Introduction
An immense advantage that bacteria have is that most of them are capable of rapid growth rates. Many of them can divide and produce a new generation under optimal environmental and nutrient conditions every twenty to thirty minutes. They are capable, as are all forms of life, of exponential or logarithmic growth where the numbers of a species double each generation (e.g. 1, 2, 4, 8, 16, 32, 64 etc.).
Bacteria have the same basic nutritional requirements as all life which includes a source of oxygen, carbon (carbon dioxide, monoxide or methane etc), Nitrogen (protein or nitrogen gas etc), a source of energy (sunlight, sulphur, sugar, protein, fats etc), water (in some resting stages of cells, bacterial spores can go without water for long periods of time, however they don’t grow or metabolize), calcium, potassium, phosphorous, magnesium and sodium. A source of minerals, such as cobalt, zinc and iron etc are also required. Bacteria synthesize every complex molecule from the basic minerals.
‘In a unique environment every gene in bacteria has been designed for maximum survival called its Niche’www1. Microbes that require cold temperatures to survive are called Psychrophiles (i.e. in refrigerators). Those which survive and grow between temperatures 25OC and 45OC are called Mesophiles (the body temperature of warm-blooded animals) and those which grow at optimal level between 50OC and 80OC are called Thermophiles. Finally those which grow and survive at extreme temperatures from 80OC to 100OC are called Extreme Thermophiles or hyperthermophiles.
Psychrophiles have largely unsaturated fatty acids in their plasma membranes which makes them adapted to their environment. ‘The degree of unsaturation of a fatty acid correlates with its solidification temperature or thermal transition stage (i.e., the temperature at which the lipid melts or solidifies); unsaturated fatty acids remain liquid at low temperature but are also denatured at moderate temperature’. www2. Within the membrane of the Thermophiles are saturated fatty acids which are stable at high temperatures; however they too solidify at relatively high temperatures.
The fluidity of the membrane is affected by the fatty acids in the membrane, whether they are solid or liquid, this affects its ability to function.
Enzymes are contained within Psychrophiles which continue functioning although at condensed rate, near temperatures of 0O. Psycrophile proteins/membranes adapt to low temperatures, but at 37OC (body temperature of warm blooded animals) they do not function.
Thermophiles are adapted to their function of temperatures over 60OC in a number of ways. They have a high Cytosine and Guanine content in their DNA which is at least as high as the optimum temperature for the organisms’ growth. ‘But this is not always the case, and the correlation is far from perfect, so thermophile DNA must be stabilized in these cells by other means. The membrane fatty acids of thermophilic bacteria are highly saturated allowing their membranes to remain stable and functional at high temperatures. The membranes of hyperthermophiles, virtually all of which are Archaea, are not composed of fatty acids but of repeating subunits of the C5 compound, phytane, a branched, saturated, "isoprenoid" substance, which contributes heavily to the ability of these bacteria to live in superheated environments. The structural proteins (e.g. ribosomal proteins, transport proteins (permeases) and enzymes of thermophiles and hyperthermophiles are very heat stable compared with their mesophilic counterparts. The proteins are modified in a number of ways including dehydration and through slight changes in their primary structure, which accounts for their thermal stability’ www2.
Results
After completing the experiment the following results were obtained.
The effect of temperature on growth
The table below shows the effect of temperature on growth of five different bacteria.
Key
0 = Growth absent
+1 = Very faint growth
+2 = Medium growth
+3 = Heavy growth
As you can see from the table of results Escherichia coli grew most at 37oC, medium at 25oC but not at the higher temperatures, neither at 4oC.
Bacillus stearothermophilus again grew mostly at 37oC but not heavily, very faint growth at 25oC and none at temperatures 4oC and 45oC. Strangely though there appears to be growth at 55oC from this organism.
Bacillus subtilis grew best at the moderate temperatures, very faintly at the lower temperatures and not at all at the highest temperature of 55oC.
Pseudomonas fluorescens grew the most at the lower temperatures and grew least as the temperature increased.
Finally, Enterococcus faecalis also grew the greatest at 37oC with medium growth shown on both 45oC and 25oC. No growth at the highest temperature but very faint growth shown at 4oC.
Discussion
As you can see from the results of the experiment, the five different bacterium’s can now be categorised into psychrophiles, mesophiles and thermophiles.
Escherichia coli appears to be a mesophile as it grows in moderate temperatures, growing greatest at human body temperature of 37oC, indicating that this organism is obviously present within our human bodies.
Bacillus stearothermophilus appears to be a thermophile but also is able to grow in mesophile conditions. There was very faint growth at 55oC indicating that if the temperature had increased further, more growth will have occurred.
Bacillus subtilis also appears as a mesophile as it grows greatest within moderate temperatures of 37oC and 45oC, yet fails to grow at extreme temperatures of 55oC and faintly at low temperatures.
The only psychrophile from these five bacterium’s is Pseudomonas fluorescens as this grows very heavily at low temperatures of 4oC and 25oC, yet faintly and none at when the temperature increases to 37oC and above.
Finally another mesophile is the Enterococcus faecalis as this also grows best at 37oC and at moderate temperatures it showed medium growth. It showed very little growth at 4oC and no growth at the extreme of 55oC.
Results
The effect of heat on survival of bacteria
The following tables indicate the effect of heat on the survival of the five different bacterium’s.
Escherichia coli
Key
+ = bacteria survived
Pseudomonas fluorescens
Enterococcus faecalis
Bacillus subtilis
Bacillus stearothermophilus
References
• Campbell, Reece, Biology (2005)
• Dr T.G. Cartledge, Introduction to biology module booklet
• www1 -
• www2 -
