Growth data answers one question: under which conditions do microorganisms multiply best? Read the trend first, find the highest value, then decide what the data can and cannot claim.
This lesson is part of microorganisms. If you need a reminder of the variables in an experiment, see identifying variables and fair comparisons.
How do I read a growth table?
Use three steps every time: trend, peak, limit.
- Trend. Does the value rise, fall or rise then fall as the condition changes?
- Peak. Which condition gives the highest value? That is the optimum among the values tested.
- Limit. What happens at the extremes? Some values may drop to zero or to a very low count.
Worked example: bacteria on agar plates
A student places the same amount of bacteria on five identical agar plates. After 24 hours at different temperatures, the colonies are counted.
| Temperature (°C) | Number of colonies |
|---|---|
| 5 | 4 |
| 20 | 46 |
| 37 | 120 |
| 50 | 31 |
| 70 | 0 |
Trend. The number rises from 5 °C to 37 °C, then falls at 50 °C and 70 °C.
Peak. The highest count, 120 colonies, is at 37 °C. So 37 °C is the optimum temperature among those tested.
Limit. At 70 °C there are no colonies, which suggests the bacteria cannot grow or were killed at this temperature.
Assumptions. Each plate began with the same amount of bacteria, the same agar and the same 24 hours. Without these, the comparison would not be fair.
Writing the conclusion
A good conclusion names the optimum, describes the trend and stays inside the data. “The bacteria grew best at 37 °C, of the temperatures tested, and did not grow at 70 °C.”
Notice the phrase “of the temperatures tested”. The true optimum might be 40 °C, which was not included. The data cannot say.
The mistake that costs marks
The slip is to overclaim. Saying “bacteria cannot grow below 20 °C” is wrong, because the table shows 4 colonies at 5 °C.
| Step | Wrong | Right |
|---|---|---|
| Peak | Bacteria like warm places | The highest count, 120, was at 37 °C |
| Low end | No growth at low temperatures | Growth was slow at 5 °C, with 4 colonies |
| High end | Heat is bad for bacteria | No colonies at 70 °C |
Use numbers from the table in your answer. A conclusion that quotes a value is easier to credit than a vague one.
Check yourself
Using the table above, suggest one reason why food kept in a refrigerator at about 5 °C spoils more slowly than food left on the counter at 37 °C.
Answer
At 5 °C the count was only 4 colonies, compared with 120 at 37 °C. So the microorganisms multiply far more slowly at low temperature.
This means the food takes longer to reach the level of microorganisms that spoils it.
What to study next
Growth conditions also explain why some applications, such as yoghurt making, work best at set temperatures. Continue with explaining beneficial applications, then use the graph evidence and fair-comparison lab to practise with new data.
To go through data questions with a teacher, see online one-to-one Science tuition.