Antibiotic Sensitivity

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.

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.

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.

Antibiotics

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.

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

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