بسم الله الرحمن الرحيم. Antimicrobial Chemotherapy to Control Microorganisms. 2013-2014. Diagnostic Medical Microbiology-Laboratory Manual. Antimicrobial Control Agents.
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بسم الله الرحمن الرحيم
Antimicrobial Chemotherapy to Control Microorganisms
Diagnostic Medical Microbiology-Laboratory Manual
Based on their origin, there are 2 general classes of antimicrobial chemotherapeutic agents:
Antibiotics:substances produced as metabolic products of one microorganism which inhibit or kill other microorganisms.
Antimicrobial Chemotherapeutic Chemicals:chemicals synthesized in the laboratory which can be used therapeutically on microorganisms.
Additive (indifferent) effect
Examples of commonly used antimicrobial chemotherapeutic agents arranged according to their mode of action:
2. Semisynthetic Penicillins :
Are effective against gram-positive bacteria but are not inactivated by penicillinase. Example Methicillin.
3. Semisynthetic broad spectrum Penicillins :
Areeffective against a variety of gram-positive and gram-negative bacteria
but are inactivated by penicillinase. Example Ampicillin.
4.Semisynthetic broad spectrum penicillins combined with beta lactamase inhibitors such as clavulanic acid and sulbactam :
Although the clavulanic acid and sulbactam have no antimicrobial action of
their own, they inhibits penicillinase thus protecting the penicillin from
degradation. Example Augmentin (Amoxicillin plus Clavulanic acid).
B. Cephalosporins (produced by the mold Cephalosporium):
Cephalosporins are effective against a variety of gram positive and gram-negative bacteria and are resistant to penicillinase (although some may be inactivated by other beta-lactamase enzymes similar to penicillinase).
Four "generations" of cephalosporins have been developed over the years in an attempt to counter bacterial resistance :
1.First generation cephalosporins :
2.Second generation cephalosporins :
3.Third generation cephalosporins :
4.Fourth generation cephalosporins :
c. Monobactems: They work only against Gram-negative bacteria, beta lactam antibiotics resistant to beta lactamase. The only commercially available monobactam antibiotic is Aztreonam.
d. Vancomycin (produced by the bacterium Streptomyces):
Vancomycin and teichoplanin are glycopeptides that are effective against gram-positive bacteria.
Vancomycin isdrug of "last resort",used only after treatment with other antibiotics had failed.
e. Bacitracin (produced by the bacterium Bacillus):
Bacitracin is used topically against gram-positive bacteria.
f. Ethambutol (a synthetic chemical):
Ethambutolis bacteriostatic against actively growing M.tuberculosis bacilli, it works by obstructing the formation of cell wall, That Inhibits incorporation of mycolic acid.
Antimicrobial agents that alter the cytoplasmic membrane.
Alteration of the cytoplasmic membrane of microorganisms results in leakage of cellular materials :
a. Polymyxin B (produced by the bacterium Bacillus):
Polymyxin B is used in severe Pseudomonas infections.
b. Amphotericin B (produced by the bacterium Streptomyces):
Amphotericin B is used for systemic fungal infections.
c. Nystatin(produced by the bacterium Streptomyces):
Nystatin is used mainly for Candida yeast infections.
d. Imidazoles (produced by the bacterium Streptomyces):
The imidazoles are antifungal antibiotics used for yeast infections, dermatophytic infections, and systemic fungal infections. Example Miconazole.
Antimicrobial agents that inhibit protein synthesis
These agents prevent bacteria from synthesizing structural proteins and enzymes.
a. Agents that block transcription (prevent the synthesis of mRNA from DNA):
Rifampins(produced by the bacterium Streptomyces):
Rifampinsare effective against some gram-positive and gram-negative bacteria and Mycobacterium tuberculosis. They inhibit the enzyme RNA polymerase.
b. Agents that block translation (alter bacterial ribosomes to prevent mRNA from being translated into proteins):
1. Agents that bind irreversibly to the 30s ribosomal subunit and cause a misreading of the mRNA (the aminoglycosides produced by the bacterium Streptomyces). Examples include Streptomycin & Amikacin.
2. Agents that bind reversibly to the 30s ribosomal subunit and interfere with the binding of charged tRNA to the bacterial ribosome. Examples include Tetracycline & Doxycycline ( producedby the bacterium Streptomyces). They are effective against a variety of gram-positive and gram-negative bacteria.
3. Agents that bind reversibly to the 50s ribosomal subunit and prevent the release of uncharged tRNA from the bacterial ribosome. Erythromycin is used against gram positive bacteria and some gram-negative bacteria.
c. Antimicrobial agents that interfere with DNA synthesis:
Quinolones (synthetic chemicals): The quinolones block bacterial DNA replication by inhibiting the DNA gyrase, the enzyme needed by bacteria to produce their circular DNA. They are broad spectrum and examples include Norfloxacin, *Nalidixicacid & Ciprofloxacin.
Nalidixic acid : antibiotic used only to treat UTI caused by Gram negative bacteria.
d. Co-trimoxazole (synthetic chemicals):
Co-trimoxazoleis a combination of sulfamethoxazole and trimethoprim,which blocks the bacterial synthesis of folic acid needed to make DNA bases.
The basic steps for the Bauer-Kirby method
of antimicrobial susceptibility testing are:
Preparea standard turbidity inoculum ((0.5) McFarland Standard ) of the test bacterium so that a certain density of bacteria will be put on the plate.
Inoculate a (90 mm diameter, 4 mm agar depth of Mueller-Hinton agar plate) with the standardized inoculum so as to cover the entire agar surface with bacteria.
Placestandardized antibiotic-containing discs on the plate.
Incubate the plate at 37oCfor 24 hours.
Measurethe diameter of any resulting zones of inhibition in millimeters.
Determineif the bacterium is susceptible, intermediate, or resistant to eachantimicrobial agent using a *standardized table.
*Standarizedtable according to manufactory pamphlet.
Pour cooled Mueller Hinton agar into sterile Petri dishes
on a level, horizontal surface to give a uniform depth of
about 4 mm and cool to room temperature.
3. Hold Muller-Hinton plate half or partially open and streak the plate using the
wet cotton swab covering all the area even at the sides.
4. Place the plates aside for about 10 – 15 minutes at room temerature. Allow
the inoculum to dry.
5. Using a sterile forceps or needle, apply a set of suitable antibiotic disks. Five to six disks for each plate, or 8 disks if you use the automatic dispenser.
6.Let the plates stand for 10 - 15 minutes at 4oC, then incubate in inverted position at 37oCfor 18-24 hours.
7. Using a ruler or caliper, measure the zone of inhibition around each
antimicrobial disk and record it.
8. Consult the special chart provided by the manufacturer of the antimicrobial
disks and interpret results as Sensitive, Resistant, or Intermediate.
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