Answer to Question #346186 in Microbiology for Letso

Question

Explain the procedures for isolation of microorganisms from soil using selective and differential media...

Answer 1

Selective media allow certain types of organisms to grow, and inhibit the growth of other organisms. The selectivity is accomplished in several ways. For example, organisms that can utilize a given sugar are easily screened by making that sugar the only carbon source in the medium. On the other hand, selective inhibition of some types of microorganisms can be achieved by adding dyes, antibiotics, salts or specific inhibitors which affect the metabolism or enzyme systems of the organisms. For example, media containing potassium tellurite, sodium azide or thallium acetate (at concentrations of 0.1 - 0.5 g/l) will inhibit the growth of Gram-negative bacteria. Media supplemented with penicillin (5-50 units/ml) or crystal violet (2 mg/l) will inhibit the growth of Gram-positive bacteria. Tellurite agar, therefore, is used to select for Gram-positive organisms, and nutrient agar supplemented with penicillin can be used to select for Gramnegative organisms.

Differential media are used to differentiate closely related organisms or groups of organisms. Owing to the presence of certain dyes or chemicals in the media, the organisms will produce characteristic changes or growth patterns that are used for identification or differentiation. A variety of selective and differential media are used in medical, diagnostic and water pollution laboratories, and in food and dairy laboratories. Three of the more common selective and differential media are described below and will be used in the laboratory exercise.

Selective and Differential Plating Procedure:

1. Obtain or pour plate of specified type of selective or differential media

2. Inoculate plate using a loop, swab or hockey stick.

3. Incubate 24-48 hours at optimal growth temperature of organism

4. Observe plates for appropriate reactions