Effect of Heat on Bacteria Growth | Experiment

Aims

Five bacteria have been allocated in the form of broth cultures. The five bacteria include Escherichia coli, Pseudomonas fluorescens, Enterococcus faecalis, Bacillus subtilis and Bacillus stearothermophilus. The first experiment will study the amount of growth for these five bacteria when incubated at different temperatures. Five nutrient broths will be aseptically inoculated with a loop full of each bacterium and placed for incubation at the correct temperature.

In a separate experiment, the five bacteria will be exposed to high temperatures for set periods of time and then incubated. Five nutrient broths will be placed in each 4 water baths of different temperatures for 5 minutes. Each bottle will then be inoculated with a drop of culture and placed back in the designated water bath, each bottle will be removed from the water bath at timed intervals and this will enable the observation of the effect of heat on the survival of bacteria.

The observations will be recorded and used to help determine the classification of the five bacteria as Psychrophiles, Mesophiles or Thermophiles and recognize why.

Results

Heavy Growth

• From the table, it can be observed that E. coli grows best at approximately 37°C but can grow below up to 25°C and just above to 45°C.
• P. fluorescens grows well between 5-37°C but growth is best at around 25°C. Small growth at 45oC and shows no growth at 55°C.
• E. faecalis grows best at approximately 37°C with slight growth at 25°C and 45°C.
• B. subtilis grows well between 25-37°C where best growth can be seen at approximately 37°C.
• B. stearothermophilus showed no growth at any of the temperatures.

Experiment 2 – Table 2 showing the effect of heat on the viability of bacteria after timed intervals

No Growth

• The first E. coli test results show that it was unable to survive 90 minutes into 60°C and no growth was recorded for 80°C and 100°C. The second E. coli test showed that there was no growth after 5 minutes into 60°C.
• E. faecalis recorded no growth at 30 minutes at 40°C, then growth reoccurred 90 minutes into 40°C till 2 minutes into 80°C and no growth was recorded there after. Second E. faecalis test recorded growth at 30 minutes at 40°C, however no growth 90 minutes into 60°C, growth at 2 and 5 minutes into 80°C and no growth afterwards.
• P. fluorescens survived at 2 minutes into 60°C and no growth was recorded thereafter. The second test shows that it grew till 5 minutes into 60°C, no growth at 30 and 90 minutes, then growth at 2 and 5 minutes into 80°C.
• B. subtilis survived right up until 30 minutes into 100°C where no growth was recorded for 30 minutes and 90 minutes. Result was similar for the second test, however no growth was recorded at 90 minutes into 80°C and growth at 2 minutes into 100°C.
• B. stearothermophilus tests show random growth at different intervals at different temperatures. First test showed growth at 2 minutes and 30 minutes at 40°C and 2 minutes and 90 minutes at 60°C, no growth was recorded elsewhere. Second test showed growth at 5, 30 and 90 minutes of 40°C and no growth elsewhere.

Discussion

Mesophiles generally like to grow when the temperature ranges from 10°C to 52°C; these Mesophiles are then split into groups of optimum growth. Group 1, which has a optimum growth temperature between 20-30°C and group 2, which has a optimum growth temperature between 35-45°C. (Carpenter, 1972). From the results it can be said that E. coli is a Mesophile because it was unable to grow when incubated at 5°C and 55°C however grew between the ranges of 25°C and 45°C, which specifies within the growth range of Mesophiles. The results also show that the highest amount of growth was at 37°C that would put E. coli in the second group of optimum growth temperature. (Carpenter, 1972) From table 2 its possible to see that E. coli was able to survive for a long period of time when exposed to 60°C but eventually destroyed after 30 minutes, this is why E. coli is such a problematic cause of illness in humans because the optimum growth temperature of E. coli is the same as the internal temperature in the human body. (Campbell & Reece, 2004) Table 1 shows that E. faecalis most growth was at 37°C which was moderate, this is because E. faecalis optimum growth is at 35°C (Hacker & Dobrindt, 2006) There was no growth at 5°C and 55°C which makes this bacteria a Mesophile. Table 2 shows that E. faecalis can survive exposure to high temperatures better than E. coli as it survives 2 minutes at 80°C but is soon destroyed after. This is an issue as it causes the majority of human enterococcal infections such as urinary tract and abdominal infections. (Willey et al, 2007) Psychrophiles are usually found in lakes and uncultivated soils, where growth can occur at temperatures of 0°C (Carpenter, 1972) and looking at table 1, P. fluorescens is the only bacteria in the test to grow at 5°C which suggests that it could also grow below this temperature therefore can be considered a Psychrophiles. P. fluorescens optimum growth temperature is between 25-30°C (Baron, 1996) which shows in the results as there was heavy growth at 25°C. P. fluorescens survived 2 minutes into 60°C and was destroyed thereafter, survival at this temperature can cause dairy products such as milk to spoil through the production of proteases and lipases. (Baron, 1996) Although B. subtilis shows signs of growth at 45°C its optimum growth temperature is between 30-37°C (Carpenter, 1972) and from the table its possible to see that there was heavy growth at 37°C making it a Mesophile. B. subtilis is naturally found in soil and vegetation (Campbell & Reece, 2004) therefore the bacteria needs to be able to withstand various temperature conditions. This stability can be seen in table 2 where the bacteria is able to survive 5 minutes at 100°C and is then destroyed.