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When you grow bacteria in a flask, how does their number change over time, and what do they need to grow?

Describe the phases of microbial population growth and the conditions microorganisms require to grow

A focused answer to the O-Level outcome on microbial growth. The four phases of a growth curve, the conditions microorganisms need, and how to estimate population size by counting.

Generated by Claude Opus 4.89 min answer

Reviewed by: AI editorial process; not yet individually human-reviewed

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  1. What this dot point is asking
  2. The answer
  3. Examples in context
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What this dot point is asking

This outcome asks you to describe how a population of microorganisms grows over time and the conditions they need. The classic picture is the growth curve with its four phases, and the practical point is that controlling conditions such as nutrients, temperature, pH and oxygen is what keeps a culture growing.

The answer

The four phases of growth

When bacteria are grown in a closed flask of nutrient broth, the number of cells follows a characteristic curve with four phases:

  • Lag phase. The cells adjust to the new conditions, take up nutrients and make enzymes. The number stays roughly constant; they are active but not yet dividing fast.
  • Log (exponential) phase. Nutrients are plentiful and waste is low, so the cells divide as fast as possible. The number roughly doubles at regular intervals and rises steeply.
  • Stationary phase. Nutrients run low and waste builds up, so the rate of division equals the rate of death. The number stays roughly constant.
  • Death (decline) phase. Nutrients are exhausted and toxic waste has accumulated, so cells die faster than new ones form, and the number falls.

The conditions microorganisms need

For a culture to grow, several conditions must be right:

  • Nutrients. A food source such as sugars, plus nitrogen and minerals, to build new cells.
  • Temperature. Warm enough for enzymes to work quickly, but not so hot that the cells are killed.
  • pH. Close to the optimum for the organism, because enzymes are sensitive to acidity.
  • Oxygen. Supplied for aerobic microorganisms; excluded for those harmed by it.
  • Sterility. The culture must be kept free of unwanted contaminating microorganisms.

Why the log phase matters

The log phase is when cells are healthiest and most productive, so industrial processes are often designed to keep cultures in this phase for as long as possible by topping up nutrients and removing waste.

Examples in context

Example 1. Keeping a culture in log phase. In a continuous industrial fermenter, fresh nutrients are added and waste removed so the cells never reach the stationary phase. Keeping them in the productive log phase maximises the output of the desired product.

Example 2. Why milk spoils. Bacteria in warm milk pass through the same growth phases, reaching huge numbers in the log phase and spoiling the milk. Refrigeration slows growth by lowering the temperature, lengthening the lag phase and slowing the log phase.

Try this

Q1. Name the four phases of microbial growth in order. [2 marks]

  • Cue. Lag, log (exponential), stationary, then death (decline).

Q2. Explain why the population stays roughly constant in the stationary phase. [2 marks]

  • Cue. Nutrients are running low and waste is building up, so the rate of cell division equals the rate of cell death.

Q3. State three conditions needed to grow a microbial culture. [3 marks]

  • Cue. Suitable nutrients, suitable temperature, and suitable pH (oxygen and sterility are also acceptable).

Exam-style practice questions

Practice questions written in the style of SEAB exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

Original6 marksDescribe the four phases of growth shown by a population of bacteria grown in a closed flask of nutrient broth.
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Examiners want the four named phases in order, each with what is happening to the population.

In the lag phase, the number of cells stays roughly constant as the bacteria adjust to the new conditions, take up nutrients and make the enzymes they need; they are active but not yet dividing rapidly.

In the log (exponential) phase, the bacteria divide as fast as they can because nutrients are plentiful and waste is low, so the number roughly doubles at regular intervals and rises steeply.

In the stationary phase, growth levels off because nutrients run low and waste builds up; the rate of division equals the rate of death, so the number stays roughly constant.

In the death (decline) phase, the number falls because nutrients are exhausted and toxic waste has accumulated, so cells die faster than new ones form.

What markers reward: the four phases named in order (lag, log, stationary, death), each correctly described, ideally with the reason for the change in number.

Original4 marksState four conditions that must be controlled to grow a microbial culture successfully, and explain why each matters.
Show worked answer →

The answer should give four genuine conditions with a reason for each.

Nutrients must be supplied because the microorganisms need a source of food, such as sugars and nitrogen, to grow and reproduce. A suitable temperature is needed because enzymes work best in a particular range and the cells die if it is too hot. A suitable pH is needed because enzymes are sensitive to acidity. Oxygen must be controlled because many microorganisms need it for aerobic respiration, while some are harmed by it.

What markers reward: four valid conditions such as nutrients, temperature, pH and oxygen (or sterility), each with a correct reason linked to growth, enzymes or respiration.

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