Quantitative culture is a microbiological technique used to count the number of microbes in a sample. The analyst dilutes the sample and incubates it with nutrients to allow the microbes to grow and form colonies, which are then counted to determine the number of microorganisms present. This method can also identify the presence of specific species of microbes.
Quantitative culture is a term that belongs to the field of microbiology. It describes a variety of techniques 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 microbes present, and therefore 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 large 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 to easily readable levels.
Microbiological samples can be anything that may contain microbes. Examples include food, blood or water. Some of these samples very often contain many microbes, such as ground meat, or have few or no microbes, such as 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 size of sample, such as 1 mL of water. This is so he or she can find out how many microbes are in that mL and then calculate how many microbes are present in the sample as a whole or sample source. If the analyst thinks the total microbial count in that mL of sample will be high, he dilutes to the point where the result will be at levels that are not too high to read.
A quantitative culture method involves one mL of the sample or dilution mixed with nutrients in a solid medium. Then the analyst incubates it at a specific temperature and time, to help the microbes multiply. From each initial microbe present in the sample that can grow under those conditions, a visible patch of many microbial cells, called a colony, is present on the medium. The analyst then simply counts them and multiplies this number by dilutions if necessary to find out how many microorganisms are present in the initial mL of sample.
Knowing 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 culturing, a microbiologist can find out not only how many microorganisms are present in the sample, but also how many of certain individual species. To do this, he must mix the sample with special nutrient media, on which only certain types of microbes grow, instead of a general purpose medium that allows many 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 microbiological testing methods, while 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.
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