Transcript Antimicrobial Medications


Discovery of antimicrobial drugs
› Paul Erlich (1909)found the first pharmaceutical
effective for treatment of syphilis: Salvarsan
 Arsphenamine highly toxic
› Sulfonamide was the first sulfa drug
 In vitro derivative of Prontosil dye
 effective against streptococcal infections
 Bayer Labs, 1939 Nobel prize in Medicine
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Discovery of antibiotics
› Penicillin discovered by Alexander Fleming
 Identified mold Penicillium that produced a
bactericidal substance that was effective
against a wide range of gram + microbes
 Inhibits cell wall synthesis
 Mass production of penicillin during WWII
› Streptomycin (1943) isolated from soil bacterium
Streptomyces griseus by Selman Waksman
 Bacteriostatic
 Inhibits protein synthesis by binding to ribosome
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Development of
new generation of
drugs
› In 1960s scientists
alteration of drug
structure gave them
new properties
 Penicillin G altered to
create ampicillin
 Broadened spectrum
of antimicrobial killing
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Most modern antibiotics come from
organisms living in the soil
› Includes bacterial species Streptomyces and
Bacillus as well as fungi Penicillium and
Cephalosporium
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To commercially produce antibiotics
› Strain is grown until maximum antibiotic
concentration is reached
› Drug is extracted from broth medium
› Extensively purified
› May be chemically altered
 Termed semi-synthetic
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Selective toxicity
› Antibiotics cause greater harm to
microorganisms than to human host
› Toxicity of drug is expressed as therapeutic
index
 Lowest dose toxic to patient divided by dose
typically used for treatment
 High therapeutic index = less toxic to patient
 Narrow therapeutic index = more toxic, monitor
closely
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Antimicrobial action
› Bacteriostatic drugs
 Inhibit bacterial growth
 rely on host immunity
› Bacteriocidal drugs
 Kill bacteria
 Most useful in situations when host defenses
cannot control pathogen
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Spectrum of activity
› Antimicrobials vary with respect to range of
organisms controlled
 Narrow spectrum
 Work on narrow range of organisms
 Gram-positive only OR Gram-negative only
 Advantage: effects pathogen only
 Disadvantage: requires identification of pathogen
 Broad spectrum
 Advantage: Work on broad range of organisms
 Disadvantage : disruption of normal flora

Effects of combinations of antimicrobial
drugs
› Combination sometimes used to treat
infections
 Synergistic: whole is > sum
 Antagonistic: whole is < sum
 Additive: whole is the sum

Tissue distribution, metabolism and
excretion
› Drugs differ in how they are distributed,
metabolized and excreted
› Half-life: Rate of elimination of drug from
body
 Time it takes for the body to eliminate one half
the original dose in serum
 Half-life dictates frequency of dosage
› Patients with liver or kidney damage tend to
excrete drugs more slowly

Adverse effects
› Allergic reactions
› Toxic effects
› Suppression of normal flora
› Antimicrobial resistance
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Mechanism of action include:
› Inhibition of cell wall synthesis
 Penicillins, Cephalosporins, Vancomycin, Bacitracin
› Inhibition of protein synthesis
 Aminoglycosides, tetracyclines, macrolides, chloramphenicol,
lincosamides
› Inhibition of nucleic acid synthesis
 Fluoroquinolones, rifamycins
› Inhibition of metabolic pathways
 Sulfonamides, trimethoprim
› Interference with cell membrane integrity
 Polymyxin

Inhibition of cell wall synthesis
› Antimicrobials that interfere with
the synthesis of peptidoglycan
› These drugs have very high
therapeutic index
› Antimicrobials of this class
include
 β lactam drugs (penicillin,
cephalosporin)
 Vancomycin
 Bacitracin

Drugs vary in spectrum
 Some more active against Gram (+)
 Some more active against Gram (-)
› Resistance through production of β-
lactamase enzyme
› Penicillins + β lactamase inhibitor
 Augmentin = amoxicillin + clavulanic acid

Vancomycin
› Inhibits formation of glycan
chains
 Does not cross lipid membrane
of Gram (-)
› Important in treating infections
caused by penicillin resistant
Gram (+) organisms
› Given intravenously due to
poor GI absorption
› Acquired resistance most
often due to alterations in side
chain of NAM molecule
 Prevents binding of
vancomycin to NAM
component of glycan
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Bacitracin
› Interferes with transport of PTG precursors
across cytoplasmic membrane
› Toxicity limits use to topical applications
› Common ingredient in non-prescription firstaid ointments

Inhibition of protein synthesis
› Structure of prokaryotic ribosome acts as target
for many antimicrobials of this class
› Drugs of this class include
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Aminoglycosides
Tetracyclins
Macrolids
Chloramphenicol
Lincosamides
Oxazolidinones
Streptogramins
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Aminoglycosides
› Irreversibly binds to 30S
ribosomal subunit
 Blocks initiation translation
 Causes misreading of
mRNA
› Not effective against
anaerobes, enterococci
and streptococci
› Often used in synergistic
combination with βlactam drugs
› Examples include
 Gentamicin, streptomycin
and tobramycin
› Side effects with
extended use include
 Nephrotoxicity
 Otto toxicity

Tetracyclins
› Reversibly bind 30S
ribosomal subunit
 Blocks attachment of tRNA
to ribosome
 Prevents continuation of
protein synthesis
› Narrow range: Effective
against certain Gram
(+) and Gram (-)
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Macrolids
› Reversibly binds to 50S
ribosome
 Prevents continuation of
protein synthesis
› Effective against variety of
Gram (+) organisms
› Often drug of choice for
patients allergic to
penicillin
› Macrolids include
 Erythromycin, clarithromycin
and azithromycin
› Resistance can occur via
modification of RNA target

Chloramphenicol
› Binds to 50S ribosomal
subunit
 Prevents peptide bond
formation
› Wide spectrum
› Drug of last resort
› Rare but lethal side
effect is aplastic
anemia
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Lincosamides: clindamycin
› Binds to 50S ribosomal subunit
 Prevents continuation of protein synthesis
› Inhibits variety of Gram (+) and Gram (-)
organisms
 Useful in treating infections from intestinal
perforation
 Especially effective against Bacterioides fragilis and
Clostridium difficile
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New class effective against β-lactams and
vancomycin resistant Gram (+) forms
› Oxazolidinones
 Binds 50S ribosomal subunit
 Interferes with initiation
› Streptogramins
 Bonds to two different sites on 50S ribosomal
subunit
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Fluoroquinolones
› Inhibit action of topoisomerase DNA gyrase
 Topoisomerase maintains supercoiling of DNA
› Broad-Spectrum: Effective against Gram (+)
and Gram (-)
› Examples include
 Ciprofloxacin and ofloxacin
› Resistance due to alteration of DNA gyrase
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Rifamycins
› Block prokaryotic RNA polymerase
 initiation of transcription
› Rifampin most widely used rifamycins
› Broad-spectrum: Effective against many Gram
(+) and some Gram (-) as well as
Mycobacterium
› Treatment of
› Tuberculosis
› Hansen’s disease
› N. meningitidis meningitis
› Resistance develops rapidly
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Folate inhibitors
 Mode of actions to
inhibit the production
of folic acid
 Mimic PABA
› Antimicrobials in this
class include
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Sulfonamides
Trimethoprim
› Human cells lack
specific enzyme in folic
acid pathway
› Resistance due to
plasmid
› Polymixn B most
common
 Common ingredient in
first-aid skin ointments
› Binds membrane of
Gram (-) cells
 Alters permeability
 Also binds eukaryotic
cells
 Limits use to topical
application
Susceptibility of organism to specific
antimicrobials is unpredictable
 Often drug after drug tried until favorable
response was observed
 Better approach
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› Determine susceptibility
› Prescribe drug that acts against offending
organism
 Best to choose one that affects as few others as possible
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MIC = Minimum
Inhibitory
Concentration
 Quantitative test to
determine lowest
concentration of
specific antimicrobial
drug needed to
prevent growth of
specific organism
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Kirby-Bauer disc
diffusion method
› qualitative
determination of
susceptibility
› Discs impregnated with
specific concentration
of antibiotic placed on
plate and incubated
› Clear zone of inhibition
around disc reflects
susceptibility
size of clearing zone indicates if
susceptible or resistant
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E-test
› Uses strips
impregnated with
gradient
concentration of
antibiotic
› Test organism will grow
and form zone of
inhibition
 Zone is tear-drop
shaped
 Zone will intersect strip
at inhibitory
concentration
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Mechanisms of resistance
› Drug inactivating enzymes
 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
ribosomal subunits
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Mechanisms of resistance
› Decreased uptake of the drug
 Alterations in porin proteins
decrease permeability of cells
› Increased elimination of the
drug
 Some organisms produce efflux
pumps
 Tetracycline resistance
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Acquisition of resistance
› Can be due to spontaneous mutation
 vertical evolution
› Or acquisition of new genes
 horizontal transfer
 Plasmid mediated
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Spontaneous mutation
› Example of spontaneous mutation
 Resistance to streptomycin is result a change
in single base pair encoding protein to which
antibiotic binds
› When antimicrobial has several different
targets it is more difficult for organism to
achieve resistance through spontaneous
mutation

Acquisition of new genes through gene
transfer
› Most common mechanism of transfer is
through conjugation
 Transfer of R plasmid
 Plasmid often carries several different
resistance genes
 Organism acquires resistance to several different
drugs simultaneously

Examples of emerging antimicrobial
resistance
› Enterococci
 Intrinsically resistant to many common
antimicrobials
 Some strains resistant to vancomycin
 VRE: Vancomycin resistant enterococcus
 Many strains achieve resistance via transfer of
plasmid
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Staphylococcus aureus
› Common cause of nosocomial infections
› Becoming increasingly resistant
 Most strains acquired resistance to penicillin
 Until recently most infections could be treated
with methicillin
 MRSA  methicillin resistant Staphylococcus aureus
 many of these strains still susceptible to vancomycin
 VISA vancomycin intermediate Staphylococcus aureus
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Streptococcus pneumoniae
› Has remained sensitive to penicillin
 Some strains have now gained
resistance
 Resistance due to modification in genes
coding for penicillin-binding proteins
 Acquisition via DNA mediated
transformation
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Slowing emergence and spread of
resistance
› Responsibilities of physicians and healthcare
workers
 Prescribe antibiotics for specific organisms
 Educate patients on proper use of antibiotics
› Responsibilities of patients
 Follow instructions carefully
 Complete prescribed course of treatment
 Misuse leads to resistance
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Slowing emergence and spread of resistance
› Importance of an educated public
 Greater effort made to educate public about
appropriateness and limitations of antibiotics
 Antibiotics have no effect on viral infections
 Misuse selects antibiotic resistance in normal flora
› Global impacts of the use of antimicrobial drugs
 Organisms which develop resistance in one country can be
transported globally
 Many antimicrobials are available as non-prescription basis
 Use of antimicrobial drugs added to animal feed
 Produce larger more economically productive animals
 Also selects for antimicrobial resistant organisms
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Available antiviral drugs
effective specific type of
virus
› None eliminate latent virus
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Targets include
› Viral uncoating
› Nucleoside analogs
› Non-nucleoside polymerase
inhibitors
› Non-nucleoside reverse
transcriptase inhibitors
› Protease inhibitors
› Neuraminidase inhibitors
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Viral uncoating
› Drugs include amantadine and rimantadine
› Mode of action is blocking uncoating of
influenza virus after it enters cell
 Prevents severity and duration of disease
› Resistance develops frequently and may
limit effectiveness of drug
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Nucleoside analogs
› Incorporation of analog results in termination of
growing nucleotide chain
› Examples of nucleoside analogs
 Zidovudine (AZT)
 Didanosine (ddI)
 Lamivudine (3TC)
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Non-nucleoside polymerase inhibitor
› Inhibit activation of viral polymerases by binding
to site other than nucleotide binding site
 Example = foscarnet and acyclovir
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Non-nucleoside reverse transcriptase
inhibitor
› Inhibits activity of reverse transcriptase by
binding to site other than nucleotide binding site
 Example = nevirapine, delavirdine, efavirenz
 Used in combination to treat HIV
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Protease inhibitor
› Inhibit HIV encoded enzyme protease
 Enzyme essential for production of viral
particles
 Examples = indinavir and ritonavir
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Neuraminidase inhibitor
› Inhibit neuraminidase enzyme of influenza
 Enzyme essential for release of virus
 Examples = zanamivir and oseltamivir
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Target for most
antifungal medications
is plasma membrane
› Ergosterol
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Include
› Polyenes
› Azoles
› Allylamines
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Other targets
› Cell wall synthesis
› Cell division
› Nucleic acid synthesis

Cell wall synthesis
› Echinocandins
 interfere with synthesis of fungal cell wall
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Cell division
› Griseofulvin
 Exact mechanism unknown
 Appears to interfere with action of tubulin
 Selective toxicity may be due to increased uptake by fungal
cells
 Used to treat skin and nail infections
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Nucleic acid synthesis
› Flucytosine
 Inhibits enzymes required for nucleic acid synthesis
 Flucytosine converted to 5-fluorouricil
Many antiparasitic drugs most likely
interfere with biosynthetic pathways of
protozoan parasites or neuromuscular
function of worms
 Example of parasitic drugs includes
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› Malarone
 Synergistic combination of atovaquone and proguanil
HCl
 Interferes with mitochondrial electron transport and
disruption of folate synthesis