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...