Transcript Document

Control of microbial growth
Antimicrobial Classes
• Disinfectants
– Products aimed at reducing by at least five powers
of 10 (99,999 %) the number of
microorganisms/virus present on inanimate
objects
• Antiseptics
– Products aimed at reducing by at least five powers
of 10 (99,999 %) the number of
microorganisms/virus present on live tissue
Antimicrobial Classes (Cont’d)
• The drugs
– Antibiotic or Antibacterial
• Against bacteria
– Antifungal
• Against fungi
– Antiviral
• Against viruses
Disinfectants and Antiseptics
• Ideal characteristics
– Broad action spectrum
– Powerful; low amounts required for a high
efficacy
– Low toxicity in humans
– Not corrosive
– Stable
– Hydrophilic and hydrophobic
– Low surface tension
– No odor or pleasant odor
The Drugs: Antibiotics
Definitions:
• Literal: Anti (against) biotic (life)
• Old def.: Any compound synthesized by a
microorganism that inhibits or kills other
bacteria
• New def.: Any compound that inhibits or kills
bacteria
Desired Characteristics
1. High selective toxicity:
– Must kill or inhibit the target organism with a
minimum of deleterious effects on the host
• Penicillin:
– Targets cell wall
• Cyanide:
– Target: electron transport of eukaryotes/prokaryotes
Desired Characteristics (Cont’d)
2. High toxic dose (LD50)
– Concentration of the compound that is toxic to
the host
• Penicillin
• Cyanide
3.Low therapeutic dose (MIC or MBC)
– Concentration of the compound required for the
clinical treatment of an infection
• Penicillin
• Table salt
The Therapeutic Index
• Toxic Dose/Therapeutic dose
– Want a therapeutic index that is?
Action Spectrum
• Narrow:
– Efficacy restricted to only a few types of
microorganisms
• Ex. Acts only on Gram -
• Broad:
– Efficacy is good for a wide variety of
microorganisms
• Ex. Acts on Gram + and -
Antibacterial targets
Cell wall synthesis
ß-lactams
DNA synthesis
Quinolones
Transcription
RNA synthesis
Macrolides
Metabolism
A
B
Translation
Protein synthesis
Aminoglycosides
Macrolides
Tetracyclines
Chloramphenicol
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Modes of Action
Viable
count
#
Direct
count
Time
• Bacteriostatic:
–
Inhibits growth
– Non-lethal
– Reversible
• Bacteriocidal • Bacteriolytic
– Kills
– Irreversible
– Kills
– Cell lysis
– Irreversible
The Beta-Lactams
• Bacteriolytic
• Inhibit cell wall synthesis
– Act only on actively growing bacteria!
Penicillines
They all contain a
beta lactam ring
Cephalosporins
Monobactams
Carbapenems
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Quinolones
• Bacteriocidal
– Inhibit DNA synthesis
– Broad spectrum
– Side effects:
• Severe gastrointestinal problems
– Ex. Ciprofloxacin
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Tetracyclines
• Bacteriostatic
– Inhibits protein synthesis
– Broad spectrum
– Side effects:
• Hepatic toxicity
• Renal toxicity
• Vitamin deficiency
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Macrolides
• Bacteriostatic
– Inhibits protein synthesis
– Narrow spectrum
– Side effects
• Diarrhea
• Hepatic dammages
– Ex. Erythromycin & Clarithromycin
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Aminoglycosides
• Bacteriocide
–
–
–
–
Narrow spectrum
Inhibits protein synthesis
High level of toxicity
Side effects:
• Allergies
• Renal dammages
• Deafness
• Ex. Gentamycin, streptomycin
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Antimicrobial Therapies
• Empirical
– The infectious agent is unknown
– Broad spectrum antibiotic is recommended
• Definitive
– The infectious agent has been identified
– A specific therapy is chosen
– Narrow spectrum antibiotic is recommended
• Prophylactic
– Prevent an initial infection or reinfection
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Kirby-Bauer Disc Diffusion Assay
• Agar is inoculated with test bacteria
• Antibiotic impregnated discs are laid on the
agar
• The antibiotic diffuses in the
medium creating a gradient
• Following the incubation the
zones of inhibition are measured
• The sizes of the zones of
inhibition are compared to those established
to determine whether the organism is
sensitive or resistant
Inhibitory Diameters Vs Conc.
27mm = MIC
< 27mm = Conc. > MIC
> 27mm = Conc. < MIC
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Determination of Efficacy
MIC/MBC
• Minimal Inhibitory Concentration
Cultures with different
concentrations of antibiotic
100
50
25
MIC=12μg/ml
Sub culture without antibiotics
MBC=50μg/ml
• Minimal Bacteriocidal Concentration
12
6
3
0
E-test
In Vivo Susceptibility
• The in vivo concentration is not constant!
– Influenced by human physiology
– A range of concentrations is maintained
• (CMax-CMin)
– The concentration at the infection site must be
higher than the MIC
• If <MIC = resistance
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Sensitivity In Vivo
• Sensitive pathogen
– MIC is lower than the lowest conc. maintained in vivo
• Resistant pathogen
– MIC is higher than the highest concentration
maintained in vivo
• Intermediate sensitivity pathogen
– MIC is between the lowest and the highest
concentration maintained in vivo
• A combination of antibiotics is recommended
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Example
• Antibiotic “A” conc. in vivo = 5-40µg/ml
– Thus:
• MIC≤ 5 µg/ml = Sensitive
• MIC≥ 40µg/ml = resistant
• MIC between 5 -40 µg/ml = intermediate sensitivity
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Decimal Reduction Time
• D value
– Time required to kill 90% of microorganisms
– Time required to reduce the population by a
factor of 10
– Time required to reduce the population by one
log10
Decimal Reduction Time
1 X 106
# Bacteria
1 log
1 X 105
1 X 104
D
120
D
100
=12min
1 X 103
5
10
15
20
25
Time (min.)
30
35
40
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Problem
• At 75oC it takes 18 min. to reduce a
population of microorganisms from 109 to 106
• What is the value of D75 ?
• 18 minutes to go from 109 to 106
– 3 log
– 3 log = 3D75
– Therefore 3D75 = 18minutes D75=6minutes