BIO 171 Microbiology Module 4. Portage Learning

BIO 171 Microbiology Module 4. Portage Learning.Module 4.1: Microbial Growth and Control - Growth Media • Growth media is a liquid or solid formulated 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.