What Is a Quantitative Culture?

Quantitative culture is a term that belongs in the field of microbiology. It describes a variety of techniques that microbiologists use to count how many microbes are present in a particular sample. Some analytical techniques only identify the presence of certain microorganisms, not the amount of the microbes present, and so these methods are not quantitative.

When a microbiologist provides microbes with enough nutrients to grow and multiply, this process is called culturing. Microbes are very small and can be present in high numbers in samples. It is virtually impossible for a microbiologist to know what is in the sample, and at what concentrations, without allowing the microorganisms to grow up to easily readable levels.

Microbiological samples can be anything that could possibly contain microbes. Examples include foodstuffs, blood or water. Some of these samples most often contain lots of microbes, like ground meat, or have few or no microbes, like blood. The analyst's job is to place the sample in a growth medium for culture that can show how many individual microbes are present.

The analyst typically removes a particular measure of the sample, such as 1 ml of water. This is so he or she can find out how many microbes are in that ml and thereby calculate how many microbes are present in the sample as a whole, or in the source of the sample. If he or she thinks that the total microbial count in that ml of sample will be high, the analyst performs a dilution down to the point where the result will be at levels that are not too high to read.

One method of quantitative culture involves a ml of the sample or the dilution mixed up with nutrients in a solid medium. Then the analyst incubates this at specific temperature and time, to help the microbes multiply. From each initial microbe present in the sample that can grow under those conditions, one visible blob of many microbial cells, called a colony, is present on the medium. The analyst then simply counts these and multiplies this number by the dilutions if necessary, to find out how many microorganisms are present in the initial ml of sample.

Knowledge of how many microbes are present in a sample, through quantitative culture, is a useful indication of the cleanliness of a sample. Urine, for example, in healthy people should not have any microbial contamination, and higher levels of contamination indicate the intensity of the infection. Through quantitative culture, a microbiologist can not only find out how many microorganisms are in the sample, but also how many of certain individual species. To do this, he or she has to mix the sample with special nutrient media, upon which only certain types of microbes grow, instead of a general media that allows lots of microbes to grow.

Some microbiological techniques allow existing microbes to grow before testing for their presence. As the initial amount of microorganisms in the sample multiplies, the numbers grow. These methods of microbiological analysis, although they can help identify the existence of specific microorganisms in a sample, cannot accurately count the levels of the microbes, and are not part of quantitative culture.