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How do we grow and count microorganisms safely?

The structure of bacteria, aseptic technique, culturing microorganisms, the growth curve, and methods of measuring growth.

A focused answer to WJEC A-Level Biology Unit 3, covering bacterial structure, aseptic technique, culturing microorganisms in batch and continuous culture, the bacterial growth curve, and methods of measuring microbial growth.

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  1. What this dot point is asking
  2. Bacterial structure
  3. Aseptic technique and culturing
  4. The growth curve and measuring growth
  5. Examples in context
  6. Try this

What this dot point is asking

WJEC wants you to describe bacterial structure, explain aseptic technique, describe how microorganisms are cultured, interpret the bacterial growth curve, and carry out and explain methods of measuring microbial growth.

Bacterial structure

Aseptic technique and culturing

Microorganisms are grown on a sterile nutrient medium (agar or broth) supplying carbon, nitrogen, vitamins and minerals. In batch culture a fixed amount of medium is inoculated and growth follows the full curve until nutrients run out; in continuous culture fresh medium is added and culture removed at the same rate, so the population stays in the log phase, which is useful for large-scale production of products such as antibiotics and insulin.

The growth curve and measuring growth

The closed-culture growth curve has four phases: lag (cells adjust and make enzymes, little division), log (exponential growth at the maximum rate), stationary (birth rate equals death rate as nutrients fall and waste rises), and death/decline (nutrients exhausted and toxic waste accumulates, so numbers fall). Growth is measured by:

  • a viable count, counting colonies grown from serial dilutions, which counts only living cells;
  • turbidity (optical density), measuring cloudiness with a colorimeter, a quick estimate of total biomass;
  • a total count, counting all cells (living and dead) in a haemocytometer under a microscope.

Examples in context

Example 1. Industrial penicillin production. Penicillium mould is grown in giant fermenters under continuous, sterile conditions with controlled oxygen, pH and nutrients, so the culture stays productive far longer than a batch would. This shows why continuous culture and rigorous aseptic technique matter in biotechnology, a standard WJEC application.

Example 2. Monitoring food spoilage. Food scientists take viable counts of bacteria in milk over time at different temperatures. The serial-dilution method shows that bacterial numbers stay low for longer when milk is refrigerated, giving the quantitative evidence behind use-by dates and the cold chain.

Try this

Q1. State two features of aseptic technique. [1 mark]

  • Cue. Flaming the container neck and working near a Bunsen flame (or sterilising equipment in an autoclave).

Q2. Explain why a continuous culture keeps bacteria in the log phase. [2 marks]

  • Cue. Fresh nutrients are added and waste and cells removed, so conditions stay ideal and reproduction continues at the maximum rate.

Q3. A culture diluted ×1000\times 1000 gave 3535 colonies from 0.1 cm30.1 \text{ cm}^3. Calculate the number of viable cells per cm3\text{cm}^3. [3 marks]

  • Cue. 35÷0.1=35035 \div 0.1 = 350; 350×1000=350 000350 \times 1000 = 350\,000, that is 3.5×105 cm−33.5 \times 10^5 \text{ cm}^{-3}.

Exam-style practice questions

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

WJEC 20174 marksDescribe the four phases of a bacterial growth curve in a closed (batch) culture.
Show worked answer →

In the lag phase, bacteria adjust to the medium, synthesising enzymes and DNA; there is little increase in number.

In the log (exponential) phase, nutrients are plentiful and bacteria divide at their maximum rate, so the population grows exponentially.

In the stationary phase, the rate of reproduction equals the rate of death as nutrients run low and waste builds up, so the number stays roughly constant.

In the death (decline) phase, nutrients are exhausted and toxic waste accumulates, so the death rate exceeds the reproduction rate and numbers fall.

Markers reward all four phases named and correctly described.

WJEC 20225 marksA 1 cm cubed sample of culture was diluted by a factor of 1000 and 0.1 cm cubed spread on agar, giving 60 colonies. Calculate the number of viable bacteria per cm cubed in the original culture, and explain why a viable count differs from a total count.
Show worked answer →

Each colony grew from one viable bacterium, so the diluted, plated sample contained 60 viable cells in 0.1 cm cubed.

Cells per cm cubed of the diluted sample =60÷0.1=600= 60 \div 0.1 = 600.

Correcting for the 1000 times dilution, the original culture contained 600×1000=600 000600 \times 1000 = 600\,000 viable bacteria per cm cubed (which is 6×1056 \times 10^5).

A viable count measures only living cells, because only living cells form colonies, whereas a total count (for example using a haemocytometer) counts all cells, living and dead, so it gives a higher figure.

Markers reward dividing by the plated volume, multiplying by the dilution factor, the value of 6×1056 \times 10^5, and viable versus total count.

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