2. Definition
Antibiotics: Natural substances produced by certain groups of
microorganisms that kill or inhibit the growth of other
microorganisms.
Most of the natural antibiotics that are being used in medicine are produced
by three unrelated groups of microbes:
• Penicillium and Cephalosporium molds
• Actinomycetes, mainly Streptomyces species
• Bacillus species
3. Features of Antimicrobial Drugs
1. Most modern antibiotics come from species of microorganisms
that live in the soil
2. Should have selective toxicity (Cause greater harm to
microorganisms than to host)
3. Some antibiotic are Bacteriostatic: Inhibits growth of
microorganisms
4. Some are Bactericidal: Kills microorganisms
4. 5. They have a variety of Spectrum of Activity
• Some kill or inhibit only limited range : Narrow-spectrum
antimicrobial
• While others kill or inhibit wide range of microorganisms: Broad-
spectrum antimicrobial
• If effective against a single organism or disease, they are referred
to as limited spectrum
5. 6. Effects of Combining Drugs can be:
• Combinations are sometimes used to fight infections.
• Synergistic: action of one drug enhances the activity of
another
• Antagonistic: activity of one drug interferes with the action of
another
6. 7. Adverse Effects
1. Allergic Reactions: some people develop hypersensitivities
to antimicrobials
2. Toxic Effects: some antimicrobials toxic at high
concentrations.
3. Suppression of normal flora: when normal flora killed,
other pathogens may be able to grow to high numbers
8. • Some microorganisms inherently resistant.
• Production of enzymes which inactivate or modify antibiotics.
• Changes in the bacterial cell membrane, preventing the uptake of an
antimicrobial.
• Modification of the target so that it no longer interacts with the
antimicrobial.
• Development of metabolic pathways by bacteria which enable the
site of antimicrobial action to be by passed.
Antimicrobial resistance
9. • Production of enzymes which inactivate or modify antibiotics,
• Changes in the bacterial cell membrane, preventing the uptake of an
antimicrobial.
• Modification of the target so that it no longer interacts with the antimicrobial.
• Development of metabolic pathways by bacteria which enable the site of
antimicrobial action to be by passed.
Antimicrobial resistance
Bacteria become resistant to antimicrobial agents by a
number of mechanisms, the commonest being:
10. ANTIMICROBIAL SUSCEPTIBILITY TESTING
A test done to check the effectiveness of a drug against the bacteria and
to select the best drug that acts against the bacteria.
• To guide the clinician in selecting the best antibiotic.
• To control the use of inappropriate antibiotics in clinical practice.
• To accumulate epidemiological information on the resistance of
microorganisms within the community.
• To reveal the changing trends in the local isolates.
12. Turbidity standard equivalent to McFarland 0.5
• This is a Barium sulphate standard against which the turbidity of the test
and control inocula can be compared. When matched with the
standard,
Shake the standard
immediately before use.
13. Kirby-Bauer modified disc diffusion
• Using discs of correct antimicrobial content, an inoculum which gives
confluent growth, and a reliable Mueller Hinton agar.
• Incubation at 35 OC for 16–18 hours
• Zone sizes are measured in mm and interpreted using CLSI (Clinical
Laboratory Standard Institute) standards.
Measurement of zone sizes
Using the Interpretative Chart of CLSI,
interpret the zones
• Resistant, Intermediate or Susceptible
14. Stokes disc diffusion technique
• Both the test and control organisms are inoculated on the same
plate.
• The zone sizes of the test organism are compared directly with that
of the control.
Stokes disc diffusion technique
QCdone by American Type
Culture Collection (ATCC)
Control strains
15. • Used to determine the (MIC) of
antibiotic to inhibit the microorganism.
• Achieved by dilution of antibiotic in either agar or broth media.
16. Patient's organism is added to
tubes containing decreasing
amounts of the antibiotic
Incubation
At 37°C overnight
Lowest concentration of drug that
inhibits growth is Minimum Inhibitory
Concentration (MIC)
Lowest concentration of drug that kills
the growth of bacteria is Minimum
Bactericidal Concentration (MBC)
18. • Varying concentrations of the antibiotics are coated on the wells.
• During testing, multiple microtiter plates are filled with a broth composed
of bacterial suspension.
• The plate is then placed into an incubator overnight at 35oC.
• The minimum inhibitory concentration (MIC) is determined by the first
well where there is no visible growth.
19. • Serial dilutions of the drug are prepared in agar and poured into
plates.
• Many strains can be inoculated on each plate containing an
antibiotic dilution.
E-Test (Epsilometer test)
• Manufactured by bioMérieux, is a manual in
vitro diagnostic to detect MIC.
• Different concentration of an antibiotic are the
on the same strip.
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