Streaking Plates for Isolation

Additional readings: Tortora Chapter 6

Read the streak plate method or streaking for isolation in Tortora et. al and in LeBoffe and Pierce.   Watch the following movie which demonstrates the technique of "streaking a plate" and then practice the technique on a piece of paper with a pencil and then using an empty petri dish. Click the image to watch the movie.
Now streak a plate from the culture slants provided and then another from the broth culture.  (Be sure to flame between each of the steps!)
 
 

The following movies will show you examples of good streaking results and unacceptable streaking results. Click the images to watch these movies.

Discussion

What exactly is a colony?

It is a visible mass of bacteria which arises from a single "mother" cell.

Since all of the cells in a colony derive from a single original cell through repeated binary fission; all of the cells in that colony should be genetically identical.

Therefore an ISOLATED colony represents a pure source of an organism from which a pure culture can be started. This is only true if the colony is isolated. If a colony touches another one it is no longer considered pure since cells are probably "mixing it up" so to speak.

STREAKING FOR ISOLATION is always a first step in purifying and identifying microorganisms from an infection site. Streaking for isolation will also allow you to tell if you are working with a pure culture, since all colonies should look identical in such a case.

Bacteria grow by binary fission and many medically important bacteria will divide once every 20 - 30 minutes under ideal growth conditions. The time it takes for a cell to go from one division to the next is called the GENERATION TIME. This can be determined in culture by either counting bacteria or measuring the optical density of a broth culture and determining the time interval needed for doubling the population. Because of this, the generation time is often called the DOUBLING TIME.

If the generation time is known and if the inoculum size is known one can determine the number of bacteria in a culture after a given incubation time. For instance, if 10 Staphylococci fell from your nose into the potato salad you were making at noon; how many bacteria would be in that potato salad for dinner at 6 PM? Assume good growth conditions and a generation time of 20 minutes. You can determine the answer with following equation:
Final # of bacteria= (X) (2 n)
where X = initial number of bacteria

n = the number of generations

Therefore: (10 Staphylococci) ( 2 18 ) = 2,621,440 Staphylococci
PROBLEM:
Using the above situation determine the number of bacteria after 1, 2,3,4,5 and 6 hours. Graph your result both logarithmically and arithmetically as shown in Tortora et. al.
As indicated above, the generation time is not a constant. Rather it is sensitive to a number of variables. Such things as TEMPERATURE, FOOD SUPPLY, AMOUNT OF WATER, pH and ACCUMULATION OF WASTE PRODUCTS will strongly influence the rate of growth.
 
 

What are the phases of bacterial growth?

What does a typical bacterial growth curve look like?

What are some of the ways that bacteria can be counted?

What are plate counts?

How is turbidity used to measure bacterial growth?

What is the difference between the pour plate method and the spread plate method for measuring bacterial growth?