What is Macconkey Agar? What is its composition? How does it differ from other media? What are its uses? And what are some common ways to prepare it?
We will take a closer look at these ideas to ensure that you have all the information you need to master MaccOnkey Agar with confidence.
What Is MacConkey Agar?
MacConkey Agar is a selective medium used to isolate gram-negative enteric bacteria; in other words, those bacteria which cause gastrointestinal infections such as diarrhea.
The principle behind the use of MacConkey Agar is based on the ability of certain types of bacteria to ferment maltose into acid (acetic) and gas (carbon dioxide).
These acids and gasses can then be detected by the growth of colonies of lactose-fermenting bacteria on the surface of the plate.
This method of detection was first described by Dr. Joseph Lister in 1884.
MacConkey Agars are named after their creator, Dr. John McConkie who developed this culture in 1909.
He called his new agar “McConkey’s Selective Enrichment Medium for Enterobacteriaceae.”
How Does MacConkey Agar Differ From Other Media?
There are many different kinds of media available for the isolation and identification of bacterial pathogens. The most commonly used media are:
- Blood agar – Blood agar is an excellent medium for isolating streptococci and staphylococci.
It also allows for the recovery of both aerobic and anaerobic organisms.
- Chocolate agar – Chocolate agar is a good medium for the isolation of coliforms and E. coli.
It is useful when there is concern about contamination with fecal material.
- Eosin methylene blue agar – Eosin methylene blue (EMB) agar is a selective medium for the isolation of Gram-positive cocci.
It is often used for the recovery of Staphylococcus species.
- Erythromycin base agar – Erythromycin base (ERY) agar is a very popular medium for the isolation of Streptococcus pneumoniae.
It is generally not recommended for use in clinical laboratories because it has been shown to select for resistant strains.
- Gram stain – Gram stains are used to identify bacteria using the presence or absence of specific cell wall components.
They are particularly helpful in identifying members of the family Enterobacteriaceae and the genus Neisseria.
- Lysine iron agar – Lysine iron agar is a selective solid medium designed for the isolation of Salmonella spp.
It is composed of lysed blood agar supplemented with ferrous sulfate and potassium tellurite.
- Methyl red/violet agar – Methyl red/violet (MRV) agar is a chromogenic medium used for the presumptive identification of Vibrio cholerae.
It contains bile salts and resins, which allow for the differentiation between non-cholera vibrios and V. cholerae.
- Mueller Hinton agar – Mueller Hinton agar is a general-purpose medium for culturing a variety of microorganisms including bacteria, fungi, yeasts, and molds.
It is widely used in microbiological studies of water quality.
- Oxoid nutrient broth – Oxoid nutrient broth is a general-purpose liquid medium that supports the growth of various microorganisms.
- Oxoid nutrient agar – Oxoid nutrient agar is a semi-solid medium that supports the growth and multiplication of a wide range of microorganisms.
It is used primarily as a vehicle for transporting specimens from one location to another.
- Phenylethanol naphthalene black agar – Phenylethanol naphthalene black agar is a selective differential medium for the detection of Vibrio parahaemolyticus.
It consists of phenylethyl naphthalene black agar plus thiosulfate citrate bile salt sucrose (TCBS).
- Phenylethanol naphthalene black agar – This medium is similar to the above but uses phenylethanol instead of phenyl ethanol and naphthalene instead of naphthalene.
- Salmon meat peptone agar – Salmon meat peptone agars are used to isolate Listeria monocytogenes.
They are prepared by adding salmon meat extract to brain heart infusion agar.
- Salmon meat trypticase soy agar – Salmon meat trypticase soy agar is a selective culture medium for the isolation of Listeria monocytogenes.
It consists of trypticase soy agar diluted 1:10 with distilled water containing 0.5% sodium chloride.
- Sterile sheep blood agar – Sterile sheep blood agars are used to detect Haemophilus influenzae. They are prepared by mixing sterile sheep blood with Brain Heart Infusion Broth.
- Trypticase soy agar – Trypticase soy is a general-purpose solid medium used to grow a wide variety of organisms.
It is commonly used in conjunction with other media such as chocolate agar and MacConkey agar.
- Yeast extract glucose agar – Yeast extract glucose agars are used to identify yeast species. They consist of dextrose agar plus yeast extract.
- Yeast extract maltose agar – Yeast Extract Maltose agars are used to differentiate between Candida albicans and C. glabrata.
They consist of deoxycholate citrate agar plus yeast extract maltose.
What Makes MacConkey Agar Different?
The most important difference between MacConkey agar and standard laboratory media is its ability to distinguish between gram-negative and positive bacteria.
The reason for this distinction is that many clinical laboratories use only Gram staining to determine whether or not an organism is gram-negative or positive.
However, it is now known that there are several different types of gram-negative bacteria.
For example, some strains of E. coli can be identified using biochemical tests alone, while others require additional testing.
In addition, certain species of Pseudomonas aeruginosa can also be distinguished by their colony morphology on MacConkey agar, which is usually round and smooth.
What Is MacConkey Agar Used For?
MacConkey agar is used to identify various bacterial species. However, it should not be confused with standard laboratory media, since it does not support the growth of all bacteria.
Instead, it is designed specifically to distinguish between gram-negative bacteria and gram-positive cocci.
Because of this specificity, it is often used to identify Enterobacteriaceae, especially Escherichia coli, Salmonella spp., Shigella spp., and Yersinia spp.
Since these organisms are gram-negative, they will form characteristic colonies on MacConkey agars.
In addition, the presence of lactose fermentation indicates that the bacterium is a member of the Enterobacteriaceae family.
Other common members include Aeromonas hydrophila, Citrobacter freundii, Klebsiella pneumoniae, Proteus mirabilis, Providencia rettgeri, Serratia marcescens, and Vibrio cholerae.
Other organisms that may be found on MacConkey agar include Campylobacter jejuni, Clostridium difficile, Corynebacterium sp., Haemophilus influenzae, Neisseria gonorrhoeae, Peptococcus sp., Pneumocystis carinii, Staphylococcus aureus, Streptococcus pyogenes, Treponema pallidum, Ureaplasma urealyticum, and Viruses.
Composition Of MacConkey Agar
The composition of MacConkey Agar is as follows:
- 17gm Peptone (Pancreatic digest of gelatin)
- 3gm Protease peptone (Meat and casein)
- 10gm Lactose monohydrate
- 1.5gm Bile salts
- 5gm Sodium Chloride
- 0.03gm Neutral red
- 0.001g crystal violet
- 13.5gm agar
- Distilled water to make 1 liter
Preparation Of MacConkey Agar
To prepare MacConkey Agar, you will need to take the following steps:
- Suspend 49.53g of the dehydrated medium inside 1000ml of distilled water
- Heat the mixture to boiling, ensuring that the medium is completely dissolved.
- Sterilize the mixture by opting to autoclave at 121 degrees centigrade, 15 lbs pressure for 15 minutes
- Cool the solution to between 45 and 50 degrees
- Mix the entire thing well
- Pour the mixture onto sterile Petri plates
You can then use the MacConkey Agar to locate and isolate any gram-negative bacteria which may be present.
Result Interpretation On MacConkey Agar
When interpreting a MacConkey Agar test, you need to look for the following characteristics:
- Red coloration around the bacterial colony indicates the presence of lactic acid bacteria
- A pinkish coloration indicates the presence of coliforms
- Blue/green coloration indicates the presence or absence of sulphite reducing clostridia
- Yellow coloration indicates the presence/absence of enterococci
- White coloration indicates the presence /absence of streptococci
- Lack of coloring indicates the presence of yeasts
If you find yourself unable to interpret the result, you should consider performing another test such as the API 20E system.
This allows you to identify more than 100 different types of microorganisms, including many pathogenic species.
The morphology of the colonies grown on MacConkey agar varies according to the type of bacteria present.
For example, some strains of E. coli produce small, round, smooth colonies whereas others grow into large, wrinkled colonies.
The shape of the colonies also depends upon the conditions under which they are grown.
For example, when incubated at 37 degrees centigrade, Enterobacter cloacae produces circular, mucoid colonies, whereas when incubated at 44 degrees centigrade, the same strain forms long, filamentous colonies.
Are There Any Limitations To MacConkey Agar?
In theory, there are no limitations to using MacConkey agar in laboratories – this remains one of the most commonly used media in microbiological research.
However, it is important to remember that it cannot be used to detect all Gram-negative bacteria.
For instance, if you were looking for an organism such as Pseudomonas aeruginosa or Acinetobacter baumannii, you would need to supplement your test with other methods.
You could do this by growing the sample on blood agar, chocolate agar, thiosulphate citrate bile salt sucrose (TCBS), or trypticase soy broth supplemented with 5% sheep’s blood.
Another limitation is that MacConkey agar can sometimes cause limited growth of bacteria, and it may be the case that you do not have enough of a sample to perform a test that can offer definitive results.
MacConkey Agar can be a very useful addition to any lab, and learning how to use and make the most of it is an important part of identifying gram-negative bacteria quickly and easily.
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