PRESERVING BACTERIAL CULTURES
Two common methods of preserving microbial cultures are deep freezingand
lyophilizationor (freeze drying). Deep freezing is a process in which a pure cul-
ture of microorganisms is placed in a suspending liquid and frozen quickly at
temperatures ranging from −50 to −95 degrees Celsius. With this type of freez-
ing method, cultures can usually be thawed and used after several years.
Lyophilization, or freeze drying, quick freezes suspended microorganisms at
temperatures from −54 to −72 degrees Celsius while water is removed by using
a high-pressure vacuum. While under the vacuum the container is sealed with a
torch. The surrounding microbes in the sealed container can last for years. The
organisms can be retained and revived by hydrating them and placing them into
a liquid nutrient medium.GROWING BACTERIAL CULTURES
Bacteria normally reproduce by a process called binary fission:
1.The cell elongates and chromosomal DNA is replicated.
2.The cell wall and cell membrane pinch inward and begin to divide.
3.The pinched parts of the cell wall meet, forming a cross wall completely
around the divided DNA.
4.The cells separate into two individual cells.(^108) CHAPTER 6 Microbial Growth and Controlling It
Fig. 6-1. Streak plate method used
to isolate bacteria.