Streak Plate Method- Principle, Methods, Significance, Limitations
- In microbiology, streaking is a technique used to isolate a pure strain from a single species of microorganism, often bacteria.
- The dilution or isolation by streaking method was first developed by Loeffler and Gaffky in Koch’s laboratory, which involves the dilution of bacteria by systematically streaking them over the exterior of the agar in a Petri dish to obtain isolated colonies which will then grow into the number of cells or isolated colonies.
- Streaking is rapid and ideally a simple process of isolation dilution.
- The technique is done by diluting a comparatively large concentration of bacteria to a smaller concentration.
- The decrease of bacteria should show that colonies are sufficiently spread apart to effect the separation of the different types of microbes.
- Streaking is done using a sterile tool, such as a cotton swab or commonly an inoculation loop.
- Aseptic techniques are used to maintain microbiological cultures and to prevent contamination of the growth medium.
Principle of Streak Plate
- The streak plate method is a rapid qualitative isolation method.
- The techniques commonly used for isolation of discrete colonies initially require that the number of organisms in the inoculums be reduced.
- It is essentially a dilution technique that involves spreading a loopful of culture over the surface of an agar plate.
- The resulting diminution of the population size ensures that, following inoculation, individual cells will be sufficiently far apart on the surface of the agar medium to effect a separation of the different species present.
- In the streaking procedure, a sterile loop or swab is used to obtain an uncontaminated microbial culture. The process is called “picking colonies” when it is done from an agar plate with isolated colonies and is transferred to a new agar or gelatin plate using a sterile loop or needle.
- The inoculating loop or needle is then streaked over an agar surface.
- On the initial region of the streak, many microorganisms are deposited resulting in confluent growth or the growth of culture over the entire surface of the streaked area.
- The loop is sterilized by heating the loop in the blue flame of the Bunsen burner, between streaking different sections, or zones and thus lesser microorganisms are deposited as the streaking progresses.
- The streaking process will dilute out the sample that was placed in the initial region of the agar surface.
Methods of Streak Plate
- There are many different types of methods used to streak a plate. There are two most commonly used streak patterns, a three sector “T streak” and four-quadrant streak methods.
- Picking a technique is a matter of individual preference and can also depend on how large the number of microbes the sample contains.
- The three-phase streaking pattern is known as the T-Streak.
- The streaking is done using a sterile tool, such as a cotton swab or commonly an inoculation loop.
- The inoculation loop is first sterilized by passing it through a flame.
- When the loop is cool, it is dipped into an inoculum such as a broth or patient specimen containing many species of bacteria.
- The inoculation loop is then dragged across the surface of the agar back and forth in a zigzag motion until approximately 30% 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.
- 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.
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 180 degrees, and inoculating the other half of the plate without sterilizing the loop or dragging bacteria from the previous section.
Discontinuous Streaking Method
- Sterilize the inoculating loop in the bunsen burner by putting the loop into the flame until it is red hot. Allow it to cool.
- Pick an isolated colony from the agar plate culture and spread it over the first quadrant (approximately 1/4 of the plate) using close parallel streaks or insert your loop into the tube/culture bottle and remove some inoculum. You don’t need a huge chunk.
- Immediately streak the inoculating loop very gently over a quarter of the plate using a back and forth motion.
- Flame the loop again and allow it to cool. Going back to the edge of area 1 that you just streaked, extend the streaks into the second quarter of the plate.
- Flame the loop again and allow it to cool. Going back to the area that you just streaked, extend the streaks into the third quarter of the plate.
- Flame the loop again and allow it to cool. Going back to the area that you just streaked, extend the streaks into the center fourth of the plate.
- Flame your loop once more.
Significance of Streak Plate Method
- The streak plate technique is the most popular method for isolating specific bacteria from a sample containing a mixture of microorganisms.
- Streak plate technique is used to grow bacteria on a growth media surface so that individual bacterial colonies are isolated and sampled.
- Samples can then be taken from the resulting isolated colonies and a microbiological culture can be grown on a new plate so that the organism can be identified, studied, or tested.
- When the bacteria are streaked and isolated, the causative agent of a bacterial disease can be identified.
Limitations of Streak Plate
Streak plating is a microbiology laboratory method that has two major disadvantages.
- Firstly, users will not be able to grow obligate anaerobes using this method.
- Secondly, only organisms that were viable in the original sample are able to be grown.
- Benson, H. J. (2005). Benson’s microbiological applications: Laboratory manual in general microbiology. Boston: McGraw-Hill Higher Education.
- James G. Cappuccino, Chad T. Welsh (2017). Microbiology: A Laboratory Manual, 11th Edition. Pearson Publications.
- Western Nevada College Biology 251 Laboratory Manual;”Three Streaks for Bacterial Isolation”; Dr. Steve Carman; 2009