Antimicrobial Control Agents. Mr. Shadi ALashi. Antimicrobial control agents. Usually, microbial controls are used to avoid contamination of pure cultures, prevent infection, or treat existing diseases. A microbiocidal effect kills microorganisms.
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.
2. Antimicrobial Chemotherapeutic Chemicals:chemicals synthesized in the laboratory which can be used therapeutically on microorganisms.
If a choice is available, a narrow spectrum is preferable since it will cause less destruction to the body's normal flora. In fact, indiscriminate use of broad spectrum antibiotics can lead to super-infection by opportunistic microorganisms.
UVcauses damage to nucleic acids by inducing covalent bonds between adjacent thymine bases, resulting in thymine dimerization. The thymine dimers change the structure of DNA, preventing DNA replication and RNA transcription.
Narrow spectrum OR broad spectrum.
Old age OR child.
Male OR female.
Pregnant OR lactating women.
In patient OR out patient.
Type of microorganisms.
Site of infection.
Several tests may be used to tell a physician which antimicrobial agent is most likely to combat a specific pathogen:
1. Tube dilution tests:
3. Epsilometer test:
Additive (indifferent) effect
Antimicrobial susceptibility testing by the disk diffusion method (Kirby-Bauer) & Antibiotic profil
Purpose of Procedure
To test isolated bacteria for its susceptibility to antimicrobial agents
In general a pure growth of the isolate.
• Sterile cotton swabs.
• Mueller Hinton Agar plates.
• Appropriate antibiotic discs or dispenser.
• McFarland Standard (0.5).
• Pure culture of the test organism.
The basic steps for the Bauer-Kirby method of antimicrobial susceptibility testing are:
a. Prepare a standard turbidity inoculum ((0.5) McFarland Standard ) of the test bacterium so that a certain density of bacteria will be put on the plate.
b. 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.
c. Place standardized antibiotic-containing discs on the plate.
d. Incubate the plate at 37oCfor 24 hours.
e.Measure the diameter of any resulting zones of inhibition in millimeters (mm).
f. Determine if the bacterium is susceptible, intermediate, or resistant to eachantimicrobial agent using a *standardized table.
* Standarized table 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.
Adjust the concentration of the inoculum by comparing the turbidity of Normal Saline tube to that of a 0.5 McFarland standard.
Note: If the turbidity of the inoculum is greater than that of 0.5 McFarland
standard, dilute with sterile normal saline. If the turbidity of the standard is
greater than the inoculum, add more of the test organism.
2. Dip a sterile cotton swab into the adjusted inoculum tube and drain excess
fluid by pressing the swab against the walls of the test tube.
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.