What is XLD agar? How does it differ from other media? What are its advantages and disadvantages?
To answer these questions and more, we took a closer look at all you need to know about Xylose Lysine Deoxycholate (XLD) agar.
What Is Xylose Lysine Deoxycholate (XLD) Agar?
Xylose Lysine Deoxycholate (XLD) agar is an agar medium that has been formulated for the isolation of lactic acid bacteria (LAB).
It was developed by Dr. J.S. Kornberg in the late 1960s as a replacement for de Man Rogosa Sharpe (MRS) agar.
The name “deoxycholate” comes from the fact that this agar contains sodium deoxycholate, which is used as a detergent agent to help remove unwanted proteins from the surface of the LAB colonies during colony formation.
The composition of xylose lysine deoxycholate (Xld) agar includes:
- A mixture of carbohydrates (xylose, glucose, sucrose, maltose, lactose), amino acids (lysine, arginine, proline, ornithine, phenylalanine), and salts (potassium phosphate, magnesium sulfate, calcium chloride, potassium hydroxide, sodium bicarbonate, to name a few).
- Sodium deoxycholate (0.2% w/v)
- Magnesium sulfate heptahydrate (1.8%)
- Potassium phosphate monobasic (10%), dibasic (20%) or tribasic (30%).
How Does XLD Agar Differ From Other Media?
As mentioned earlier, XLD agar is a replacement for MRS agar. This means that it can be used to isolate LAB from food samples.
However, it should not be confused with MRS agar because they have different purposes.
While MRS agar is designed to grow only LAB, XLD agar supports the growth of both LAB and yeasts.
In addition, XLD agar also differs from MRS agar in terms of its pH and salt concentration.
For example, while MRS agar has a pH range of 5.5 – 7.0, XLD agar has a pH between 6.0 – 7.4.
Moreover, MRS agar contains 0.3% NaCl whereas XLD agar contains 1.8%.
Advantages Of Using XLD Agar
There are several advantages associated with using XLD agar over other media, and these include:
Since XLD agar is less viscous than MRS agar, it is easier to pour into Petri dishes.
This makes it much easier to spread the agar evenly across the surface of the plate.
Easy Growth Of Bacteria And Yeast
Unlike MRS agar, XLD agar allows the growth of both Lactobacillus species and yeast.
This makes it ideal for isolating LAB from foods such as dairy products, meat, bread, wine, etc.
Unlike most agars, XLD agar promotes the formation of large colonies within 48 hours.
This makes it easy to distinguish between bacterial and fungal colonies.
Disadvantages Of Using XLD Agar
While there are many benefits associated with using XLD, there are some drawbacks too. These include:
Compared to other media such as MRS agar, the cost of XLD agar is quite high. This is due to the fact that it requires special equipment to prepare.
Moreover, since it is made up of chemicals, it needs to be stored properly.
Difficult To Prepare
Because XLD agar is prepared using chemicals, it takes time to make.
Therefore, if you do not have access to specialized laboratory equipment, then it will be difficult to use.
Not Suitable For All Types Of Samples
Although XLD agar is suitable for isolating LAB, it may not work well for all types of samples.
For instance, it does not support the growth of certain strains of LAB.
What Are Some Applications Of XLD Agar?
XLD agar is widely used in a number of scientific applications, and these include:
One of the main uses of XLD agar includes food safety.
Since this medium supports the growth of both bacteria and fungi, it can be used to identify pathogens in food samples.
It can also be used to determine whether a food sample is contaminated by pathogens.
Another important application of XLD agar involves diagnosing diseases.
The reason why this is possible is that this medium supports the growth not just of pathogenic bacteria but also of nonpathogenic ones.
Therefore, it can be used for identifying the presence of disease-causing organisms in patients.
Another common use of XLD agar relates to research.
Scientists often use this medium to study the effects of various factors on the growth of microorganisms.
By doing so, they can find out which conditions promote or inhibit their growth.
In addition to the above, XLD agar can also be used in other areas, including:
This medium is commonly used in microbiology laboratories.
Microbiologists use it to isolate and grow different kinds of microorganisms. They also use it to detect the presence of pathogens in food samples.
This medium is also used in biotechnological processes. Biotechnologists use it to culture cells, tissues, and organs.
In addition, they use it to produce enzymes and proteins.
Scientists also use XLD agar to culture cells and tissue.
When culturing cells and tissue, scientists usually need to add nutrients and other components into the medium.
Scientists sometimes use XLD agar when studying the nutritional requirements of microorganisms.
They use this medium to test the ability of microorganisms to utilize carbohydrates, amino acids, vitamins, etc.
This medium is also useful in pharmaceutical industries. Pharmaceutical companies use it to develop new drugs.
XLD Agar Culture Characteristics
The following are some characteristics of XLD agar that enable you to distinguish between good and bad colonies:
- Colonies appear as small roundish shapes with smooth edges. If this is the case, then your colony is likely to be good. However, if your colony has irregular edges, then it is probably a poor one. If your colony appears as a flat surface, then it is also most likely a poor one. This is because the growth of bacteria tends to be inhibited by high concentrations of salt.
- If your colony appears as a large mass, then it is most probably a poor one. This happens because the growth of bacteria is inhibited by low temperatures.
- Colonies are white in color – this can mean that your colony contains no bacteria. On the other hand, if your colony is yellow, orange, pink, red, purple, blue, green, black, brown, gray, or any other colors, then your colony may contain bacteria.
- Colonies are slightly raised from the surface of the agar. This can indicate that your colony contains many bacteria. However, if your colonies have sunken centers, then your colony may lack bacteria.
- Colonies do not spread over the entire surface of the plate. If your colony spreads over the entire surface of your plate, then it is a good one. If your colony does not spread over the entire plate, then it is probably poor.
How To Prepare XLD Agar
To prepare XLD agar, follow the steps below:
- Add 1% xylose lysine deoxycholate (Xld) to distilled water.
- Stir until all the ingredients dissolve completely.
- Pour the mixture into sterile Petri dishes.
- Allow the mixture to solidify at room temperature.
- Store the plates in an incubator at 37° C.
- Incubate the plates for 24 hours.
- Observe the plates daily.
- Record your observations in a lab notebook.
- Make sure that you record the date and time of each observation – this will be crucial when it comes to comparing the different results and transforming raw data into a valid, legitimate hypothesis.
Are There Any Limitations That Can Affect The Results Of XLD Agar?
As with any experiment or scientific medium, there are limitations that can affect the results of XLD agar. Some of these include:
It is essential that the medium be stored at 4° C. If the medium is stored at higher temperatures, then it may lose its effectiveness.
It is recommended that you store the medium for no more than 3 months. After 3 months, the medium may become inactive.
If you keep the medium in a refrigerator, make sure that the temperature does not exceed 10° C.
The medium should always be kept away from direct sunlight, or you risk the results becoming contaminated.
What Factors Will Impact Experiments That Use XLD Agar?
There are many factors that will affect the outcome of experiments using XLD agar. These include:
Amount Of XLD Added
The amount of XLD added to the medium determines how well the bacteria will grow.
Too little XLD will result in poor growth, whereas too much XLD will inhibit growth.
It is important that you incubate the plate for 24 hours before recording your observations.
This allows enough time for the bacteria to grow on the medium.
You must observe the colony every day during the first week of incubation.
Then, you can reduce the number of days that you observe the colony to once per week.
Too large a volume of medium will cause the bacteria to grow faster, resulting in dense colonies.
A smaller volume of medium will allow the bacteria to grow slowly, resulting in sparse colonies.
Different bacterial strains require different amounts of nutrients.
Therefore, if you use one strain of bacteria, you need to adjust the concentration of XLD accordingly.
The size of the inoculum affects the rate at which the bacteria grows. Smaller inoculums will take longer to grow.
Some methods of inoculating the medium are better than others.
For example, when you pour the liquid onto the surface of the medium, you might get clumps of cells growing on top of each other.
When you spread the inoculum evenly over the entire surface of the medium, however, you will get fewer clumps of cells.
XLD is a key ingredient in many media used by microbiologists, and its range of applications means that it is important to have a good understanding of its uses and principles – this will help you to solve a host of questions and experiments.
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