MadSci Network: Microbiology

Re: Growing bacteria

Date: Wed Jan 14 09:49:22 1998
Posted By: Chris Yost, Grad student Microbiology
Area of science: Microbiology
ID: 884622895.Mi

Dear Katie,

To grow bacteria you need to supply them with nutrients (in other words a food source). In the particular case of blood agar the bacteria are getting the majority of their nutrients from the blood (could be sheep' blood), other nutrients which are necessary for growth may be added to the agar. The bacteria do not use the agar for growth, in fact the agar merely provides a semi-solid surface to grow the bacteria on.

As for temperature, if you are growing bacteria on blood agar plates it usually implies that you are trying to culture bacteria which live in an animal. Therefore you would want to incubate the plates at the same temperature as the internal environment of the animal. For example, lets say you take a sterile q-tip and scrap the inside of your cheek with the q-tip and then use this q-tip to streak a blood agar plate. Since our body temperature is usually 37 Celsius you would want to incubate the blood agar plate at 37 Celsius as the bacteria are accustomed to this temperature.

When bacteria grow on an agar plate you will not see the individual bacterial cells. Bacteria grow tremendously fast when supplied with an abundance of nutrients. What you will see on an agar plate is a colony of bacterial cells. Within the colony are millions of individual bacterial cells. The fascinating thing is that different bacteria will produce different types of colonies, some colonies may be coloured (the bacteria Serratia can produce red colonies), some colonies are circular in shape while others are irregular. The characteristics of a colony (shape, size, pigmentation, etc.) are termed the colony morphology. Colony morphology is a way scientists can identify bacteria. In fact there is a great book, commonly termed Bergey's Manual, which describes the majority of bacterial species identified by scientists, so far. This manual provides descriptions for the colony morphologies of each bacterial species.

Another method to help identify your unknown bacteria is to perform a Gram Stain. A Gram stain involves taking some of the bacteria from a colony, placing them on a microscope slide and then adding a series of dyes to the bacteria. After the dyes have been added you can look at them under a microscope. This stain will divide bacteria into two major groups, Gram positive (they will look purple under microscope) and Gram negative (they will look red under the microscope). For example a bacterium named Staphylococcus aureus is Gram positive and will look purple after a Gram stain while a bacterium named Escherichia coli is Gram negative (red after a gram stain). Recall that bacteria species can have different colony morphologies, well, individual bacterial cells may also look different. For example an individual cell of Staphylococcus aureus is circular, while an individual cell of Escherichia coli looks like a rod (long, and narrow shape).

So armed with information about a bacteria's colony morphology, cellular morphology and Gram stain and using the Bergey's Manual a person can begin to determine what bacteria they have cultured on an agar plate.

To determine if a colony of bacteria are dead you could take a sample of bacteria from the colony and place them on a fresh nutrient agar plate, if the bacteria do not grow it is likely that they are dead. There are also stains available whereby you can take a bacterial sample from the colony and place it on a microscope slide and then stain the slide using the special stains. Using a special fluorescent microscope live cells will appear to be one colour while dead cells will be another colour.

I wish you good luck in the science fair!

Chris Yost

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