SDM1 Selective and Differential Media 1
Learning Objectives
The student will
Use aseptic techniques in the safe inoculation of various forms of media.
Follow oral and written instructions and manage time in the lab efficiently.
Apply correct terminology regarding microbiological techniques, instruments, microbial
growth, biochemical testing, and media types when making observations.
Correctly perform various inoculation techniques and describe each technique’s purpose.
Make accurate observations and appropriate interpretations of biochemical test results and
use them in the identification of potentially disease causing microbes.
Background/Theory
In this course you will encounter many types of growth media. Most of the types, TSA/TSB
(Tryptic Soy Agar/Broth) and NA/NB (Nutrient Agar/Broth) for example, are all purpose or general
media. They contain a wide variety of complex carbon, nitrogen and sugar compounds. You will often
see terms like beef extract, peptone, tryptone, soytone in the list of ingredients. These components are
slurries of animal and plant tissue that have been partially hydrolyzed (broken down) so that the
components are more readily available. These media contain many different complex molecules,
however the exact amount and types of each is unknown. These media will support a wide range of
nonfastidious microbes with differing nutritional requirements.
Media that inhibit the growth of unwanted microorganisms and support the growth of the
organism of interest by supplying nutrients and reducing competition are called selective media.
(OpenStax CNX, 2018) Selective media are formulated with inhibitors such as antibiotics or high NaCl
concentration. When studying a mixed sample, selective media can be helpful. For example, if you
suspect a patient is carrying Salmonella (a pathogenic Gram negative bacillus), you may plate a stool
sample on a selective medium containing an antibiotic effective against Gram positive bacteria. By
eliminating the Gram positive organisms, the range of organisms growing on the plate will be narrowed
to Gram negatives. Thus, the variety of bacteria you will need to study is reduced.
The fact that a medium does not grow every microbe, does not make it selective. For example,
TSA is an all-purpose medium and a wide range of organisms grow on it. Certain fastidious organisms,
however, will fail to grow or grow poorly on TSA because it lacks the specific nutrients required by those
bacteria. Even so, TSA is not classified as selective. To be selective, a medium must contain a specific
substance intentionally added to inhibit certain microbes and not others.
Differential media contain substrates and indicators (often pH indicators) that make a certain
biochemical process visible. Differential media allow one to differentiate between types of organisms
growing on the plate because each has a distinct appearance based on whether or not it is carrying out a
particular biochemical reaction. “Color changes are the result of end products created by interaction of
bacterial enzymes with differential substrates in the medium or, in the case of hemolytic reactions, the
lysis of red blood cells in the medium” (OpenStax CNX, 2018). Differential media can be used to
distinguish between bacteria that can ferment a specific type of sugar and those that cannot or between
bacteria that utilize a certain electron acceptor and those that do not.
Some media are selective, some are differential and some are both. We will study Mannitol Salt
Agar and MacConkey agar as examples.
