Streaking (microbiology)
In microbiology, streaking is a mechanical technique used to isolate a pure strain from a single species of microorganism, often bacteria. Samples from a colony derived from a single cell are taken from the streaked plate to create a genetically identical microbiological culture grown on a new plate so that the organism can be identified, studied, or tested. Different patterns can be used to streak a plate. All involve the dilution of bacteria by systematically streaking them over the exterior of the agar in a Petri dish to obtain isolated colonies which contain gradually fewer numbers of cells. If the agar surface grows microorganisms which are all genetically same, the culture is then considered as a pure microbiological culture.
History
The modern streak plate method was developed in the 1880s from the efforts of Robert Koch and other microbiologists to obtain microbiological cultures of bacteria in order to study them. Prior to the adoption of streaking, pour plates were the common technique utilized by microbiologists to obtain pure strains. The dilution or isolation by streaking method was first developed in Koch's laboratory by his two assistants Friedrick Loeffler and Georg Theodor August Gaffky.Technique
Streaking is a rapid and simple process of microbe isolation through consecutive dilution. The technique is accomplished by reducing a comparatively large concentration of bacteria to a smaller concentration. The decrease of bacterial concentration should sufficiently spread apart colonies and allow for the separation of the different types of microbes in a sample. Streaking is done using a sterile tool and aseptic technique, such as a cotton swab or commonly an inoculation loop. If using a metal inoculation loop, it is first sterilized by passing it through a flame. When the loop is cool, it is dipped into an inoculum such as a microbial culture or patient specimen containing many species of bacteria. Aseptic techniques are used to maintain microbiological cultures and to prevent contamination of the growth medium.Early examples of streaking moved in a single direction across the plate, different from the back and forth "zig zag" motion seen nowadays. Many different methods have been developed to streak a plate. Picking a technique is a matter of individual preference and can also depend on how large the number of microbes the sample contains.The most common pattern used is Quadrant streaking, also called "four sectors streaking" and "four way streak method." Involves splitting the agar plate into four sections, or quadrants. To begin, a sterile loop starts in the first quarter of a plate and moves in a back and forth motion multiple times across the agar surface going from the outside of the plate into the center. Once each previous quarter is completed, the plate is turned 90 degrees and a newly sterilized inoculation loop must be used. Starting from the bottom of the previous quadrant, re-run over half of the streaks to pickup material before covering the next quarter. This process repeats moving through all four quadrants and will result in the final containing the most diluted section.
The three-phase streaking pattern, also known as the T-Streak, is recommended for beginners. However, it is also limited in applications using greater than a single culture. The plate is split by drawing a "T" to create three separate sections. Rotate the plate so that the top of the "T" is furthest from your dominant hand. Starting from this first section a sterilized inoculation loop is dragged across the surface of the agar back and forth in a zigzag motion until approximately a third of the plate has been covered. The loop then is re-sterilized and the plate is turned 90 degrees. Starting in the previously streaked section, the loop is dragged through it two to three times continuing the zigzag pattern before moving to cover a second section. The procedure is then repeated once more, being cautious to not touch the previously streaked sectors. Each time the loop gathers fewer and fewer bacteria until it gathers just single bacterial cells that can grow into a colony. The plate should show the heaviest growth in the first section. The second section will have less growth and a few isolated colonies, while the final section will have the least amount of growth and many isolated colonies.
For use in both dilutions and pure cultures, radiant streaking begins from streaking a small portion of agar on one side of the plate utilizing a sterile loop. Starting from the streaked section on the one side, make a set of vertical lines across the plate stretching to the other in a ray like pattern. Then switch to a new sterile inoculation loop and make horizontal lines crossing over the vertical as you go down the plate.
Continuous streaking is a method utilized to spread an even distribution of a sample across a plate for propagation, or increasing the size of the culture. It is implemented by starting from the outside and moving towards the inside of a plate in a single motion. This method is quick but only applicable for very diluted samples or in cases where a pure strain has already been achieved. In laboratories wishing to save material, a single plate can be divided into sections and a continuous streak used for a different material in each section. Allowing for a maximum number of samples to be streaked at one time.
Another continuous method is zig zag streaking and is also used to propagate culture samples. Starting from the side furthest from your dominant hand, move a sterilized loop back and forth across the plate. Use large motions across the entire width of the plate to cover the greatest area of the agar surface.