The spread plate technique is an important method to master in the world of microbiology, as it allows the isolation and enumeration of microorganisms that are present in a mixed culture, and distributes the different colonies evenly for easier access and analysis.
Getting to grips with the basics of the spread plate technique is crucial – and we have put together everything you need to know about the principle, procedure, and uses of this technique.
What Is Spread Plate Technique?
The spread plate technique is used to isolate and count bacteria from mixed cultures.
This is done by spreading a small amount of liquid containing the bacteria onto a solid medium. If the bacteria grow on the surface of the medium, they can be counted using standard methods.
What Are The Main Uses of The Spread Plate Technique?
This technique is important for all students who want to learn how to work with bacterial cultures.
Students working with any kind of laboratory techniques will find this technique useful. There are many uses for the spread plate technique.
Here are just some examples:
Isolating And Counting Bacteria From Mixed Cultures
This is one of the primary uses of the spread plate technique, and it involves isolating individual colonies from a mixture of different types of bacteria.
For example, say you have grown a sample of soil containing both Gram-positive and Gram-negative bacteria.
If you isolate the bacteria using the spread plate technique, then you can identify which species they belong to by examining their morphology.
Determining The Viability Of An Antibiotic
The spread plate technique is also used to determine whether an antibiotic is still effective against certain bacteria.
Simply put, you take a sample of bacteria from a patient who has recently taken antibiotics and grows them on agar plates.
Then, you add the appropriate amount of antibiotic to these plates and wait for the bacteria to develop into colonies.
Once they do, you count how many colonies appear. If the number of colonies is significantly reduced compared to a control group, then you know that the antibiotic is no longer working.
Identifying Bacterial Contamination
The spread plate technique is also useful when trying to detect bacterial contamination.
To begin with, you simply take a swab from a clean area of skin (such as your hand) and use it to streak a small portion of agar medium.
Next, you add a drop of inoculum to the center of the plate.
Finally, you cover the entire surface of the plate with another piece of sterile paper towel and let it sit for 15 minutes. After this time, you will see tiny white dots appearing on the plate.
These are the colonies of bacteria that were present on the swab. You can now use the spread plate technique to confirm that the swab was contaminated with bacteria.
How Do I Use The Spread Plate Technique? Step By Step Instructions
1. Prepare Your Agar Plates
First, prepare your agar plates. Take two pieces of filter paper and cut them into quarters. Place 1/4 inch strips of agar on top of each quarter.
Now, fold the other three quarters over the agar so that only half of the strip remains visible. This should create four equal-sized squares. Repeat this process until you have eight agar squares.
2. Streak Inoculums Onto The Agar Squares
Take a cotton swab and place it onto a Petri dish. Using a pipette or dropper, transfer approximately 5 microlitres of inoculums onto the middle of the first square of agar.
Then, carefully move the tip of the pipette across the agar so that the inoculums spread out evenly across the whole surface of the agar.
3. Cover With A Paper Towel And Wait For Colonies To Form
Cover the inoculated agar with a piece of sterile paper towel. Leave it undisturbed for fifteen minutes.
During this period, the bacteria will start to form colonies. Remove the paper towel and gently tap the agar to remove any excess liquid.
4. Count The Number Of Colonies On Each Square
Using a pair of tweezers, pick up one colony at a time and place it in a new Petri dish. Count the number of colonies on each square. Record your results on a spreadsheet.
5. Compare Results From Control Group
Compare the results from the control group to those from the test group. Which group had more colonies? Was there any difference between the groups?
If the number of colonies in the control group was much higher than in the test group, then you know the swab was contaminated by bacteria.
However, if the number of colonies in both groups were similar, then you cannot be sure which bacteria caused the problem.
If the number of colonies in your test group is lower than expected, then you know that your antibiotic is not working anymore. But what if the number of colonies is still high?
Does this mean that the antibiotics are still effective?
6. Check For Cross-Contamination
Sometimes, cross-contamination occurs during testing.
If you find that the number of colonies in one group is very different from the other group, then you know that cross-contamination occurred.
To avoid cross-contamination, make sure that all equipment used in the experiment is cleaned thoroughly before being reused.
Also, always use fresh gloves when handling the samples.
If the number of colony counts from the control and test group are similar, then you do not need to worry about contamination.
However, if the numbers are significantly different, then you may want to repeat the experiment using a larger sample size.
Final Thoughts
The spread plate technique is an important element of microbiology because it helps us understand how bacteria behave under certain conditions.
It can also help us determine whether our antibiotics are working properly, and is a must-have in any scientist’s toolkit.
Ever since I was a little girl, I have been fascinated by science. Every holiday I would ask for the newest science kit, and spend weeks learning with it.
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