Psychrophiles are found in many lakes, streams and uncultivated soils, with their main principle being that they are able to grow at 0oC.Growth temperature ranges have been known to start from as low as -5oC and some facultative Psychrophiles still grow between 30 and 35oC (Carpenter, 1972).
Mesophiles tend to have a growth temperature range of 10oC-52oC and are split into two groups; those who have an optimum growth temperature of between 20-30oC and those who have an optimum growth temperature of between 35-45oC (Carpenter, 1972).
Thermophiles are those that are able to grow at over 55oC, with many not even being able to grow below 40oC, which is why they can be found colonizing on hot springs. However, facultative Thermophiles are also able to grow at 37oC (Carpenter, 1972).
Above or below these temperature ranges the bacteria will be unable to grow, however it may still be able to survive for an amount of time depending on how well it is adapted to cope with exposure to extreme conditions. This is an important point to take into account when considering microbial control, as a lack of growth does not necessarily ensure that all of the bacteria have been eliminated
Results
Escherichia coli grows best at approx. 37oC, Bacillus Stearothermophilus grows best above 55oC, Bacillus subtilis grows best approx. between 25oC and 37oC Pseudomonas fluorescens grows best between 4oC and 25oC and Enterococcus faecalis grows best at approx. 25oC.
Escherichia coli were unable to survive after 5 minutes at 80oC and 2 minutes at 100oC.
Pseudomonas fluorescens were unable to survive after 30 minutes at 80oC and could not survive being exposed to 100oC at all.
Enterococcus Faecalis couldn’t survive after 90 minutes at 60oC or for any length of time at 80oC and above.
Bacillus subtilis couldn’t survive past 30 minutes at 100oC
Bacillus stearothermophilus could survive exposure to all temperatures for all the tested lengths of time, the most severe of conditions being 100oC for 90 minutes.
Discussion
Due to the results, it can be said that Escherichia coli is a mesophile. Its inability to grow when incubated at 4oC and 55oC shows that its growth range must be in between these two values, fitting with the idea that Mesophiles have a growth temperature range of between 10oC and 52oC. Also, its optimum growth appears to be around 37oC which puts it in the second set of Mesophiles that have an optimum growth temperature of between 35oC and 45oC (Carpenter, 1972). Despite the fact that it is most commonly found in an environment where temperature is regulated very finely, it is surprising to see that it is able to survive when exposed to 60oC for a long period of time, however was quickly destroyed after only 5 minutes at 80oC. Enterococcus faecalis shows a growth temperature range similar to E. coli, however it is shown that the most growth occurred at around 25oC, but optimal growth temperature could’ve been anything above 4oC but below 37oC making it a mesophile, however since its optimum growth temperature was inconclusive, the type of mesophile is unknown. E. faecalis couples a slightly lower optimal growth temperature than E. coli with a slightly lower survival rate, being disposed of after 90 minutes at 60oC and becoming destroyed upon first contact with 80oC and any temperatures exceeding. For the same reasons as E. coli, the results obtained show that E. faecalis can survive at prolonged exposure to higher temperatures much better than expected. The fact that E. coli can survive this exposure, and the fact that the optimum growth value is the same as the internal temperature in the human body maintained via homeostasis (Campbell & Reece, 2004), may explain why it is such a common cause of ill health, as the bacteria could lie dormant inside raw meat and if the meat is undercooked it could continue to lay dormant until it is inside the body, where conditions are right for optimum growth.
Since P. fluorescens was able to grow at 4oC, this suggests that it may still be able to grow at 0oC. Along with its inability to grow at 37oC it can be said that it is a psychrophile, and because it is able to grow above 22oC (Carpenter, 1972) it can be determined that it is a facultative psychrophile. However, I believe that some of results are an example of bad laboratory practice because a bacterium that cannot survive exposure to 60oC should not be able to survive exposure to 80oC. Also, due to its habitat P. fluorescens can easily come into contact with farm animals, and can spoil dairy products such as milk through the production of lipases and proteases. Pasteurisation would normally control this, however if the bacteria were actually able to survive up to 80oC then pasteurisation would not be enough to kill the bacteria.
Although Bacillus subtilis shows slight growth at 55oC, it grows best at 25oC and 37oC, making it a mesophile. B. subtilis is normally found in soils and in decomposing vegetation, so would have to be able to withstand the coldness of the winter and the heat of the summer. This durability is demonstrated in the results, which show an ability to survive after being exposed to 100oC, however it could not survive past 30 minutes exposure to this heat.
The results show Bacillus stearothermophilus to be an obligate thermophile because it showed no sign of growth. However, further reading suggests that B. stearothermophilus has a minimum growth temperature of between 33-37oC (Carpenter, 1972, p221 table 11-1), making it a facultative thermophile. According to table 6 it is able to survive and still grow after exposure to 100oC for 90 minutes demonstrates how it is able to colonize by hot springs.
Conclusion
E. coli, E. faecalis and B. subtilis are mesophiles and their common environment allows them to not be exposed to drastic change in temperature, meaning that they do not survive very well when exposed to extreme conditions. P. fluorescens is a psychrophile, and due to their notoriety coming from an ability to grow at 0oC it has been observed that thermal death of psychrophiles seems to be associated with the loss of integrity of their cell membranes and subsequent cell lysis (Carpenter, 1972). On the other hand, B. stearothermophilus is a thermophile, and thermophiles obviously possess unusual proteins or an unusual physicochemical situation that permits, and even favours, physiologic activity at temperatures that denature the proteins of other organisms (Carpenter, 1972)
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References
www1- http://cancerweb.ncl.ac.uk/cgi-bin/omd?bacillus+stearothermophilus
www2- http://genome.jgi-psf.org/finished_microbes/psefl/psefl.home.html
Ryan KJ, Ray CG (editors) (2004). Sherris Medical Microbiology, 4th ed., McGraw Hill, 294–5
Carpenter, 1972, Microbiology 3rd edition, W.B.Saunders Company
Campbell and Reece, 2005, Biology, 7th Edition, Benjamin Cummings.
