Microbial Growth Calculators

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Microbial growth refers to the increase in the number of microbial cells in a population rather than the increase in cell size. Bacteria reproduce by binary fission — each cell divides into two daughter cells. Under ideal conditions, populations can double in as little as 20 minutes (E. coli). The growth curve of a batch culture passes through four distinct phases — lag, log (exponential), stationary, and death — each with characteristic microbial activity. Understanding microbial growth is fundamental in microbiology, food safety, biotechnology, and infection biology.

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Binary Fission and Generation Time

Bacteria reproduce by binary fission: one cell → two cells. Generation time (g) = doubling time.
N(t) = N₀ × 2^(t/g)
g = t × log(2) / log(N/N₀) = 0.301 × t / log(N/N₀)

Four Phases of the Bacterial Growth Curve

1. Lag Phase

No increase in cell number. Cells synthesize enzymes, repair DNA, increase ribosome number to prepare for growth. Duration depends on inoculum health and medium change.

2. Exponential (Log) Phase

Cells dividing at maximum rate. Constant generation time. OD₆₀₀ and CFU increase exponentially. Most uniform phase — ideal for experiments and industrial fermentation.

3. Stationary Phase

Growth rate = death rate — population constant. Caused by nutrient depletion or waste accumulation. Secondary metabolites (antibiotics, toxins) often produced here.

4. Death (Decline) Phase

Death rate exceeds growth — viable count decreases. Energy reserves exhausted; cells lyse. Persister cells survive.

Measuring Microbial Growth

  • OD₆₀₀: Turbidimetric — fast, non-destructive, includes dead cells
  • Viable count (CFU/mL): Plate count — only live cells, 24–48 hr incubation
  • Direct microscopic count: Hemocytometer — includes dead cells
  • ATP bioluminescence: Rapid viable cell proxy

Glossary

Generation Time (g)
The time for a bacterial population to double: g = 0.693/μ = 0.301 × t / log(N/N₀). E. coli: ~20 min. Varies with species, temperature, nutrients, and growth phase. Also called doubling time.
Exponential (Log) Phase
The bacterial growth phase where cells divide at maximum rate for given conditions. Population doubles every g minutes. Cells are biochemically uniform — ideal for experiments and industrial fermentation.
Stationary Phase
The growth phase where growth rate equals death rate, maintaining a constant population. Caused by nutrient depletion or waste accumulation. Secondary metabolites (antibiotics, pigments) often produced during this phase.

Frequently Asked Questions

Lag phase: no cell division; cells adapt, synthesize enzymes, repair DNA. Exponential (log) phase: cells divide at maximum rate; population doubles every g minutes; OD and CFU increase exponentially. Stationary phase: growth rate = death rate; population constant; nutrient depletion; secondary metabolites produced. Death phase: death rate > growth rate; viable count declines; cells lyse from energy depletion.

g = 0.301 × t / (log N − log N₀) = 0.301 × t / log(N/N₀). Example: population goes from 10⁵ to 10⁸ CFU/mL in 3 hours: g = 0.301 × 3 / (8−5) = 0.903/3 = 0.301 hr = 18 minutes per doubling. Or from OD during exponential phase: plot ln(OD) vs. time; slope = μ (specific growth rate); g = ln(2)/μ = 0.693/μ.

Temperature (each species has optimal, minimum, and maximum; Q10 ≈ 2 below optimum); pH (most bacteria: 6.5–7.5 optimal; acidophiles: 2–5; alkaliphiles: 8–11); water activity (Aw; most bacteria require Aw > 0.91); oxygen (aerobes, anaerobes, facultative, microaerophilic); nutrient availability (carbon, nitrogen, phosphorus, trace elements); osmotic pressure. The most growth-limiting factor at any time (Liebig's Law) determines actual growth rate.

OD₆₀₀ (optical density at 600 nm) measures turbidity — light scattered by bacterial cells. It is fast, non-destructive, and gives continuous growth curves. OD₆₀₀ is linearly proportional to cell density in the range 0.05–0.8. Limitations: includes dead cells; different organisms have different OD-to-cell ratios; not accurate above OD₆₀₀ ≈ 1 (non-linear). For accurate viable counts, plate CFU/mL assays are required.