Antimicrobial Drugs
Different types of antimicrobial drugs:
Antibacterial drugs
Antifungal drugs
Antibiotic/Antimicrobial
Antibiotic
: Chemical produced
by a microorganism that kills or
inhibits the growth of another
microorganism
Antimicrobial agent
: Chemical
Antibiotics
• Bactericidal antibiotics kills bacteria
• Bacteriostatic antibiotics
Properties of Useful Antibiotics
• Non-toxic
• Soluble in blood (most)
• Persist in the body
• Active against: a wide range of bacteria or against
specific bacteria
Antibiotic spectrum of activity
1.
narrow
- agents that affect a limited number
of microorganisms- isoniazid, erythromycin,
penicillin
2.
broad
- inhibit a wide variety of organisms -
tetracyclin, streptomycin, chorlamphenicol,
fluoroquinolones
Antibiotic
spectrum of activity
Mechanisms of resistance
Enzymatic destruction of drug
Prevention of penetration of drug
Alteration of drug's target site
Rapid ejection of the drug
Origin and Mechanisms of
Antibiotic Resistance
Origin of resistance
Microbes that
produce
antibiotics
Mutation of sensitive strains
RESISTANCE TO
ANTIMICROBIAL DRUGS
Mechanisms of resistance
Drug inactivating enzymes Some organisms produce
enzymes that chemically modify drug
Penicillinase breaks β-lactam ring of penicillin antibiotics
Alteration of target molecule
Minor structural changes in
antibiotic target can prevent binding
Changes in ribosomal RNA prevent macrolids from binding to
SUSCEPTIBILITY OF BACTERIAL
TO ANTIMICROBIAL DRUG
Mechanisms of resistance
Decreased uptake of the drug
Alterations in porin proteins
decrease permeability of cells
Prevents certain drugs from entering
Increased elimination of the drug
Some organisms produce efflux
pumps
Increases overall capacity of organism to eliminate drug
Enables organism to resist
higher concentrations of drug
What Factors Promote
Antimicrobial Resistance?
Exposure to sub-optimal levels
of antimicrobial
Exposure to microbes carrying
Antibiotic use and abuse
Viral infections are not stopped by antibiotics
Yet doctors still prescribe (or are coerced
into prescribing) antibiotics to treat them
Inappropriate Antimicrobial Use
Prescription not taken correctly Antibiotics for viral infections
Antibiotics sold without medical supervision
Spread of resistant microbes in hospitals due to lack of
Lack of quality control in manufacture or outdated
antimicrobial
Inadequate surveillance or defective susceptibility assays Use of antibiotics in foods
Antibiotics in Foods
Antibiotics are used in animal feeds and sprayed on plants to
prevent infection and promote growth
Multi drug-resistant Salmonella typhi has been found in 4 states
Proposals to Combat
Antimicrobial Resistance
Use more narrow spectrum
Antisense agents
Complementary DNA or peptide nucleic acids that
binds to a pathogen's virulence gene(s) and
prevents transcription
The Future of
MECHANISMS OF ACTION OF
ANTIBACTERIAL DRUGS
Mechanism of action include: Inhibition of cell wall synthesis Inhibition of protein synthesis Inhibition of nucleic acid synthesis Inhibition of metabolic pathways Interference with cell membrane
MECHANISMS OF ACTION OF
ANTIBACTERIAL DRUGS
Inhibition of Cell wall synthesis
Bacteria cell wall unique in construction
Contains peptidoglycan
Antimicrobials that interfere with the synthesis of
cell wall do not interfere with eukaryotic cell
- Due to the lack of cell wall in animal cells and
differences in cell wall in plant cells
These drugs have very high therapeutic index
Low toxicity with high effectiveness
Antimicrobials of this class include
MECHANISMS OF ACTION OF
ANTIBACTERIAL DRUGS
Penicillins and cephalosporins
Part of group of drugs called β –lactams
Have shared chemical structure called β-lactam ring
Competitively inhibits function of penicillin-binding proteins
Inhibits peptide bridge formation between glycan molecules
This causes the cell wall to develop weak points at the growth sites
and become fragile.
a) Non-treated cell wall
Mechanism of Penicillin
The Beta Lactam (4 membered ring with carbonyl) binds to the
active site of the Transpeptidase enzyme
Once it is inside the enzyme’s active site, penicillin doesn’t leave,
act as structural analogs that bind to the active site of bacterial enzymes necessary to synthesize cell walls of daughter cells
MECHANISMS OF ACTION OF
ANTIBACTERIAL DRUGS
MECHANISMS OF ACTION
OF ANTIBACTERIAL DRUGS
The weakness in the cell wall
causes the cell to lyze.
Penicillins and cephalosporins are
considered bactericidal.
Penicillins are more effective
The cephalosporins
Chemical structures make them resistant to inactivation by certain β-lactamases
Tend to have low affinity to penicillin-binding proteins of Gram + bacteria, therefore, are most effective against Gram – bacteria.
Chemically modified to produce family of related compounds
First, second, third and fourth generation cephalosporins
Vancomycin
Inhibits formation of glycan chains
Inhibits formation of peptidoglycans and cell wall construction Does not cross lipid membrane of Gram -ve
Gram -ve bacteria innately resistant
Important in treating infections caused by penicillin resistant Gram +ve bacteria Must be given intravenously due to poor absorption from intestinal tract
Acquired resistance most often due to alterations in side chain of NAM molecule
Prevents binding of vancomycin to NAM component of glycan
Mechanism of Vancomycin
Forms multiple hydrogen bonds to the D-alanyl-D-alanine amino acids of
Bacitracin
Interferes with transport of peptidoglycan precursors across cytoplasmic membrane
Toxicity limits use to topical applications
Inhibition of protein synthesis
Structure of prokaryotic ribosome acts as target for many
antimicrobials of this class
Differences in prokaryotic and eukaryotic ribosomes
responsible for selective toxicity
Drugs of this class include
Aminoglycosides Tetracyclins
Macrolids
Chloramphenicol
MECHANISMS OF ACTION
OF ANTIBACTERIAL DRUGS
Aminoglycosides
Irreversibly binds to 30S
ribosomal subunit
Causes distortion and
malfunction of ribosome
Blocks initiation
translation
Causes misreading of mRNA
Not effective against
-Anaerobes (oxygen required to uptake of antibiotic)
Intracellular bacteria.
Often used in synergistic
combination with β-lactam drugs
Allows aminoglycosides to
MECHANISMS OF ACTION OF
ANTIBACTERIAL DRUGS
Examples of
aminoglycosides include Gentamicin,
streptomycin and tobramycin
Side effects with
Tetracyclins
Reversibly bind 30S ribosomal subunit
Blocks attachment of tRNA to ribosome
Prevents continuation of protein synthesis
Effective against certain Gram +ve and Gram –ve
bacteria
Newer tetracyclines such as doxycycline have longer
half-life
Allows for less frequent dosing
Resistance due to decreased accumulation by bacterial
cells (efflux resistance)
Can cause discoloration of teeth if taken as young child
MECHANISMS OF ACTION OF
ANTIBACTERIAL DRUGS
Macrolids
Reversibly binds to 50S ribosome
Prevents continuation of protein synthesis
Effective against variety of Gram +ve bacteria and those
responsible for atypical pneumonia
Often drug of choice for patients allergic to penicillin Macrolids include
MECHANISMS OF ACTION OF
ANTIBACTERIAL DRUGS
Resistance can occur via modification of RNA target
Other mechanisms of resistance include production of
MECHANISMS OF ACTION
OF ANTIBACTERIAL DRUGS
Chloramphenicol
Binds to 50S ribosomal subunit
Prevents peptide bonds from forming and blocking proteins
synthesis
Effective against a wide variety of organisms
Generally used as drug of last resort for life-threatening
infections
Inhibition of nucleic acid synthesis
These include
Fluoroquinolones Rifamycins
Fluoroquinolones
Inhibit action of topoisomerase DNA gyrase
Topoisomerase maintains supercoiling of DNA
Effective against Gram + and Gram
-Examples include
Nalidixic acid, Ciprofloxacin and ofloxacin,
norfloxacin, levofloxacin, lomefloxacin, sparfloxacin
Nalidixic acid: Relatively toxic - it accumulates in the urine so it can be used to treat urinary tract
infections
Resistance due to alteration of DNA gyrase
Rifamycins
Block prokaryotic RNA polymerase
Block initiation of transcription
Rifampin most widely used rifamycins
Effective against many Gram + and some Gram - as
well as members of genus Mycobacterium
Primarily used to treat tuberculosis and Hansen’s
disease as well as preventing meningitis after exposure to N. meningitidis
Resistance due to mutation coding RNA polymerase
Resistance develops rapidly
MECHANISMS OF ACTION
OF ANTIBACTERIAL DRUGS
Inhibition of metabolic
pathways
Relatively few
Most useful are folate
inhibitors
Mode of actions to
inhibit the production of folic acid
Antimicrobials in this
Sulfonamides (Sulfa drugs)
Inhibit folic acid synthesis Broad spectrum
Figure 5.7
MECHANISMS OF ACTION OF
MECHANISMS OF ACTION
OF ANTIBACTERIAL DRUGS
Sulfonamides
Group of related compounds
Collectively called sulfa drugs
Inhibit growth of Gram + and Gram - organisms
Through competitive inhibition of enzyme that aids in
production of folic acid
Structurally similar to para-aminobenzoic acid
Substrate in folic acid pathway
Human cells lack specific enzyme in folic acid pathway
Basis for selective toxicity only for bacteria but not human
Resistance due to plasmid
Trimethoprim
Inhibits folic acid production
Interferes with activity of enzyme following enzyme
inhibited by sulfonamides
Often used synergistically with sulfonamide
Most common mechanism of resistance is
plasmid encoded alternative enzyme
Genes encoding resistant to sulfonamide and
trimethoprim are often carried on same plasmid
MECHANISMS OF ACTION
OF ANTIBACTERIAL DRUGS
Interference with cell membrane integrity
Few damage cell membran
Causes leakage from gram negative bacteria
Polymixn B most common
Common ingredient in first-aid skin ointments
Binds membrane of Gram –ve bacterial cells
Alters permeability
Leads to leakage of cell and cell death
Also bind eukaryotic cells but to lesser extent
EFFECTS OF
COMBINATIONS OF DRUGS
Sometimes the chemotherapeutic effects of
two drugs given simultaneously is greater than
the effect of either given alone.
This is called
synergism
. For example,
penicillin and streptomycin in the treatment
of bacterial endocarditis. Damage to
EFFECTS OF
COMBINATIONS OF DRUGS
Other combinations of drugs can be
antagonistic.
For example, the simultaneous use of penicillin
and tetracycline is often less effective than
when wither drugs is used alone. By stopping
the growth of the bacteria, the
EFFECTS OF
COMBINATIONS OF DRUGS
Combinations of antimicrobial drugs should
be used only for:
1.
To prevent or minimize the emergence of
resistant strains.
Kirby-Bauer method for
determining drug susceptibility
1.
Bacteria spread on surface of agar plate
2.
12 disks, each with different antimicrobial
drug, placed on agar plate
3.
Incubated- drugs diffuse outward and kill
susceptible bacteria
ANTIMICROBIAL
SUSCEPTIBILITY TESTING
Probably the most widely used testing method is the
The basic quantitative measures of the
in vitro
activity of antibiotics are the
minimum inhibitory
concentration (MIC)
and the
minimum bactericidal
concentration (MBC)
.
The MIC is the lowest concentration of the antibiotic
that results in inhibition of visible growth (
i.e.
colonies on a plate or turbidity in broth culture)
under standard conditions.
The MBC is the lowest concentration of the antibiotic
that kills 99.9% of the original inoculum in a given
time.
8 4 2 1 0
Tetracycline (µg/ml) MIC = 2 µg/ml
Determination of MIC
Measuring Antimicrobial Sensitivity
MIC: Minimal
inhibitory
New Approaches to Antibiotic
Therapy Are Needed
Scientists work to find new antibiotic targets
in pathogens
Discovery of new and unique antibiotics is
