SELEC TIVE AND DIFFERENTIAL MEDIA
In health clinics and hospitals, it is necessary to detect microorganisms that are
associated with disease. Selective and differential media are therefore used.
Selective mediaare made to encourage the growth of some bacteria while
inhibiting others. An example of this is bismuth sulfite agar. Bismuth sulfite agar
is used to isolate Salmonella typhifrom fecal matter. Salmonella typhiis a gram-
negative bacterium that causes salmonella. Differential media make it easy to
distinguish colonies of desired organisms from nondesirable colonies growing
on the same plate. Pure cultures of microorganisms have identifiable reactions
with different media. An example is blood agar. Blood agar is a dark red/brown
medium that contains red blood cells used to identify bacterial species that
destroyred blood cells. An example of this type of bacterium is streptococcus
pyogenes, the agent that causes strep throat.
MacConkey agar is both selective and differential. MacConkey agar contains
bile salts and crystal violet, which inhibit the growth of gram-positive bacteria,
and lactose, in which gram-negative bacteria can grow.
Enrichment culturesare usually liquids and provide nutrients and environ-
mental conditions that provide for the growth of certain microorganisms, but
not others.
PURE CULTURES
Infectious material or materials that contain pathogenic microorganisms can be
located in pus, sputum, urine, feces, soil, water, and food. These infectious
materials can contain several kinds if bacteria. If these materials are placed on
a solid medium, colonies will form that are the exact copies of that same
microorganism. Acolonyarises from a single spore, vegetative cell, or a group
of the same organism that attaches to others like it into clumps or chains.
Microbial colonieshave distinct appearances that distinguish one microorgan-
ism from another.
Thestreak plate methodis the most common way to get pure cultures of bac-
teria. A device called an inoculating loopis sterilized and dipped into a culture
of a microorganism or microorganisms and then is “streaked” in a pattern over
a nutrient medium. As the pattern is made, bacteria are rubbed off from the loop
onto the nutrient medium. The last cells that are rubbed off the loop onto the
medium are far enough apart to allow isolation of separate colonies of the orig-
inal culture. (See Fig. 6-1.)
CHAPTER 6 Microbial Growth and Controlling It^107