FACTORS AFFECTING BACTERIA GROWTH - TEMPERATURE

FACTORS AFFECTING BACTERIA GROWTH - TEMPERATURE Isabel O’Brien (Year 10) Science Faculty, The Illawarra Grammar School, Western Avenue, Mangerton, 2500

Abstract

Food consumed by humans is stored and refrigerated or more commonly cooked before being consumed. In this experiment, bacteria sample Staphylococcus epidermidis was placed on 4 petri dishes. A control was incubated at 30 degrees, and the other three stored in the refrigerator at 5oC. Staphylococcus epidermidis was also boiled at 95 degrees Celsius for 10 minutes and then placed on a further three petri dishes, and then grown in an incubator at 30 degrees to test the hypothesis that bacteria do not grow outside the temperature range of 5 – 60 degrees Celsius, hence the reason food consumed by humans is stored outside of this temperature range. The study found that the boiled and refrigerated bacteria did not grow at all after 24 hours, yet the control petri dish was covered in bacteria.

Introduction

The aim of this study was to investigate the way in which temperature affects the growth of bacteria. When bacteria is grown below or above its growth range, it will not grow. But if it is grown within the growth range it will thrive.

Bacterial growth is defined as the increase in the bacterial population rather than the growth in size of individual cells. Bacteria multiply using mitosis, with the division of cells. Mitosis occurs in the parent cell, where DNA is copied and the cell splits into two daughter cells. Many factors influence the rate and growth of bacteria which include temperature, nutrients, pH, water, salt, and gaseous concentration. Temperature affects the growth of bacteria in various ways. There is a minimum and maximum temperature at which bacteria can grow, and an optimal temperature within this range where the bacteria can thrive. This range changes between different types of bacteria. Bacteria that grow in food have a growth range between around 6 to 60 degrees Celsius. The bacteria will not grow below the minimum as the membrane solidifies and nutrients cannot be transferred or above the maximum temperatures where proteins and enzymes denature. If temperature increases consistently from the minimum, bacterial growth increases until a maximum growth rate is reached, known as the optimal temperature. If the temperature continues increasing, the growth rate of bacteria declines until the maximum temperature is reached bacterial growth ceases. Availability of nutrients affects the growth of bacteria. Furthermore, pH affects bactericidal growth as well as Water availability and Gaseous concentration which are important factors in bacterial growth.

The temperature at which bacteria grow is of particular importance as bacteria can grow in food that we eat and therefore can cause us diseases and food poisoning. When food is stored in the refrigerator or heated up before eating, this is to prevent or kill bacterial growth. When we leave food out on the bench, or when food is lukewarm for long periods of

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time this is creating the optimal temperature for bacteria to grow, meaning bacteria growth is rapid at this stage. Some bacteria could be potentially pathogenic and cause disease. Bacteria that can cause food poisoning from food that humans consume is directly affected by temperature and grows best at temperatures between 5oC and 60oC. Therefore, we store food in refrigerators and freezers, to keep food below that temperature of 5 degrees where bacteria will grow in food or heat it up by cooking it past 60 degrees.

Hence, it is important for us to store food in refrigerators and to cook food, and to investigate the effects of storing food at different temperatures not within the optimal temperature range for bacteria growth.

Method

The bacteria sample of Staphylococcus epidermidis was used, 2-3 drops of the bacteria were dropped onto 4 of the dishes using the pipette and the inoculating loop was used to spread it thinly around the petri dish.

10 -15 drops of the bacteria were dropped into a test tube and placed in boiling water for ten minutes. The temperature was taken as 95 degrees Celsius. This boiled bacterium was dropped onto the remaining 3 petri dishes using the pipette and spread using the inoculating loop. The lids of the petri dishes were sealed with masking tape. The dishes were labelled as either control, boiled or refrigerated accordingly. 4 dishes were placed in the incubator (the 3 boiled and 1 control dish), and 3 dishes in a refrigerator. The dishes were left in their locations for 24 hours. Results were recorded recording percentage coverage of bacteria by estimation through observation on each petri dish and this was recorded in a results table.

Results

The petri dishes which were stored in the refrigerator, and the petri dishes stored in the incubator that had been boiled had no bacteria coverage. The control petri dish had around 85-90% coverage of bacteria.

Location % covered % covered

% covered

Dish Dish 1 Dish 2 Dish 3 Refrigerated 0% 0% 0% Incubated (boiled) 0% 0% 0% Incubated (control) 85-90% n/a n/a

100% 90% 80% 70% 60% 50% 40% 30% 20% 10% 0% Average coverage of bacteria

Refrigerated Boiled Incubated (control)

Percentage covered Dish 1 Percentage covered Dish 2

Percentage covered Dish 3

Discussion

It was hypothesised that bacteria that had been boiled or refrigerated would grow minimal bacteria, as this is outside the optimum range.

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The results therefore supported the hypothesis. This is because the bacteria cannot grow outside a range of temperatures. For Staphylococcus epidermidis this temperature range is 15oC to 45oC, however it grows best at 30oC to 37oC. This is similar to other bacteria’s such as Norovirus, Salmonella, Clostridium perfringens, Campylobacter, and other Staphylococcus bacteria such as Staphylococcus aureus which can grow in food consumed by humans and cause illness. The bacteria that were incubated were incubated at 30oC to ensure optimal conditions for growth. The reason the refrigerated bacteria did not grow was that it was grown in conditions below the temperature range that it can survive (at 5oC). Bacteria will not grow below the minimum as the membrane solidifies and nutrients cannot be transferred. The bacteria that were boiled at 95oC for ten minutes and then incubated did not grow since 95oC is outside the maximum range for most bacteria (especially Staphylococcus epidermidis) and bacteria grown above its maximum temperature range cannot survive as it is at these temperature levels where proteins and enzymes denature. This therefore supports the theory that refrigerating, and cooking (heating) food is done to prevent bacteria growth and illness from food. This also supports this theory as the control, which was grown at 30oC, or close to room temperature, means perishable food not stored in a refrigerator or not heated before consumption may provide optimum conditions for bacterial growth and may cause illness if the bacteria that has grown is consumed. Reliability in this experiment comes from the repeatability of the experiment. 3 samples were grown in both temperature conditions, and in addition a control sample was utilised. The results were repeated in each petri dish that was used. Validity in this experiment is achieved by the variables being controlled. Only one variable was changed (temperature) and other variables were able to be controlled such as bacteria sample used, sterilisation of materials, materials used, period given for bacteria to grow etc. Given more resources, more control could have been used to find more consistent results. Future studies may look at exactly what temperature bacteria stops growing in, by growing bacteria at specific temperatures clustered around 60oC and 5oC. Hence there was a clear difference between temperatures in terms of growth.

Conclusion

In summation, bacterial growth is significantly affected by temperature. Food is stored above and below certain temperature to limit bacterial growth on food that may cause sickness. If bacteria is grown below or above its growth range (for Staphylococcus epidermidis this at or below 5oC or at or above 60oC), it will not grow. If it is grown within this range (e.g., at 30oC) it will thrive.

References

Conditions needed for bacterial growth - Food safety –CCEA - GCSE Home Economics: Food and Nutrition (CCEA) Revision - BBC Bitesize 2021, BBC Bitesize, viewed 17 November 2021, <https://www.bbc.co.uk/bitesize/guides/z77v 3k7/revision/1>. CDC 2020, Foodborne Germs and Illnesses, Centers for Disease Control and Prevention, viewed 17 November 2021, <https://www.cdc.gov/foodsafety/foodbornegerms.html>. Kundrat, L 2016, Environmental Isolate Case Files: Staphylococcus epidermidis, Microbiologics Blog, viewed 17 November 2021, <https://blog.microbiologics.com/environme ntal-isolate-case-files-staphylococcusepidermidis/>.

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