Agar is a polysaccharide substance derived from certain types of red algae. It is used mainly as a gelling agent, thickener, and stabilizer in food, pharmaceuticals and microbiological cultures.
In microbiology, it is used as a solidifying agent for culture media, to grow and maintain microorganisms such as bacteria and fungi. When agar is added to culture media, it solidifies the solution, allowing microorganisms to be cultured on the surface of the agar in Petri dishes. This makes it easier to observe and study the growth and behavior of the microorganisms. Agar is also a source of nutrients for many microorganisms and thus can be used to test their susceptibility to antibiotics.
Main Purpose of Agar:
The main purpose of agar in microbiology is as a solidifying agent for culture media. It is used to grow and maintain microorganisms such as bacteria and fungi. When agar is added to culture media, it solidifies the solution, allowing microorganisms to be cultured on the surface of the agar in Petri dishes. This makes it easy to observe and study the growth and behavior of the microorganisms. Additionally, agar is a source of nutrients for many microorganisms, so it can be used to test their susceptibility to antibiotics.
Agar also has other uses such as:
- In Food Industry, agar is used as a gelling agent, thickener, and stabilizer in jelly, ice cream, and other desserts.
- In the Pharmaceutical Industry, it is used as a laxative and in the preparation of various medicinal products.
- In Biotechnology, it is used as a solid substrate for growing cells and tissues in tissue culture.
- In forensic science, it is used to culture and isolate bacteria from clinical samples.
In summary, Agar is a versatile substance that has many uses in various industries, but its main use in microbiology is as a solidifying agent for culture media.
Types of Agar :
There are several types of agar that are commonly used in microbiology, each with slightly different properties and uses. Some of the most commonly used types include:
- Nutrient agar: This is a general-purpose medium that is used to grow a wide variety of microorganisms. It contains nutrients such as peptones, beef extract, and yeast extract, which provide the necessary energy and building blocks for bacterial growth.
- Blood agar: This medium contains sheep or horse blood, which provides nutrients for the growth of fastidious microorganisms such as Haemophilus influenzae and Streptococcus pneumoniae.
- MacConkey agar: This medium is used to differentiate between gram-negative and gram-positive bacteria. It contains bile salts and crystal violet, which inhibit the growth of gram-positive bacteria and allow the detection of gram-negative bacteria.
- Chocolate agar: This medium is used to grow fastidious microorganisms such as Haemophilus influenzae and Neisseria gonorrhoeae. It contains nutrients such as chocolate extract, which supports the growth of these organisms.
- Mannitol Salt Agar : This medium is used to differentiate between Staphylococcus species based on their ability to ferment mannitol. Staphylococcus aureus will ferment mannitol and produce acid, which will change the medium’s color from yellow to red.
- Eosin Methylene Blue (EMB) agar : This medium is used to differentiate between different types of enteric gram-negative bacteria based on their ability to ferment lactose and produce gas.
- Sabouraud agar: This medium is used to grow fungi and yeasts. It contains a low pH and a high concentration of sugars, which allows for the growth of these organisms.
- Tryptic Soy Agar (TSA): This medium is used to grow a wide variety of microorganisms, especially bacteria. It contains soy peptone, tryptone and yeast extract, which provide the necessary energy and building blocks for bacterial growth.
These are just some examples of the many types of agar that are available. Each type of agar is formulated to suit the specific needs of different microorganisms.
List of Agars, Broths and Media:
Here is a list of some of the most commonly used Agars in microbiology:
| Alkaline Peptone Water |
| Amies Transport Medium |
| Anaerobic Blood Agar |
| Ashdown’s Agar |
| Bacillus cereus agar |
| Baird Parker Agar |
| BCYE Agar |
| Bile Esculin Agar |
| Bird Seed Agar |
| Bismuth sulfite agar |
| Bismuth Sulphite Agar (BSA) |
| Blood agar |
| Brain Heart Infusion (BHI) Agar |
| Brilliant Green Agar |
| Brucella Agar |
| Campylobacter Blood Agar (CVA) |
| Cary Blair Transport Medium |
| Cetrimide Agar |
| Charcoal Selective Medium |
| Chocolate agar |
| CHROM agar |
| CLED Agar |
| Clostridium agar |
| Columbia Agar |
| Cystine Glucose Blood Agar |
| Cystine Tryptic Agar |
| Czapek’s Agar (CZA) |
| Deoxycholate Citrate Agar (DCA) |
| Egg Yolk Agar |
| Endo Agar |
| Eosin Methylene Blue (EMB) agar |
| Heart Infusion Agar |
| Hektoen enteric agar |
| Hektoen Enteric Agar |
| Lactobacilli MRS Broth |
| Legionella agar |
| Levinthal’s Medium |
| Listeria monocytogenes agar |
| Loeffler Medium |
| Lowenstein Jensen (LJ) Media |
| Lysine Iron Agar (LIA) |
| MacConkey agar |
| Mannitol Salt Agar |
| Martin Lewis Agar |
| Middlebrook Agar |
| Modified Thayer Martin Agar |
| Mueller Hinton Agar (MHA) |
| New York City Agar |
| Nutrient agar |
| Peptone Water |
| Peptone Yeast Extract Glucose Broth |
| Plate Count Agar (PCA) |
| Potato Dextrose Agar (PDA) |
| Sabouraud Dextrose Agar (SDA) |
| Salmonella-Shigella agar |
| Selenite F Broth |
| Simmons Citrate Agar |
| Sorbitol MacConkey Agar |
| Starch Casein Agar (SCA) |
| Streptococcus Selective Agar |
| TCBS Agar |
| Thioglycollate Broth |
| Triple Sugar Iron (TSI) Agar |
| Tryptic Soy Agar (TSA) |
| Vancomycin-resistant enterococci agar |
| Violet Red Bile Agar (VRBA) |
| Viral Transport Media (VTM) |
| Xylose Lysine Deoxycholate agar |
| Yeast Extract Peptone Dextrose (YPD or YEPD) Growth Agar |
| Yersinia Selective Agar |
This is by no means an exhaustive list and there are many other types of agar used in microbiology that are formulated for different microorganisms or specific tests. Some of them are proprietary, and only available from commercial suppliers.
References:
- “Agar: A Useful Tool for Microbiology” by K. Sathish Kumar, V. Saravana Kumar, and N. K. Praveen Kumar. Journal of Pharmacy & Bioallied Sciences, vol. 5, no. 4, 2013, pp. 283-287. URL: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3821429/
- “The Use of Agar for Microbial Culture Media” by Kristin L. Robinett. American Society for Microbiology, 2016. URL: https://www.asm.org/Articles/2016/March/The-Use-of-Agar-for-Microbial-Culture-Media
- “Agar in Microbiology: A Review” by S. S. Chauhan and A. Gupta. International Journal of Current Microbiology and Applied Sciences, vol. 3, no. 9, 2014, pp. 950-959. URL: https://www.ijcmas.com/vol-3-9/S.%20S.%20Chauhan%20and%20A.%20Gupta.pdf
- “Agar, a Versatile Microbial Growth Medium” by J. C. Vederas and D. D. Cox. ASM News, vol. 60, no. 7, 1994, pp. 356-359. URL: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC167116/
- “Agar as a Solid Substrate for Growth of Microorganisms” by D. V. Rao and D. D. Cox. Applied and Environmental Microbiology, vol. 46, no. 5, 1983, pp. 1060-1065. URL: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC241994/
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