brane. The cell’s plasma membrane regulates substances that enter and exit
the cell during its life. Nutrients enter the cell as waste products exit the cell.
Damage to the plasma membrane proteins or phospholipids by physical or
chemical agents allows the contents of the cell to leak out. This causes the death
of the cell.
Proteins act as regulators in cellular metabolisms, function as enzymes (which
are important in all cellular activities), and form structural components in cell
membranes and cytoplasm. The function of a protein depends on its three-dimen-
sional shape. The hydrogen and disulfide bonds between the amino acids that
make up the protein maintain this shape. Extreme heat, certain chemicals and
very high or low pH can easily break some of these hydrogen bonds. This break-
age is referred to as the denaturingof the protein. The protein’s shape is changed,
thus affecting the function of the protein and ultimately bringing death to the cell.
Certain chemicals, radiation, and heat can damage nucleic acids. The nuclear
acids, DNA and RNA, carry the cell’s genetic information. If these are damaged,
the cell can no longer replicate or synthesize enzymes, which are important in
cell metabolism.Chemical Agents That Control Microbial Growth
The growth of a microorganism can be controlled through the use of a chemical
agent. A chemical agent is a chemical that either inhibits or enhances the growth
of a microorganism. Commonly used chemical agents include phenols, pheno-
lics, glutaraldehyde, and formaldehyde.PHENOLS AND PHENOLICS
Phenols are compounds derived from pheno (carbolic acid) molecules. Pheno-
lics disrupt the plasma by denaturing proteins; they also disrupt the plasma
membrane of the cell. As mentioned in Chapter 1, Joseph Lister used phenol in
the late 1800s to reduce infection during surgery.
Alcoholsare effective against bacterial fungi and viruses. However, they, are
not effective against fungal spores or bacterial endospores. Alcohols that
are commonly used are isopropanol (rubbing alcohol) and ethanol (the alcohol
we drink).(^114) CHAPTER 6 Microbial Growth and Controlling It