INTRODUCTION
     An antiseptic is a substance that will kill or halt the growth of the microorganisms that cause disease.
An antiseptic should kill germs without damaging tissues or interfering with the body's natural defenses
against disease. Such substances, which may be used directly in treatment, are distinguished from
substances called disinfectants that are too strong to be used in treatment. Disinfectants arc more
generally used to sterilize clothing, instruments, and room? Antiseptics blossomed in the 1930's with
the discovery of sulfa drugs, used to fight a variety of disease organisms in human blood. Today a
variety of antiseptics are used for surgery, skin infections, and injected wounds. Some of the more
common types include ethyl alcohol, merthiolate, mercurochrome, phenol, hydrogen peroxide, and
certain detergents.
     Antibiotics, like antiseptics, will inhibit the growth of or kill microorganisms, but whereas
antiseptics are chemicals antibiotics are substances produced by living organisms. The principle
of one microorganism attacking another one is known as "antibiosis". A number of microorganisms
such as fungi, molds, and yeast have the ability to destroy disease-causing rnicroorganisms in
humans. Antibiotics, are among the most important life-saving medicines known today. They are
used to treat tuberculosis, syphilis, typhoid fever, scarlet fever, dysentery, pneumonia, and other
diseases. Many diseases treated with antibiotics today were considered fatal only twenty years
ago. Over 400 antibiotics have been discovered, but only a few have proven safe to administer to
humans. The most common types in use today include: penicillin, streptomycin, chloromycin,
aureomycin, terramycin, erythromycin, polymyxin, and bacitrycin. The search for new antibiotics
must continue since bacteria tend to produce strains resistant to the currently used drugs.
     Today scientists have developed routine procedures for testing the "killing ability" of various
antiseptics and antibiotics. The current method involves growing a field of bacteria on an agar
plate. Next 4 disks are soaked in different antiseptics or antibiotics of known strength. One of
the disks is now placed on each quadrant of the agar plate. When incubated for 24-48 hours
the bacteria can be observed growing on the surface of the agar. Any antiseptics or antibiotics
that can kill that strain of bacteria will create a zone of no growth around that disk. This "zone
of inhibition" can then be measured with a ruler to compare the degree of effectiveness of each
antiseptic and antibiotic. Use the following to better understand the entire process of preparing
and reading the agar plates.