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Name: Olga
Status: Student
Age: 15
Location: N/A
Country: N/A
Date: April 2004

I am doing a science project where i am going to expose nutrient broth (or use agar plates) to the air in different locations of the house and see how much the bacteria grows. My dad however, has to be quantitative. Is there any way I can measure the amount of bacteria found in the agar plate or nutrient broth?

Measuring the number of bacteria in a solution takes advantage of a number of techniques that had to be figured out at the beginning of the study of microbes, all of which were mastered if not invented by Louis Pasteur. Pasteur is the scientist who figured out how to make milk safe for human consumption, among many other inventions. The techniques Pasteur used included sterilization of glassware and nutrient-filled agar, pouring the sterile agar into sterile petri dishes in a sterile manner, flaming or otherwise sterilizing any utensil used to move broth that contains bacteria and smear small quantities of bacteria-containing broth onto an agar surface, and accurate pipetting of those very small, precise volumes of liquid broth. One procedure commonly used to determine the concentration of bacteria in a solution at a particular moment in time (remember that bacteria have very short generation times) requires taking a known volume of the solution and diluting it serially 1:10 in a precise manner, and then spreading a small amount of a range of the diluted solutions on agar plates that are known to be sterile prior to the addition of the diluted solution.

On those plates that had a sufficiently small number of bacteria applied to them, individual colonies of bacteria will develop, which commonly have the appearance of discrete gooey mounds, which are counted. If the solution was not dilute enough, then instead of separate colonies, you may see plates that are coated with the gooey material, commonly called "lawns." Literally, in a serial 1:10 dilution that ranges far enough, you will find, for example, a series of plates that will have thousands of colonies next to a plate that has hundreds of colonies next to a third plate that has tens of colonies. By counting the number of colonies on a plate and knowing what the dilution factor and the volume applied to the plate, one can calculate the approximate concentration of bacteria in the original solution. For example, let's say that the fourth plate in a 1:10 serial dilution generated 42 colonies from a 100 microliter quantity of the diluted sample. The number of bacteria in the 100 microliter aliquot was 42 x the inverse of the dilution factor, which is 420,000; commonly, concentrations are stated with respect to milliliters, so in this case the concentration would be 4.2 million bacteria per milliliter.

If you knew the volume of the original solution of bacteria, you could then calculate the total number of bacteria in the sample by multiplying that volume by the concentration, taking care to keep the units straight.

Donald J Silvert

Well, the simplest way is to just count them! If you have less than 300 colonies (spots) of organisms on each plate you can just count the numbers. It will be difficult to distinguish between the different KINDS of organisms because many look very similar to the eye. You can get a number and then try to compare the qualitatively the different kinds between different areas of the house. You will not be able to count organisms by looking at a broth unless you take a sample and plate it.


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