Streaking is a microbiological technique that is used to obtain pure cultures of microorganisms (particularly bacteria) in the laboratory. It usually involves a series of drawing a loop (carrying an inoculum of the test organism or sample) back and forth on dried solid culture media. As the streaking of the microbe on the dried solid culture media continues, the inoculum is thinned out along the streak lines as it is distributed along the culture media (Figure 1). By streaking or spreading the inoculum on the surface of a dried solid culture medium, single colonies of the inoculum will be obtained at some parts of the culture media plate during incubation.
Streaking is just one method of obtaining pure isolated colonies of bacteria. Other methods of obtaining pure cultures in the microbiology laboratory are the spread plate and pour plate methods. In spread plate method, the test organism or sample is serially diluted in broth culture media before they are transferred onto dried solid culture media and then spread using the inoculating loop. Pour plate methods involve serially diluting the test organism or sample in liquid culture media and then transferring same into molten agar. The molten agar containing the diluted inoculum or sample is then poured onto sterile Petri dishes and allowed to set or gel. Streak plating, spread plating and pour plating are the three known methods used to obtain pure cultures in the microbiology laboratory.
Figure 1: Illustration of streaking pattern in the microbiology laboratory.
Streaking is usually started at a point known as the primary inoculum on the solid media from where further streaks are made. As the streaking continues, the number of microorganisms on the loop reduces, and this ensures that a pure culture of the organism is obtained after the entire streaking process and incubation of the agar plate. From Figure 1 as shown above, 1= flame the inoculating loop over a Bunsen burner flame, and streak a loopful of the broth, sample or test organism at point 1 as shown in the figure; 2= re-flame the inoculating loop and allow it to cool, then streak the organism or specimen from point 1 to spread the original inoculum over more of the agar surface; 3= re-flame the loop again and continue the streak to point 3; 4= re-flame the inoculating loop again and allow to cool before streaking the inoculum from point 3 to point 4. Finally, label the culture plate and incubate it at the appropriate temperature condition while making sure that the Petri dish plate is inverted.
Culturing technique is used for the propagation of microorganisms in the microbiology laboratory; and it is an important procedure required for studying the morphological characteristics of microbes especially on solid culture media. Microorganisms (bacteria in particular) are cultured in vitro in various ways and in various conditions using different growth media (inclusive of solid media or broth/liquid media). Bacterial growth on solid culture media is often the best approach for studying the physical appearance of a microbe in terms of its consistency, shape, texture, size and colour among other colonial features of microbes on solid growth medium. Culturing technique involves the inoculation of growth media with the inoculum (e.g., bacteria or fungi) or test sample suspected to contain a particular microbe; and the incubation of the culture plate under optimum condition that encourage the growth of the test organism. The cultured organism can appear in any of these forms on the solid culture media: as individual or discrete colonies and as confluent growth (i.e. film of surface growth) as shown in Figure 2.
Confluent growth is often required for performing antimicrobial susceptibility testing in the microbiology laboratory. In broth or liquid medium, bacterial growth is usually inferred by the presence of turbidity or cloudiness in the medium; and a loopful of the organism can be transferred onto a solid culture media to obtain pure cultures. Various culturing techniques are used for the propagation of microorganisms in the microbiology laboratory. Examples of some notable culturing procedure include stab culture (produced by deep inoculation of solid medium in a tube with a straight wire loop), slope or slant culture (produced on the gradient of a solid medium in tubes) and plate culture. Note: Stab cultures are often used for biochemical tests and mycological investigations in which fungi or fungal samples are stabbed deep into solid media in tubes.
Figure 2: A nutrient agar culture plate. The organism is a clinical isolate of Klebsiella pneumoniae.
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