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Antimicrobial Control Agents


Antimicrobial control agents. Usually, microbial controls are used to avoid contamination of pure cultures, prevent infection, or treat existing diseases. – PowerPoint PPT presentation

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Title: Antimicrobial Control Agents

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.
  • A microbiostatic effect prevents the
    reproduction of microorganisms.
  • Antiseptics are chemicals used on living tissues
    to inhibit the growth of microorganisms.
  • Disinfectants are chemicals used on nonliving
    surfaces to inhibit the growth of microorganisms.
  • Chemotherapeutic agents are chemicals used to
    destroy or inhibit the growth of microorganisms
    in living tissues.

  • Antimicrobial Chemotherapy is the use of
    chemicals to inhibit or kill microorganisms in
    the host.
  • Selective Toxicity This means that the agent
    used must inhibit or kill the microorganism
    without seriously harming the host.
  • 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.

  • Antimicrobial agents are
  • Cidal in action they kill microorganisms.
  • OR
  • Static in action they inhibit microbial
    growth long enough for the body's own defenses to
    remove the organisms.
  • Antimicrobial agents also vary in their spectrum
  • Broad spectrum Drugs which are effective
    against a variety of both gram-positive and
    gram-negative bacteria.
  • Narrow spectrum Drugs which are effective
    against just gram-positive bacteria, just gram
    negative bacteria, or only a few species are

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
UV causes 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.

The Action of Antimicrobial Drugs
  • Several tests may be used to tell a physician
    which antimicrobial agent is most likely to
    combat a specific pathogen

1. Tube dilution tests
  • A series of culture tubes are prepared, each
    containing a liquid medium and a different
    concentration of a chemotherapeutic agent. The
    tubes are then inoculated with the test organism
    and incubated for 18-24 hours at 37 oC . After
    incubation, the tubes are examined for turbidity
  • Minimum Inhibitory Concentration (MIC)
  • Is the lowest concentration of chemotherapeutic
    agent capable of preventing growth of the test
  • Minimum Bactericidal Concentration (MBC)
  • Is the lowest concentration of the
    chemotherapeutic agent that allows less than 0.1
    of the inoculum to survive , also called
    Minimum Lethal Concentration (MLC)

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2. The agar diffusion test (Bauer-Kirby test)
  • In this test, the in vitro response of bacteria
    to a standardized antibiotic-containing disc has
    been correlated with the clinical response of
    patients given that drug.
  • In the development of this method, a single
    high-potency disc of each chosen chemotherapeutic
    agent was used.
  • Zones of growth inhibition surrounding each type
    of disc were correlated
  • with the minimum inhibitory concentrations of
    each antimicrobial agent (as determined by the
    tube dilution test).
  • The MIC for each agent was then compared to the
    usually-attained blood level in
  • the patient with adequate dosage. Categories of
    "Resistant, "Intermediate," and "Sensitive" were
    then established.
  • Its a new technique for direct detection of
    MIC, a graduated increasing concentration of the
    antibiotic is fixed along a rectangular plastic
    test strip which is applied to the surface of an
    inoculated agar plate, after over night
    incubation a tear drop shaped inhibition zone is
  • The zone edge intersect the graded test strip at
    the MIC of the antimicrobial.

3. Epsilometer test
Disk-Diffusion Test
E Test
E Test
Effects of Combinations of Drugs
  • Additive (indifferent) effect the activity of
    two drugs in combination is equal to the sum (or
    a partial sum) of their independent activity when
    studied separately.
  • Synergistic effect the activity of two drugs in
    combination is greater to the sum of their
    independent activity when studied separately.
  • Antagonistic effect the activity of two drugs in
    combination is less to the sum (or a partial sum)
    of their independent activity when studied

Effects of Combinations of Drugs
Additive (indifferent) effect
Effects of Combinations of Drugs
Synergistic effect
Effects of Combinations of Drugs
Antagonistic 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 Specimen Requirements In general a pure
growth of the isolate. Equipment Sterile
cotton swabs. Reagents Required 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 37 oC for 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 each antimicrobial
agent using a standardized table. Standarized
table according to manufactory pamphlet.
Mueller Hinton Agar
  • Beef Extract.
  • Digest of Casein ( peptone ).
  • Starch.
  • Agar ....... 1.7 .
  • Final pH is 7.3 at 25C.
  • 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.

Mueller Hinton Agar
0.5 McFarland standard
  • McFarland standards are used as a reference to
    adjust the turbidity of bacterial suspensions so
    that the number of bacteria will be within a
    given range.
  • Were mixing specified amounts of barium chloride
    and sulfuric acid together.
  • Mixing the two compounds forms a barium sulfate
    precipitate, which causes turbidity in the
  • A 0.5 McFarland standard is prepared by mixing
    0.05 mL of 1.175 barium chloride dihydrate
    (BaCl22H2O), with 9.95 mL of 1 sulfuric acid
  • Turbidity of 0.5 McFarland standard is
    approximately equal 1.5X108 CFU/mL of bacterial

McFarland standards
  • Procedure
  • 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
  • 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
  • 4. Place the plates aside for about 10 15
    minutes at room temerature. Allow the inoculum to

Procedure continue
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 37 oC for 18-24 hours.
Procedure continue
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
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