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LearnYBI11WBI14 · Unit 4: Energy, Environment, Microbiology and ImmunityMicrobiology, immunity and forensicsMicrobial growth and forensics
Microbiology, immunity and forensics · Subtopic 3 of 3

Microbial Growth and Forensics

Microorganisms are cultured on agar or in broth using aseptic technique. A growth curve has a lag phase (enzymes being made), log (exponential) phase (doubling each generation), stationary phase (births = deaths; nutrients limited, waste builds up) and death phase. N=N0×2nN=N_0\times2^n. Counting: serial dilution then plate count (colonies × dilution factor ÷ volume plated); haemocytometer; turbidity (colorimeter). Forensics: time of death is estimated from body temperature (falls ~1.5 °C per hour to surroundings), rigor mortis (starts 2–6 h, lasts ~36 h, as ATP runs out), decomposition by bacteria and enzymes, and forensic entomology (blowfly larvae stages), plus a predictable succession of insects on the body. DNA profiling identifies victims and suspects.

Edexcel YBI11 WBI14≈ 45 minRevision 1 · updated 2026-11-02
Section 1 of 14

Learning objectives

  • Describe culturing and counting microorganisms.
  • Explain the phases of a growth curve.
  • Calculate population size from growth and dilution data.
  • Describe methods to estimate time of death.
Pause and think

Why do bacteria stop growing exponentially in a flask?

Section 2 of 14

Core idea

Start hereFrom one to millions

A single bacterium dividing every 20 minutes could produce millions of cells by teatime.

The big idea

Bacterial populations grow exponentially until resources run out; forensic scientists use biology to estimate time of death.

Estimating time of death

Several clues combined.

Worked examplePlate count

45 colonies from 0.1 cm³ of a 10⁻⁵ dilution.

Goal
Cells per cm³ originally.
Strategy
Colonies ÷ volume × dilution factor.
  1. 450.1=450 cm−3\frac{45}{0.1}=450\ \text{cm}^{-3}
    In the diluted sample.
  2. 450×105=4.5×107450\times10^5=4.5\times10^7
    Undo the dilution.
Answer4.5 × 10⁷ cells cm⁻³.
✓ CheckMuch larger than the colony count ✓.

Summary

  • Growth curve phases.
  • N = N₀ × 2ⁿ.
  • Time of death evidence.
Key words
Generation time
Time for a bacterial population to double.
Forensic entomology
Using insects to estimate time of death.
HistoryForensic entomology. A 13th-century Chinese coroner found a murderer when flies gathered on the one sickle with invisible traces of blood.
Fun factDoubling every 20 minutes. E. coli can divide every 20 minutes; in ideal conditions one cell could give over 2 million in 7 hours.
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