BIO 171 Microbiology Module 4. Portage Learning.Module 4.1: Microbial Growth and Control
- Growth Media
• Growth media is a liquid or solid formulated
... [Show More] to support microbial growth.
The media contains essential nutrients to provide the microbe with a source
of carbon/energy, which, in turn, encourages its expansion. The nutrient
composition often includes a rich source of sugars, amino acids, and
vitamins. Such media can also be referred to as a nutrient broth and is
commonly used to grow microbes in a suspension. Perhaps the most
common nutrient broth in the lab, LB media (lysogeny broth), is a
nutritionally rich liquid known for its ability to grow a vast array of microbes.
However, there are also occasions where a researcher may want to
specifically control what types of microbes can grow. For instance, a
researcher may want to establish conditions where various types of bacteria
will grow, while others will not (selective). On the other hand, a researcher
may want to establish conditions where several types of microbes can be
grown simultaneously while being able to distinguish one type from another
(differential). Altogether, these conditions would require specialized media.
- Selective Media
• Selective media allows for only the growth of certain microbes and, by
extension, restricts the growth of all others. Such an approach can be
accomplished in a variety of ways including using limiting amounts of
nutrients, varying degrees of pH (being either very acidic or very basic
media), or various chemical additives that limit unwanted microbial growth
(i.e. antibiotics). Selective media is often used in medical laboratories for the
cultivation of human pathogens such as Neisseria meningitides, the
potential causative agent of meningitis. Because Neisseria meningitides is a
relatively slow-growing organism, other bacteria, mold, or fungi often
outgrow the sample—remember medical samples often contain numerous
unknown microbes! —therefore, additives are included in the growth media
1
Wednesday, September 8, 2021
to specifically inhibit such foreign microbes and ensure the isolated growth
of Neisseria meningitides.
- Differential Media
• Differential media distinguishes between two, often related, microbes. For
instance, two microbes Escherichia coli and Salmonella are both Gramnegative but can be distinguished by the presence (or absence) of lactose
fermentation. If grown on the same differential media, E. coli ferments
lactose and turns the culture red. In contrast, Salmonella does not ferment
lactose, and the culture remains white/tan in color.
- Enriched Media
• Enriched media is used to grow fastidious microorganisms—organisms
with complex growth requirements such that if absent will not grow. Thus,
enriched medias contain the essential nutrients required for the growth of
this subset of microorganisms. Medias can be a combination of selective,
differential, and enriched.
Module 4.2: Agar Plates
- Growth media comes in two forms: liquid and solid. Solid growth media, often
contained within a sterile petri dish, is in its simplest form liquid growth media
that has had a hardening agent added. The addition of a polysaccharide
derived from seaweed (algae) extract creates a solid medium called agar.
Agar is used to create a solid, smooth surface on which microbes can grow.
As they grow, microbes form colonies that take on the appearance of
individual isolated dots. However, as the microbe increases in number, the
isolates merge into one another. Once the microbe has grown to cover the
entire plate, this is referred to as a lawn. As we will see below, various forms of
agar plates exist in order to aid in the growth and classification of a broad
range of microorganisms. [Show Less]