Growth Dynamics of E. coli in Varying Concentrations of Nutrient Broths, pH, and
in the Presence of an Antibiotic


Dvora Szego,
Elysia Preston
Darcy Kmiotek,
Brian Libby
Department of Biology
Rensselaer Polytechnic Institute
Troy, NY 12180

Abstract

The purpose in this experiment of growth dynamics of E. coli in varying media
was to determine which media produces the maximum number of cells per unit time.
First a control was established for E. coli in a 1.0x nutrient broth. This was
used to compare the growth in the experimental media of 0.5x and 2.0x, nutrient
broths; nutrient broths with an additional 5.0mM of glucose and another with
5.0mM lactose; nutrient broths of varying pH levels: 6.0, 7.0, and 8.0; and
finally a nutrient broth in the presence of the drug/antibiotic chloramphenicol.
A variety of OD readings were taken and calculations made to determine the
number of cells present after a given time. Then two graphs were plotted, Number
of cells per unit volume versus Time in minutes and Log of the number of cells
per unit volume versus Time growth curve. The final cell concentration for the
control was 619,500 cells/mL. Four media, after calculations, produced fewer
cells than that of the control, these were: Chloramphenicol producing 89,3 01
cells/ml; glucose producing 411,951 cells/mL; lactose producing 477,441 cells/mL
and finally pH 6.0 producing 579,557cells/mL. The remaining four media, after
calculations, produced cell counts greater than the control: 2X with 1,087,009
cells/mL; 0.5X with 2,205,026 cells/mL; pH 8 with 3,583,750 cells/mL and finally
pH 7.0 with 8,090,325 cells/mL. From these results the conclusion can be made
that the environment is a controlling factor in the growth dynamics of E. coli.
This was found through the regulation of pH and nutrient concentrations. In the
presence of the drug/antibiotic, chloramphenicol, cell growth was minimal.

Introduction

E. coli grows and divides through asexual reproduction. Growth will continue
until all nutrients are depleted and the wastes rise to a toxic level. This is
demonstrated by the Log of the number of cells per unit volume versus Time
growth curve. This growth curve consists of four phases: Lag, Exponential,
Stationary, and finally Death. During the Lag phase there is little increase in
the number of cells. Rather, during this phase cells increase in size by
transporting nutrients inside the cell from the medium preparing for
reproduction and synthesizing DNA and various enzymes needed for cell division.
In the Exponential phase, also called the log growth phase, bacterial cell
division begins. The number of cells increases as an exponential function of
time. The third phase, Stationary, is where the culture has reached a phase
during which there is no net increase in the number of cells. During the
stationary phase the growth rate is exactly equal to the death rate. A
bacterial population may reach stationary growth when required nutrients are
exhausted, when toxic end products accumulate, or environmental conditions
change. Eventually the number of cells begins to decrease signaling the onset
of the Death phase; this is due to the bacteria’s inability to reproduce (Atlas
331-332).
The equation used for predicting a growth curve is N=N0ekt. N equals the
number of cells in the culture at some future point, N0 equals the initial
number of cells in the culture, k is a growth rate constant defined as the
number of population doublings per unit time, t is time and e is the exponential
number. The k value can be easily derived by knowing the number of cells in a
exponentially growing population at two different times. K is determined using
the equation k=(ln N-ln N0 )/t, where ln N is the natural log of the number of
cells at some time t, ln N0 is the natural log of the initial number of cells
and t is time. This equation allows one to calculate the numbers of cells in a
culture at any given time. The reciprocal of k is the mean doubling time, in
other words, the time required for the population to double, usually expressed
as cells per unit volume. (Edick 61-62)
Temperature is the most influential factor of growth in bacteria. The
optimal temperature of E. coli is 37C, which was maintained throughout the
experiment. Aside from temperature, the pH of the organisms environment exerts
the greatest influence on its growth. The pH limits the activity of enzymes
with which an organism is able to synthesize new protoplasm. The optimum pH of
E coli growing in a culture at 37C is 6.0-7.0. It has a minimum pH level of 4.4
and a maximum level of 9.0 required for growth. Bacteria obtains it nutrients
for growth and division from their environment, thus