E.
coli Growth Inhibition As A Result Of Temperature Changes
The use of Escherichia coli in laboratory settings
has helped vastly increase the pool of knowledge regarding bacterial growth
requirements and metabolic pathway utilization. For the binary fission used by
prokaryotes to increase population size, sufficient nutrients for the production
of building blocks along with an adequate energy source are required. E. coli is an organism that can
synthesize from a carbon source all of their requirements for growth such as
amino acids, pyrimidines, purines, and other necessary nutrients. (Todar, 2012)
While there are different metabolic processes by which a cell can produce
precursor metabolites and energy, some are much more efficient than others.
When an optimal carbon source is not available, cells can still generate the
macromolecules needed for growth but the rate of growth is slowed. Even when the
ideal conditions for maximum cell growth are present, inevitably the growth
rate will slow as resources are depleted. Once the limiting resource
contributing to cell growth has vanished, cell breakdown will begin and
eventually the cell population will enter the death phase. There are other
environmental factors that can also adversely affect the rate of cell growth,
such as pH, oxygen availability, the specific nutrients available, and
temperature (Schussler, et all, 2012). Temperature is an important variable
concerning almost all chemical reactions. Especially regarding organisms that
have acclimated to the environment inside the human body, a lower temperature
can cause an inhibited or even a lack of cell growth.
To
gain concrete knowledge of the optimal temperature for cell growth, an
experiment was designed to provide data regarding the growth differential of E. coli at cooler temperatures rather than
the assumed optimal temperature of 37 degrees Celsius, the internal temperature
of the human body. A strain of wild type, nonpathogenic E. coli was grown in glucose as the organism to be subjected to
experimental conditions. For the experiment, a growth medium of M9 was used.
Glucose was added as a variable to elicit growth of the organism in both
experimental temperatures, and as a control the organism was also grown in the
M9 culture without any glucose. The two experimental flasks consisted of M9,
glucose, and the E. coli strain. Both
were agitated during the growth phase. One flask was cultured at 25 degrees
Celsius and the other at 37 degrees Celsius, each for 90 minutes total. The
control of M9 without glucose but also with the E. coli strain was also agitated and grown at 25 degrees Celsius,
also for 90 minutes. The experimental cultures were duplicated so that each M9,
glucose, and E. coli specimen replicated
so there were two separate flasks for each temperature variable. Every flask
was tested for culture population using the optical density calculated by a
spectrophotometer. All the experimental and control flasks had the absorbance
level measured at the beginning of the experiment to establish a baseline, and
again after 30 minutes, at 60 minutes, and at 90 minutes. The density of the E. coli population was reflected in
higher absorbance levels measured of the optical density by the
spectrophotometer, providing a linear progression of cell growth and in certain
populations, consequent die off, over a total of 90 minutes.
The
results of the experiment demonstrated that the control of M9 and E. coli with no glucose did not really
grow. The averaged optical density from the multiple control flasks was around
0.6 at baseline and stayed the same at each 30-minute increment; this was to be
expected because there was no carbon source for the organism to utilize for
growth. The cultures of E. coli grown
with M9 and glucose at 25 degrees Celsius did not demonstrate very much growth,
with an averaged baseline of 0.6 that rose only to 0.825 at its highest
measurement which was at 60 minutes. The following measurement at 90 minutes
showed the culture population began decreasing, with an average absorbance
value of 0.7. The most growth was seen, as anticipated, in the culture grown at
37 degrees Celsius. The baseline measurement of optical density was actually
below 0.5 but over the consecutive 30-minute intervals, the culture growth
increased to an optical density of 0.2, significantly higher growth than that
seen when the culture was grown at 25 degrees Celsius.
This
study does confirm that E. coli grows
best at a warm 37 degrees Celsius, as compared to a cooler room temperature
level such as 25 degrees Celsius. The incorporation of agitation, which should
provide optimal growth of a bacterial species by maximizing the rate of reactions
in metabolic processes, did ensure that both of the experimental cultures had
the benefit of maximum possible growth, thereby equalizing that particular
variable. The control culture of M9 and E.
coli, which did not show any population increase at all, demonstrated that
the glucose added to the cultures was necessary for the possibility of any kind
of growth. The E. coli cultures grown
at25 degrees demonstrated only barely discernible growth and even entered a
die-off phase much more quickly than would be expected, based on the extensive
growth seen by the same organism at 37 degrees Celsius, suggesting that
chemical reactions were so slowed by the 12-degree difference that not even the
limiting resource was used up before the bacterial population lost what little
ability it had for growth. Because the cooler temperature of 25 degrees Celsius
severely inhibited the maximal growth potential for E. coli, it would be a logical conclusion that most enteric flora
which has acclimated to the internal human body would be unable to demonstrate much
growth of any kind if faced with a variable such as a slightly cooler
environment. There are some bacteria species that do thrive in more hostile or
extreme environments, but those are not organisms that have evolved residing in
the human body such as E. coli. This
experiment is a valid reaffirmation of previously acquired knowledge regarding
the abilities and limitations of prokaryotes that live in close association
with the human body. Additional study similar to this one could include a wider
range of temperatures instead of just two differing temperatures, and also
testing different strains of E. coli,
especially pathogenic strains such as E.
coli 0157:H7, foodborne strain that causes severely debilitating illness.
References:
Schussler E, Hudson, J.,
Rowe E., Lemieux M., and Naswa S.(2012) Cell Growth and Metabolism. University
of Tennessee, Knoxville. Organization and
Function of the Cell, Biology 140 Laboratory Manual. 17-23.
Todar K. Online
Textbook of Bacteriology, University of Wisconsin [internet] 2008-2012
[cited 2013 March 6] Available from: http://textbookofbacteriology.net/nutgro_2.html
