Five bacteria have been incubated at certain temperature levels and the amount of growth observed in each case was recorded. In a separate instance, the bacteria were also exposed to high temperatures for set periods of time and then incubated. This exposure to high temperatures allows an observation of the effect of heat on the survival of the bacteria. These observations will help us to classify the five bacteria as Psychrophiles, Mesophiles or Thermophiles and understand why this is so.
Bacillus stearothermophilus is a species of gram-positive bacteria found in soil, hot springs, arctic waters, ocean sediments and spoiled food products (www1). Bacillus subtilis is a Gram-positive, rod-shaped, nonpathogenic bacterium which lives in soil (www1) Pseudomonas fluorescens encompasses a group of common, nonpathogenic saprophytes that colonize soil, water and plant surface environments. As its name implies, it produces a soluble, greenish fluorescent pigment, particularly under conditions of low iron availability (www2).
Escherichia coli are normally found in the small intestine of mammals and can cause humans to suffer from food poisoning. Enterococcus faecalis also known as Group D Streptococcus (cocci) is a Gram-positive commensal bacterium inhabiting the gastrointestinal tracts of humans and other mammals (Ryan KJ, Ray CG, 2004). As is evident from these bacteria, through natural selection and adaptation different species of micro organisms have developed their own growth temperature range, having a minimum, an optimum and a maximum temperature at which the culture is able to grow. The optimum growth temperature, commonly defined as the temperature at which most rapid multiplication occurs, is often only a few degrees below the maximum (Carpenter, 1972). These cardinal points allow bacteria to be classified according to their growth temperature:
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
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.