduction; thorough mixing is critical at each step.

Frequently Asked Questions

A CFU is a single viable microorganism (or clump) capable of dividing and forming a visible colony on an agar plate. CFU is measured by plating serial dilutions of a sample on agar, incubating, and counting colonies: CFU/mL = colonies / (dilution factor × volume plated mL). Only plates with 30–300 colonies give statistically reliable counts. CFU measures viable, culturable cells — it underestimates total cell count because dead cells and viable but non-culturable (VBNC) cells do not form colonies.

Serial dilutions step-wise reduce concentration to a countable range. Standard 1:10 dilution: transfer 1 mL into 9 mL diluent, mix well → 10⁻¹; repeat for 10⁻², 10⁻³, etc. Mix each tube thoroughly before transferring. Plate 0.1 mL from at least 3 consecutive dilutions on duplicate plates. After incubation, count plates with 30–300 colonies. Example: 164 colonies on 10⁻⁴ dilution plate (0.1 mL plated): CFU/mL = 164/(10⁻⁴ × 0.1) = 1.64 × 10⁷. Consistent mixing at each dilution step is the most critical factor for accurate results.

Below 30: Poisson statistics mean each individual colony represents a large percentage of the total — one missed or extra colony introduces >3% error; counts are unreliable. Above 300: physical crowding of colonies causes merging and undercounting of actual cells; nutrient depletion affects colony appearance. The 30–300 range (approximately 10–300 colonies per plate; the exact range varies slightly between methods) balances statistical precision with physical countability. When multiple dilutions produce plates in range, average the CFU/mL from each. If no plate falls in range, use the closest plate and note the deviation.

CFU/mL: counts viable, culturable bacteria by plating; requires 18–24 hour incubation; most accurate for measuring infectious dose and growth effects; labor-intensive. OD₆₀₀ (optical density at 600 nm): measures light scattering by all cells (viable + dead); instantaneous; non-destructive; allows continuous monitoring; requires instrument calibration. OD₆₀₀ ≈ 1.0 corresponds to ~8 × 10⁸ E. coli/mL (varies by strain and instrument). Both are needed for complete characterization: OD₆₀₀ for growth kinetics; CFU for viability during antibiotic treatment, UV exposure, or other killing treatments.