Bacterial Growth Curve

Bacteria, like all living organisms, grow. The time it takes for a population of bacteria to double in number is called the "growth rate." The doubling time is a fixed characteristic of each type of bacteria, and it can be used for identification.

Below is a diagram of the bacterial growth curve. When a culture of microorganisms is transferred into a new container for counting, there is an initial "lag phase." During this period, the bacteria are adjusting to their new surroundings, new food source, and new temperature. They are not yet multiplying in numbers. Then, the "exponential" or "log" phase begins. During this phase of growth, the bacteria are multiplying like mad! They have abundant food and not enough toxins are around to harm them. They make as many new bacterial cells as they can. Eventually, the bacteria run out of room, space, or they have produced toxins that are limiting their growth. This begins the "stationary phase," during which the bacterial numbers are not really changing. The bacteria are still making new cells, but the same amount are dying as are being made, so the curve is flat. Finally, the number of bacteria dying becomes greater than those being created, and the numbers decline in the "death phase."

 

Activity: Using the simulated data below (this is not from an actual experiment, but the data is typical of what you would find for a species of bacteria like Escherichia coli,which multiplies fairly quickly), graph a bacterial growth curve and calculate the growth rate.

When microbiologists count bacteria, they often can't count all the cells in the sample. Instead, they dilute the sample by taking a small volume of sample and mixing it with a larger volume of clean, sterile solution. (Make sure you have completed the Dilution Worksheet before doing this exercise!) The scientists then spread the mixture onto an agar plate (a substance made from seaweed that looks like Jell-O-- bacteria grow on it really well!) and count the number of dots (called colonies) that grow. The number of colonies that grow on each plate are called "Plate counts." Most scientists do more than one plate count for each sample and take the average of their results.

In the simulated data below, samples were taken from the growing bacteria culture over time, diluted, then grown on a plate. You should average the three plate counts given, then calculate the number of cells that were in the actual sample based on the dilution factor given. For example, the first set of plate counts are: 200, 205, 213. The average of these numbers is 206. The dilution factor is 10^4. That means that for each cell counted on the plate, there were 10^4 or 10,000 living cells in the culture. So, there were 206 x 10^4 cells in the culture at the beginning of the experiment (when time = 0). BUT, that number is not in scientific notation properly! In order to record this number correctly, it should be 2.06 x 10^6.

Please fill in the table below with the correct numbers.

Time (minutes) Dilution Factor Plate Counts Average Plate Count Number of living cells in the culture

0

10^4 200,205,213    
15 10^4

195,215,200

   
30 10^4 192,203,214    
45 10^4 270,255,266    
60 10^5 42,37,40    
90 10^5 80,83,78    
120 10^5 175,168,185    
150 10^6 42,32,37    
210 10^6 50,55,53    
270 10^6 54,49,57    
330 10^6 51,55,48    
390 10^5 260,253,249    
420 10^5 180,176,183    

 

Now, graph the results of the experiment on graph paper. Put time (in minutes) on the x-axis and numbers of living bacteria on the y-axis. This graphing exercise works best with semi-log graph paper, if you can use that. The students will have to understand logarithms (base 10) and be able to plot their numbers on semi-log paper. On semi-log paper, the x-axis is normally gridded with even spacing; the y-axis, however, has less and less space in between the lines as you go up from 1 to 10. It then gets large again and decreases from 11-20, etc. The semi-log paper is necessary to have the graph turn out with the exponential phase as a straight line so the students can visually calculate a growth rate. This graphing exercise can also be done on a computer with a program that can plot the graph for them, if you wish to use technology in the classroom.

The students can also calculate the growth rate using this formula:

k= [log (xt) - log (x0)] / 0.301 * t

k= the number of population doublings in one hour (called the growth rate constant)

x0= the number of cells/milliliter at the beginning of the log phase

xt= the number of cells/ml at some later time

t= the number of hours between the beginning and the later time

1/k= the Generation Time (the amount of time it takes for a population to double in number)