1.3.2: Types of Media - Physical Characteristics

Liquid Media -

Figure 1. Sterile broth media are shown on the left, and broth media with bacterial growth are shown on the right. Credit: Sherry Smith

Liquid media are commonly referred to as broths. Broths contain nutrients that have been dissolved in liquid water and do not contain agar. Both plant-based and animal-based ingredients may be used to produce broths. Some common broths you may encounter this semester in laboratory session include nutrient broths, brain heart broths, and tryptic soy broths. Broths may be used in flasks, bottles, or test tubes.

There are some advantages as well as some disadvantages to utilizing broths for culturing microorganisms. A few of the advantages presented with the use of broths include:

1. Broths permit the growth of large numbers of bacteria within a single container. Bacterial growth in a broth can be detected due to cloudiness. Sterile broths remain clear, while broths containing bacteria are cloudy in appearance.

   The culture tube on the left in Figure 1 contains sterile broth solution, indicated by the clear color, while the broth on the right contains a sample of the bacterium Staphylococcus aureus. The bacteria growing in the broth tube on the right caused the solution to become cloudy.

2. Another advantage of utilizing broths is the ability to grow organisms at various concentrations of oxygen. Organisms vary in their ability to tolerate the presence of molecular oxygen, O₂. Oxygen levels are highest at the surface of the liquid and decrease as the depth of the broth increases.
3. Broths can be used to conduct a variety of tests and microbiological techniques such as carbohydrate fermentation tests and serial dilutions, both of which we will explore in more detail in later laboratory exercises.

Semi-Solid Media -

Figure 2. Motility test positive for motility. Source: VeeDunn, Flickr, CC BY 2.0.

Semi-solid media are media that contain nutrients to meet the nutritional requirements of the microbe and a small percentage of agar, usually 0.5% or less. The semi-solid media allows for some movement of bacteria through the media if the bacteria are motile. Semi-solid media are commonly used for motility tests.

Motility tests are performed by introducing the bacteria of interest to the media within a test tube using an inoculation needle. The needle with the sample is stabbed in the center of the tube. If growth remains near the site of inoculation, the bacteria are deemed non-motile. If growth appears to radiate outward from the site of inoculation, the bacteria are deemed motile.

Solid Media -

Figure 3. Agar slant. Credit: Sherry Smith

Solid media are prepared like semi-solid media, with the exception that it contains a higher concentration of agar. Solid media contain the required nutrients and 1.5%–2% agar. Solid media may be placed in test tubes or petri plates.

When solid media is introduced to test tubes, the tubes will be placed in a slanted position, which allows the media to solidify along the side of the tube, producing what is referred to as a slant. Slants provide a means of storing microbes for a longer period in cold storage.

Figure 4. Sterile nutrient agar plate. Credit: Sherry Smith

When the media is poured into a petri plate and allowed to solidify, it produces a large surface area for the bacteria to be inoculated. Solid media in petri plates are used to perform several important isolation techniques, including the streak plate method. Solid media allow for the isolation of an individual species and allow the microbiologist to observe differences in the morphology, or form and appearance, of the colonies growing on the surface.

Figure 5. Petri plate with bacterial growth. Blue colonies are ampicillin-resistant Escherichia coli, while white colonies are ampicillin-susceptible. Credit: Sherry Smith

Petri plates may also be used to observe differences in colony appearance, including color, size, shape, and other visible characteristics. These observations can help microbiologists compare bacterial growth and begin making decisions about how to further test or identify microorganisms.