Why streaking for isolation

She cools the loop by touching it to the agar, then dips the loop into the sample and spreads it back and forth to cover a section of the plate. She sterilizes the loop, cools it, and inoculates a second, adjacent section of the plate by dragging the loop through the first section several times and covering the second section using a zigzag motion. This picks up a small number of bacteria from the first section and transfers them to the second section.

The number of times this basic procedure is repeated depends on the streak plate method used. Despite the method, the original sample is used to inoculate the first section of the plate only. Streak plate methods vary by the number of agar sections streaked. The T-streak method uses three sections: the upper half and two equally sized bottom sections. The initial inoculum is placed in the top half of the plate. Bacteria are dragged from the top section to one of the bottom sections, then from that bottom section to the other.

In the quadrant method, four equally sized sections are streaked. The continuous streaking method typically involves inoculating the top half of the plate, rotating it degrees, and inoculating the other half of the plate without sterilizing the loop or dragging bacteria from the previous section. Cynthia Ruscitto has been writing professionally since Choice of which growth medium is used depends on which microorganism is being cultured, or selected for.

Growth media are usually forms of agar, a gelatinous substance derived from seaweed. Spread plates are simply microbes spread on a media plate. Microbes are in a solution, and can be diluted. They are then transferred to a petri dish with media specific for the growth of the microbe of interest. The solution is then spread uniformly through a number of possible means, the most popular is the use of sterile glass beads that are shook on top of the media, spreading the microbe-containing liquid evenly on the plate.

Also common is the use of a bent-glass rod, often referred to as a hockey stick, due to its similar shape. The glass rod is sterilized and used to spread the microbe-containing liquid uniformly on the plate. Diagrams a The diagram on the left shows the four quadrants and their zig-zag streaked patterned mentioned in steps two through four.

Note the thickest, densest amount of bacteria in quadrant one no isolated, individual colonies and the correspondingly fewer number of colonies in quadrants two, three and four. With isolated colonies in quadrant four, that would be easy to obtain without touching any other colony using a sterile loop to start an experiment! Source: Elte. Show More. Related Articles. Panthers Unite! One Comment. Through subsequent isolation of a single colony, where all units are derived from a common mother cell, the second streaking procedure generates a relatively clonal bacterial population, suitable for further characterization or inoculation into broth.

Subscription Required. Please recommend JoVE to your librarian. The initial streak-plate may contain colonies originating from cells with different genetic makeup or depending on sample purity from different bacterial species Figure 2 A. Through subsequent isolation of a single colony, where all units are derived from a common mother-cell, the second streaking procedure generates a relatively clonal bacterial population, suitable for further characterization or inoculation into broth Figure 2 B.

Figure 2: A pure culture can be generated from a mixed sample through isolation of a single, secluded colony. A Growth of a single bacterial cell CFU generated a clonal colony, separated from those of other species and strains. This CFU was used for subsequent streaking onto a new plate B A second plate, where the bacterial population consists solely of cells derived from the initial CFU. The ability to obtain and cultivate a pure bacterial colony is essential, both in clinical and academic settings.

Streak plating enables the isolation of a relatively clonal cell population, originating from a shared CFU, that may be of particular interest during diagnosis or for additional characterization of the isolate. A sample is streaked onto a suitable agar-based nutrient medium and incubated until colonies become visible. An isolated colony is subsequently harvested and re-streaked onto a second plate. To learn more about our GDPR policies click here.

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Ensure that agar plates, sample solution s and either a box of pre-sterilized plastic inoculation loops or a metal loop plus a Bunsen flame, are close at hand. Disposable, plastic loops are typically pre-sterilized. Allow the wire to cool down by raising the lid of the plate only slightly to prevent contamination and tapping it against the solidified medium.

Protocol Preparation of media Identify and prepare a solid medium typically containing 1. Mix the medium in a bottle able to hold twice the final volume to avoid overflow when autoclaving. Close the cap properly as soon as the bottle is removed from the autoclave. Preparation of culture plate s Mark the base of sterile Petri dishes typically x 15 mm on either the side or the bottom with the experimenter's name, the date, and media type.

Should foam appear along the edges, this should be swiftly removed using a regular pipette and a sterile tip. Immediately place all lids back onto the dishes to prevent contamination.

Streak plating Submerge a sterile loop into the desired inoculum and immediately disperse the collected sample onto the first quadrant of the plate using a zig-zag motion Figure 1, I. Close the lid and re-sterilize the inoculation loop or collect a new sterile disposable loop.

Make strokes radiating from the first quadrant containing a relatively dense bacterial population towards the second quadrant of the plate Figure 1, II.