The most popular and widely used medium used in microbiology laboratories
is the solidifying agent agar. Agar is a complex polysaccharide derived from red
algae. Very few microorganisms can degrade agar, so it usually remains in a
solid form. Agar media are usually contained in test tubes or Petri dishes. The
test tubes are held at a slant and are allowed to solidify on an angle, called a
slant. A slant increases the surface area for organism growth. When they solid-
ify in a vertical tube it is called a deep. The shallow dishes with lids to prevent
contamination are called Petri dishes. Petri dishes are named after their inventor,
Julius Petri, who in 1887 first poured agar into glass dishes.CHEMICALLY DEFINED MEDIA
For a medium to support microbial growth, it must provide an energy source,
as well as carbon, nitrogen, sulfur, phosphorous, and any other organic growth
factors that the organism cannot make itself, source for the microorganisms
to utilize.
Achemically definedmedium is one whose exact chemical composition is
known. Chemically defined media mustcontain growth factors that serve as a
source of energy and carbon. Chemically defined media are used for the growth
of autotrophic bacteria. Heterotrophic bacteria and fungi are normally grown on
complex media, which are made up of nutrients, such as yeasts, meat, plants, or
proteins (the exact composition is not quite known and can vary with each mix-
ture). In complex media, the energy, carbon, nitrogen, and sulfur needed for
microbial growth are provided by protein. Proteins are large molecules that some
microorganisms can use directly. Partial digestion by acids and enzymes break
down proteins into smaller amino acids called peptones. Peptones are soluble
products of protein hydrolysis. These small peptones can be digested by bacteria.
Different vitamins and organic growth factors can be provided by meat and
yeast extracts. If a complex medium is in a liquid form it is called a nutrient
broth. If agar is added, it is called a nutrient agar. Agar is not a nutrient; it is a
solidifying agent.ANAEROBIC GROWTH
Because anaerobic organisms can be killed when exposed to oxygen, they must
be placed in a special medium called a reducing medium. Reducing media con-
tain ingredients like sodium thioglycolate that attaches to dissolved oxygen and
depletes the oxygen in the culture medium.(^106) CHAPTER 6 Microbial Growth and Controlling It