Microbial growth and counting: study guide

Key ideas

Bacteria are counted by diluting a culture and counting colonies, and they grow by doubling. A few formulas turn plate counts into concentrations and growth curves into generation times.

• CFU per mL = colonies counted ÷ (dilution factor × volume plated in mL). Count plates with roughly 30 to 300 colonies.
• In a serial dilution the factors multiply: three tenfold steps give 10⁻³.
• The number of generations is n = log₂(N ÷ N₀), and the generation time is g = elapsed time ÷ n.
• Exponential growth has a growth rate constant k = ln 2 ÷ g, and N = N₀ × 2ⁿ gives the count after n generations.

Key equations

• CFU/mL = colonies ÷ (dilution × volume plated)
• n = log₂(N ÷ N₀)
• g = t ÷ n
• k = ln 2 ÷ g
• total dilution = product of the steps

Worked example

A plate of the 10⁻⁶ dilution, with 0.1 mL spread, has 150 colonies, so the culture holds 150 ÷ (10⁻⁶ × 0.1) = 1.5 × 10⁹ CFU/mL. A culture growing from 1 × 10⁵ to 1.6 × 10⁶ cells in 2 h made log₂(16) = 4 generations, so g = 120 ÷ 4 = 30 min.

Common mistakes

• Forgetting to divide by the volume plated, which is often 0.1 mL rather than 1 mL.
• Using plates with too few or too many colonies, which gives noisy or crowded counts.
• Using natural logarithms for the number of generations without dividing by ln 2.

Practice

Test yourself with practice questions on this topic: Microbial growth and counting