Microorganisms are grown in the lab in culture media. Culture media can be solid, semi-solid, or liquid. Nutrient media are prepared for the purpose of growing specific microorganisms.

Any culture medium contains a nitrogen source, a protein source, and a source of electrolytes, at a minimum including sodium chloride. Agar solidifies culture media, but it is possible to prepare a culture medium with or without it.

When agar is added to a culture medium, it is known as a solid nutrient medium, or an agar nutrient medium. When agar is not added to a culture medium, it is known as a nutrient broth or a nutrient liquid medium.

The difference between a nutrient agar medium and a nutrient broth is the presence or absence of agar.

What is a nutrient agar medium?

Nutrient agar is a general-purpose medium used for culturing bacteria. It is suitable for the growth of non-fastidious microorganisms, microbes that don’t need any special conditions or special nutrition to grow.

A prime example of one of the non-fastidious species of bacteria that grows in nutrient agar is E. coli. Others include staph and Pseudomonas.

Nutrient agar mix contains, of course, agar as a thickening agent, along with vitamins, a nitrogen source, carbohydrates.

What is a nutrient broth?

Nutrient broth contains the same ingredients as nutrient agar except agar as a solidifying agent. Otherwise, the composition of a nutrient broth is the same as a nutrient agar. Both nutrient agar and nutrient broth have an amber color in the culture vessel. Nutrient broth is almost always easier to prepare than nutrient agar.

A classic experiment for biology students that uses nutrient broth is determining the oxygen requirements of different species of bacteria. In lecture, they learn the principle that some bacteria are obligate aerobes, that is, they require oxygen. Students learn that other bacteria are facultative aerobes, that is, they grow best with but not absolutely require oxygen.

Still other bacteria are facultative anaerobes, preferring anaerobic conditions but tolerating the presence of oxygen, and others are obligate anaerobes that die in the presence of atmospheric oxygen.

These requirements can be easily observed when bacteria are grown in tubes filled with nutrient broth.

What are the differences between nutrient agar and nutrient broth?

Students need to know the key differences between nutrient agar and nutrient broth:

• Nutrient agar is a solid medium used for growing a wide variety of bacteria. Nutrient broth is a liquid medium used for growing a wide variety of bacteria.
• Nutrient agar contains peptone (solid protein broken down in the early stages of digestion), beef extract, yeast extract, sodium chloride, agar, and water. Nutrient broth contains peptone, beef extract, yeast extract, sodium chloride, and water, but no agar.
• Nutrient agar contains agar to create a gel. Nutrient broth does not contain agar, and remains a liquid.
• Nutrient agar can be poured into Petri plates, while nutrient broth is not, to prevent spillage and evaporation. Both nutrient agar and nutrient broth can be used to grow cultures in test tubes and flasks.

Common questions students have about nutrient agar and nutrient broth

Working with nutrient agar and nutrient broth is an important lab skill. Students don’t necessarily get everything to go to their satisfaction with their first attempt. Failure to read instructions concerning temperature, concentration, pH, and contamination produce results that lead to student questions, such as the following.

1. Why didn’t I get bacteria colonies on my streak plate?
2. Students may get colonies on their streak plates that don’t look like others. They may get colonies of bacteria where they did not streak the plate. And they may get no bacteria growth at all.

Typically, these problems arise from contamination with airborne bacteria or fungi. The problem can be that the bacteria in the source culture were dead. Dead bacteria look a lot like live bacteria. Sometimes the problem is that the student did not prepare the right growing medium.

The way to correct this problem is to make sure you are providing optimal growing conditions only while bacteria are actively growing. Only let the Petri dish, test tube, or flask stay at room temperature for 24 hours. Then refrigerate it for study later. Never leave a culture on the counter or lab desk over the weekend.

1. Why is my culture growing so slowly?
2. Fungi and yeasts can take 3 to 7 days to grow noticeably. Bacteria grow much faster. The problem can be that you aren’t actually growing bacteria.

Otherwise, the problem can be an error in mixing the nutrient agar or nutrient broth, not giving bacteria all the nutrients they need in the concentrations that they need them.

If you are growing aerobic bacteria, increasing the surface area of the growing medium (using a Petri dish) will result in faster growth. If you are growing anaerobic bacteria, a containment vessel with a small surface exposed to air is preferable.

1. I had bacteria growing in culture but they disappeared!
2. Some species of bacteria, especially Bacillus, lyse when they die. Fortunately, they usually leave spores behind, and you can revive the culture. If you cannot get the spores to grow, then you must start over with a new sample.
3. The plate has colonies that don’t look like the others? What should I do?
4. Your Petri plate has become contaminated. Take a loop and transfer a sample of the most common bacteria to another plate, and start over.

Modern Biology sells nutrient agar and nutrient broth.

Every Modern Biology kit includes all the reagents and test materials teachers need for their laboratory exercise. There’s no ordering separate reagents, fussing about missing shipments, or checking out lab materials from the supply room. Modern Biology supplies the safe, non-toxic, reliable reagents and measurement materials you need for every laboratory exercise. And because every Modern Biology experiment is available at a fixed cost, it’s easier to budget your supply cost for each class for each term.