2. Antibiotics
• Antibiotics are antimicrobial agents produced by microorganisms that kill
or inhibit other microorganisms. This is the microbiologist's definition. A
more broadened definition of an antibiotic includes any chemical of
natural origin (from any type of cell) which has the effect to kill or inhibit
the growth of other types cells. Since most clinically-useful antibiotics are
produced by microorganisms and are used to kill or inhibit infectious
Bacteria, we will follow the classic definition. Pharmacologists refer to any
antimicrobial chemical used in the treatment of infectious disease as as
antibiotic.
• Antibiotics are low molecular-weight (non-protein) molecules produced as
secondary metabolites, mainly by microorganisms that live in the
soil. Among the molds, the notable antibiotic producers are Penicillium
and Cephalosporium, which are the main source of penicillin and its
relatives. In the Bacteria, the Actinomycetes, notably Streptomyces
species, produce a variety of types of antibiotics including streptomycin,
erythromycin, and the tetracyclines. Some Bacillus species produce
antibiotics such as polymyxin and bacitracin.
3. Antibiotics
• The most important property of a clinically-useful antimicrobial agent,
especially from the patient's point of view, is its selective toxicity, i.e., the
agent acts in some way that inhibits or kills bacterial pathogens but has
little or no toxic effect on the animal taking the drug This implies that the
biochemical processes in the bacteria are in some way different from
those in the animal cells, and that the advantage of this difference can be
taken in chemotherapy.
• Antibiotics may have a cidal (killing) effect or a static (inhibitory) effect on
a range of microbes. The range of bacteria or other microorganisms that
are affected by a certain antibiotic is expressed as its spectrum of action.
Antibiotics which kill or inhibit a wide range of Gram-positive and Gram-
negative bacteria are said to be broad spectrum . If effective mainly
against Gram-positive or Gram-negative bacteria, they are narrow
spectrum. If effective against a single organism or disease, they are
referred to as limited spectrum.
4. Antibiotics
• Natural penicillins, such as Penicillin G or Penicillin V, are produced by Penicillium
chrysogenum. They are effective against streptococcus, gonococcus and
staphylococcus, except where resistance has developed. They are considered
narrow spectrum since they are not effective against Gram-negative rods.
• Semisynthetic penicillins first appeared in 1959. A mold produces the main part of
the molecule which can be modified chemically. Many of these compounds have
been developed to have distinct benefits or advantages over penicillin G, such as
increased spectrum of activity (e.g. effectiveness against Gram-negative rods), or
effectiveness when administered orally. Amoxycillin and Ampicillin have
broadened spectra against Gram-negatives and are effective orally.
• Although nontoxic, penicillins occasionally cause death when administered to
persons who are allergic to them. In the U.S. there are 300 - 500 deaths annually
due to penicillin allergy. In allergic individuals a part of the penicillin molecule
attaches to a blood protein which initiates an IgE-mediated inflammatory
response.
6. KIRBY-BAUER TEST for Antibiotic
Susceptibility
The Kirby-Bauer test for antibiotic
susceptibility, called the disc diffusion test, is
a standard that has been used for years. It
has been replaced in clinical labs by
automated tests. But the K-B is still used in
some labs, or used with certain bacteria that
automation does not work well with. The
bacterium is swabbed on the agar and the
antibiotic discs are placed on top. The
antibiotic diffuses from the disc into the agar
in decreasing amounts the further it is away
from the disc. If the organism is killed or
inhibited by the concentration of the
antibiotic, there will be NO growth in the
immediate area around the disc: This is
called the zone of inhibition. The zone sizes
are looked up on a standardized chart to give
a result of sensititive, resistant, or
intermediate. Many charts have a
corresponding column that also gives the
MIC (minimal inhibitory concentration) for
that drug. In this experiment we will
determine the susceptibility of Staph species
to various antibiotics.
7.
8. INTERPRETATION
• Place the metric ruler across the zone of inhibition, at the widest diameter, and
measure from from one edge of the zone to the other edge.
• The disc diameter will actually be part of that number. If there is NO zone at all,
report it as 0 even though the disc itself is around 7 mm.
• Zone diameter is reported in millimeters, looked up on the Zone Size Interpretive
Chart, and result reported as S (sensitive), R (resistant), or I (intermediate).
• Record the results in Table 1
