What Is Agar?

Agar is a type of sugar in a gelatinous form made from algae and typically used to grow bacteria in a lab. Bacteria eat the agar and help scientists perform culture tests.

Agar is one of those materials that can be found in science laboratories. It is, in fact, one of the most distinctive of those materials mostly because of its unique smell. But what is agar? The best way to define agar is by talking about its chemical makeup. Agar is a polymer that is composed of subunits of galactose sugar.

According to the Biology-Online Dictionary:

Agar (noun): a gelatininous material derived from algae, specifically used as a culture medium of bacteria and other cells for diagnostic or laboratory experiments purposes.

The agar that we find in science laboratories has been dissolved first in boiling water and then cooled. That is why it looks gelatinous.

How Do We Obtain Agar? 

Agar is obtained from red algae in California or in different parts of East Asia. Agar is a component of the cell wall of those algae.

“Happiness and bacteria have one thing in common; they multiply by dividing!” — Rutvik Oza

But laboratory agar is not found in nature in the same state it is used in a science lab. First, it needs to be given its gelatinous texture but going through the process we have explained (boiled and then cooled).

What Is Agar Used For? 

Actually, agar is used for many different purposes. Its main purpose is to work as a culture medium for several different microorganisms (chiefly, bacteria).

But agar has other, more every day uses. For example, did you know that agar is also used in order to thicken ready-made sauce or soup? Well, it is. And it is also used in the manufacturing of different beverages, ice cream, and jelly; and, beyond the food industry, also in fabrics and cosmetics.

The reason why agar is used so widely is that, unlike good old gelatin, it can be used to culture bacteria. The problem with gelatin is that is can be degraded by bacteria. In other words, bacteria can eat gelatin but it cannot eat agar.

“Bacteria: the only culture some people have.” — Hesiod

But that is not the only reason why agar is preferable to regular gelatin. Agar is also used because is bother stronger and firmer than gelatin.

Having said all that, gelatin is still used for culturing bacteria when agar is not available.

What Kind of Agar Are There?

There are several types of agar available. Each of them has its own characteristics. Some of them are suitable for student use, while others should just be used by professional scientists mostly because there is a risk of contamination from them.

  • Blood agar: this type of agar is obtained from animal blood (mostly from sheep). Therefore, blood agar has animal blood cells. Blood agar can be used to grow most bacteria. This type of agar is not suitable for student use.
  • Chocolate agar: this type of agar is obtained from sheep blood. It is used for the growth of Haemophilus because it has the necessary factors to do so (X and V). Chocolate agar is a nutrient culturing medium. As such it can be used to grow species of organisms such as Neisseria and Hemophilus. The only problem is no hemolysis data can be obtained from this type of agar. So, if you need to differentiate between different Hemophilus species, you will need to perform other tests. Chocolate agar is not suitable for student use.
  • Luria Bertani agar or LB agar: this is actually not a type of agar as such but, rather, a subtype. LB agar is mostly used in microbiology. It is good for routine cultivation. LB agar can be used to grow microorganisms with the exception of fastidious organisms like Neisseria and Hemophilus. LB agar is suitable for student use.
  • MacConkey agar: this can only be used to grow negative bacteria. MacConkey is actually a powder. There are two versions of MacConkey agar: one type has added sugar lactose in it, while the other comes with no added sugars at all. E.coli can be cultured on MacConkey agar. This can be easily seen because of e.coli forms in red colonies. So, if you notice red colonies forming this means that e.coli are using sugar to develop. MacConkey agar is not suitable for student use.
  • Miller’s LB Agar: this is actually a variation of Luria Bertani agar or LB agar. The components are exactly the same as LB agar. The only difference is the proportion. Miller’s LB agar is suitable for student use but only as the generic formula is used.
  • Neomycin agar: this type of agar has an antibiotic called neomycin. Neomycin is usually found is different ointments and medications including also eye drops. This antibiotic has been used since it was first discovered in 1949 by Selman Waksman. Neomycin is not man-made, it is produced by a bacterium known as Streptomyces fradiae. What are the effects of neomycin agar? Mostly, it kills gram-positive and gram-negative bacteria. Some people are allergic to it so there is a potential of it being toxic. Neomycin agar can be used to culture microorganisms anaerobically. But it stops any gram-negative bacilli and staphylococci, which in turn it allows species of Streptococcus to grow. Neomycin is not suitable for student use.

“Bacteria keeps us from heaven and puts us there.” — Martin H. Fischer

  • Non-nutrient agar: although non-nutrient agar cannot be used for culturing bacteria, it is good for the growth of other microorganisms. Non-nutrient agar is not suitable for student use.
  • Nutrient agar: this type of agar grows many kinds of bacteria and fungi. Having said that, not all bacteria can grow on nutrient agar. The nutrient in this type of agar is a combination of beef broth and yeast extracts. Sometimes nutrient agar is too rich for some bacteria and too deficient for others. This type of agar is suitable for student use.
  • Sabouraud agar: this type of agar has an aminoglycoside antibiotic known as gentamicin. Because of its low pH, sabouraud agar kills most bacteria. This type of agar is not suitable for student use.
  • Tryptic soy agar: this type of agar is good for growing many microorganisms. It is mostly used for colony morphology.
  • Xylose lysine deoxycholate agar or XLD agar: this type of agar is used with stool samples for the growth of gram-negative bacilli. This type of agar is not suitable for student use.