When a medical professional suspects that a patient is ill due to the presence of a pathological (“bad guy”) microbe, a clinical sample is obtained from the patient. A clinical sample is material taken from a human (or an animal, in the case of veterinary medicine), such as a throat swab, or urine specimen.
Normal Flora and Clinical Samples
Depending on the area of the body where the clinical sample was obtained, the sample may contain many different types of microbes, some of which could be normal flora. Normal flora are microbes that live on and in the human body, usually without causing harm, and often providing some benefit. But the physician is only interested in the identity of the harmful bacteria that is causing the illness.
Since several different types of normal flora may be present in the clinical sample, in addition to a possible pathogen, all microbes in the sample must be isolated and identified. In order to find and identify the pathogen, it must first be grown in a pure culture.
Cultivating Microorganisms to Obtain Pure Cultures
A pure culture consists of cells that are all of the same type, either all having resulted from one parent cell, or having arisen from a group of related cells. There are several different microbiology methods that scientists can use to isolate pure cultures of bacteria. Two common methods are streak plate and pour plate techniques.
Streak Plating Isolation Technique
Streak plating is a procedure in which the clinical sample is smeared onto a section of agar (a type of bacterial growth medium) in a Petri dish and then spread out in progressive steps that, if done correctly, result in the growth of individual colonies appearing once the plate has been incubated.
A bacterial colony is a dot of microbes on the growing medium that arose from a single parent cell, so all the cells in a colony are of the same type. Each colony can then be sampled and cultivated so that all types of bacteria in the sample can ultimately be identified. Identification of the bacteria is accomplished after pure cultures are obtained, by using additional tests.
Streak plating can only be used to grow microbes that can tolerate oxygen, since all cultured microbes in a streak plate grow in the surface of the agar.
Pour Plating Isolation Technique
When pour plates are used to isolate bacteria, a sterile liquid growing medium is combined, in a test tube, with material from the clinical sample. Then a small portion of this mixture is put into a second tube of liquid agar, further diluting the clinical sample. The mixture is progressively diluted in this fashion, each time a small amount of liquid from the previous dilution being added to a new tube of sterile liquid growing medium.
Once the clinical sample has been sufficiently diluted, the mixture is poured into a petri dish in combination with warm liquid agar. The agar is a gelling agent, which transforms the mixture into a jello-like solid upon cooling. After incubation, the plate will have separate colonies growing both on the surface and within the agar. Using this method allows both aerobic and anaerobic bacteria to be cultivated, and ultimately identified.
To learn more about microbiology, see Todar’s Online Book of Bacteriology or visit the Virtual Microbiology Classroom of the science education website Science Prof Online.
Sources
- Bauman, R. (2007). Microbiology with Diseases by Taxonomy. Pearson Benjamin Cummings.
- Leboffe, M. J. and Pierce, B. E. (2010) Microbiology Laboratory Theory and Application, Third Edition. Morton Publishing Company.
- Perry, J. and Stanley, J. (1997) Microbiology Dynamics & Diversity. Saunders College Publishing.
Tami Port - Tami Port is a college professor of cell and microbiology and creator of ScienceProfOnline.com, a free science education website.
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