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Importance of Antimicrobial Medications
§  Imagine a world without antimicrobial
medications
•  people with common diseases (e.g., bacterial
pneumonia, severe staphylococcal infections)
usually died
•  penicillin in 1940s
•  changed that
•  Misuse coupled with
evolution of microbial
resistance threatens
these medications
The dawn of antibiotics
§  Paul Erlich (1910)
•  Wanted to find the “magic
bullet” for syphilis
•  proposed the idea of the
blood brain barrier
•  Worked at staining tissues
and first to come up with
the idea behind “selective
toxicity”
•  Nobel Prize in 1908
A Glimpse of History
§  Paul Ehrlich (1854–1915)
•  Observed some dyes stain bacterial but not animal cells
•  Indicated fundamental difference between cell types
•  Searched for “magic bullet” that would kill microbial
pathogens without harming human host
•  Synthesized arsenic compounds to treat syphilis, caused
by spirochete Treponema pallidum
•  In 1910, the 606th tested compound proved effective in
laboratory animals
–  Arsphenamine, named Salvarsan
–  Potentially lethal for patients but did cure infections
previously considered hopeless
–  Proved some chemicals could selectively kill microbes
History and Development of Antimicrobial Drugs
§  Discovery of Antimicrobial Drugs
•  Salvarsan (Paul Ehrlich,1910) first documented case
•  Red dye Prontosil (Gerhard Domagk, 1932) used to
treat streptococcal infections in animals
•  No effect in test tubes; enzymes in blood split to produce
sulfanilamide, the first sulfa drug
•  Both are chemotherapeutic agents
•  Chemicals used to treat disease
•  Antimicrobial drugs, or antimicrobials
Alexander Fleming
§  A physician who studied
bacterial action of blood
and antisepsis
§  Discovered and named
Lysozyme
§  Discovered mold growing
on an agan plate(1928)
§  1945 Nobel Prize in
Physiology or Medicine
along with Chain and
Florey
Chain and Florey
§  1940 developed a system for growing Penicillium and
purifying the drug
§  Tested the drug in mice, passed all trials
§  Received the Nobel Prize in 1945 with Alexander
Fleming for their work
20.1. History and Development of Antimicrobial Drugs
•  Soon thereafter, Selman Waksman
purified streptomycin from soil
bacterium Streptomyces griseus
•  Researchers screened hundreds
of thousands of microbial strains
for antibiotics
•  Even today, pharmaceutical
companies examine soil samples
from around world
•  Discovered in 1960s that altering
structure of penicillin yields new
drugs
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Temocillin
1980
Mezlocillin
Piperacillin
Carfecillin
1975
Talampicillin
Apalcillin
Azlocillin
Pivmecillinam
Bacampicillin
Carindacillin
Sulbenicillin
Ticarcillin
1970
Epicillin
Mecillinam
Cyclacillin
Amoxicillin
Flucloxacillin
Pivampicillin
Azidocillin
Carbenicillin
1965
Dicloxacillin
Cloxacillin
Ampicillin
1960
Oxacillin
Nafcillin
Propicillin
Phenethicillin
Methicillin
6-APA
1955
1950
Penicillin V
Penicillin G
Penicillium chrysogenum
From Rolinson, George, N., (1998), Journal of Antimicrobial Chemotherapy 41, 489-603
Features of Antimicrobial Drugs
§  Selective Toxicity: cause greater harm to microbes
•  Interfere with essential structures or properties common
in microbes but not human cells
•  Toxicity is relative and expressed as therapeutic index
•  Lowest dose toxic to patient / dose used for therapy
–  Penicillin G useful, has high index; interferes with cell
wall synthesis, a process not present in humans
–  Drugs too toxic for systemic use may be used topically
§  Antimicrobial Action
•  Bacteriostatic drugs inhibit bacterial growth
•  Patient’s defenses must still eliminate
•  Bactericidal drugs kill bacteria
•  Sometimes only inhibitory
Features of Antimicrobial Drugs
§  Spectrum of Activity
•  Broad-spectrum antimicrobials affect a wide range
•  Important for treating acute life-threatening diseases
–  Especially when no time to culture for identification
•  Disrupt normal microbiota that aid in excluding pathogens
•  Narrow-spectrum antimicrobials affect limited range
•  Requires identification of pathogen, testing for sensitivity
•  Less disruptive to normal microbiota
§  Effects of Combinations
•  Some drugs interfere with each other, are antagonistic
•  Combinations where one drug enhances are synergistic
•  Combinations that are neither are additive
20.2. Features of Antimicrobial Drugs
§  Tissue Distribution, Metabolism, and Excretion
•  Antimicrobials differ in behavior in body
•  E.g., only some drugs cross from bloodstream into
cerebrospinal fluid, important in treating meningitis
•  Some unstable at low pH, must be injected
•  Rate of elimination or half-life dictates frequency
•  Patients with kidney or liver dysfunction may differ
§  Adverse Effects
•  Include allergic reactions and toxic effects
•  Suppression of normal microbiota may allow pathogens
to flourish (e.g., Clostridium difficile)
•  Important to remember that antimicrobials save
countless lives when properly prescribed and used
20.2. Features of Antimicrobial Drugs
§  Resistance to Antimicrobials
•  Certain bacteria have innate or intrinsic resistance
•  E.g., Mycoplasma lack cell wall, resist penicillin etc.
•  Outer membrane of Gram-negatives resists many drugs
•  Bacteria may develop acquired resistance
•  Spontaneous mutations
•  Horizontal gene transfer
Range of activity
§  Narrow range: target one group of microbes
§  Broad range: target a wide group of different
microbes
§  Which one is the best?
Mechanisms of Action of Antibacterial Drugs
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
wall
§  Antibacterial Cell
(peptidoglycan)
drugs target
synthesis
specific bacterial
processes and
structures
•  Cell wall
synthesis
•  Protein synthesis
•  Nucleic acid
synthesis
Cell membrane
Integrity
•  Metabolic
pathways
•  Cell membranes
Nucleic acid synthesis
A
B
Metabolic pathways
Protein
synthesis
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Cell wall
(peptidoglycan)
synthesis
β-lactam drugs
Vancomycin
Bacitracin
Nucleic acid synthesis
Fluoroquinolones
Rifamycins
A
Cell membrane
integrity
Polymyxin B
Daptomycin
B
Metabolic pathways
(folate biosynthesis)
Sulfonamides
Trimethoprim
Protein synthesis
Aminoglycosides
Tetracyclines
Macrolides
Chloramphenicol
Lincosamides
Oxazolidinones
Streptogramins
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
β-lactam drugs
Competitively inhibit enzymes that
help form peptide bridges between
adjacent glycan chains.
Vancomycin
Binds to the amino acid side
chain of NAM molecules,
blocking peptidoglycan
synthesis.
Peptidoglycan
(cell wall)
Cytoplasmic
membrane
NAG
NAM
Bacitracin
Interferes with the transport
of peptidoglycan precursors
across the cytoplasmic
membrane.
20.3. Mechanisms of Action of Antibacterial Drugs
§  Antibacterial Drugs That Inhibit Cell Wall Synthesis
•  Bacterial cell walls are unique, contain peptidoglycan
•  Great target for drugs: often have high therapeutic index
§  Penicillins, Cephalosporins, other β-Lactam Drugs
•  All have β-Lactam ring
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Penicillin
O
S
C
R
NH
CH
CH
C
N
CH3
C
CH3
CH
COOH
O
β-lactam ring
(a)
Cephalosporin
O
R
C
S
NH
CH
CH
C
N
CH2
C
O
R
β-lactam ring
(b)
COOH
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Basic Structure
Side Chain
C
CH2
C
S
NH
CH3
CH
CH
C
C
N
CH
O
CH3
COOH
β-lactam ring
Penicillin G
OCH2
Side chain varies
for derivatives
of penicillin
Penicillin V
(acid-resistant)
OCH3
Methicillin
(penicillinase-resistant)
OCH3
CI
N
CI
Dicloxacillin
(acid- and penicillinase-resistant)
O
CH3
Ampicillin
(broad-spectrum and acid-resistant)
CH
NH2
HO
Amoxicillin
(like ampicillin but more active
and requiring less frequent doses)
CH
NH2
CH
S
COONa
CH
Piperacillin
(like ticarcillin but a broader
spectrum of activity)
NH
C
Ticarcillin
(more activity against Gram-negative
rods, including Pseudomonas, but not
as effective against some Gram-positive
organisms)
O
N
O
N
O
C 2H 5
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Temocillin
1980
Mezlocillin
Piperacillin
Carfecillin
1975
Talampicillin
Apalcillin
Azlocillin
Pivmecillinam
Bacampicillin
Carindacillin
Sulbenicillin
Ticarcillin
1970
Epicillin
Mecillinam
Cyclacillin
Amoxicillin
Flucloxacillin
Pivampicillin
Azidocillin
Carbenicillin
1965
Dicloxacillin
Cloxacillin
Ampicillin
1960
Oxacillin
Nafcillin
Propicillin
Phenethicillin
Methicillin
6-APA
1955
1950
Penicillin V
Penicillin G
Penicillium chrysogenum
From Rolinson, George, N., (1998), Journal of Antimicrobial Chemotherapy 41, 489-603
Antibacterial Drugs That Inhibit Cell Wall Synthesis
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
§  The penicillins share basic structure
C
CH2
•  Side chain modified to create derivatives
•  Five general groups of penicillins
•  Natural are from Penicillium
chrysogenum
–  Narrow-spectrum, act against Grampositives and a few Gram-negatives
•  Penicillinase-resistant developed in
response to S. aureus strains
–  Some now able to produce altered
PBPs to which β-lactam drugs do not
bind (e.g., methicillin-resistant
S. aureus, or MRSA)
Basic Structure
Side Chain
C
S
NH CH
C
CH
N
O
CH3
C
CH3
COOH
C
H
β-lactam ring
Penicillin G
OCH2
Penicillin V
(acid-resistant)
OCH3
Methicillin
(penicillinase-resistant)
OCH3
CI
CI
N
O CH3
Ampicillin
(broad-spectrum and acid-resistant)
CH
NH2
HO
Amoxicillin
(like ampicillin but more active
and requiring less frequent doses)
CH
NH2
CH
S
COONa
CH
NH
C O
O
N
N
Dicloxacillin
(acid- and penicillinase-resistant)
O
C 2H 5
Ticarcillin
(more activity against Gramnegative rods, including
Pseudomonas, but not as effective
against some Gram-positive
organisms)
Piperacillin
(like ticarcillin but a broader
spectrum of activity)
Antibacterial Drugs That Inhibit Cell Wall Synthesis
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
§  The penicillins share basic structure
C
C
CH2
•  Side chain modified to create derivatives
•  Broad-spectrum act against Grampositives and Gram-negatives due to
modified side chain
–  Inactivated by many β-lactamases
•  Extended-spectrum have greater activity
against Pseudomonas species
–  Reduced activity against Grampositives; destroyed by many
β-lactamases
•  Penicillins + β-lactamase inhibitor
includes inhibitor to protect penicillin
Basic Structure
Side Chain
S
NH CH
O
C
CH
N
CH3
C
CH3
COOH
C
H
β-lactam ring
Penicillin G
OCH2
Penicillin V
(acid-resistant)
OCH3
Methicillin
(penicillinase-resistant)
OCH3
CI
CI
N
O CH3
Ampicillin
(broad-spectrum and acid-resistant)
CH
NH2
HO
Amoxicillin
(like ampicillin but more active
and requiring less frequent doses)
CH
NH2
CH
S
COONa
CH
NH
C O
O
N
N
Dicloxacillin
(acid- and penicillinase-resistant)
O
C 2H 5
Ticarcillin
(more activity against Gramnegative rods, including
Pseudomonas, but not as effective
against some Gram-positive
organisms)
Piperacillin
(like ticarcillin but a broader
spectrum of activity)
Antibacterial Drugs That Inhibit Cell Wall Synthesis
§  Penicillins, Cephalosporins, other β-Lactam Drugs
(cont…)
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
•  Interfere with peptidoglycan
synthesis
•  Weaken cell walls, leads
to cell lysis
β-lactam drugs
Competitively inhibit enzymes
that help form peptide bridges
between adjacent glycan
chains.
Vancomycin
Binds to the amino acid side
chain of NAM molecules,
blocking peptidoglycan
synthesis.
Peptidoglycan
(cell wall)
Cytoplasmic
membrane
NAG
NAM
Bacitracin
Interferes with the transport
of peptidoglycan precursors
across the cytoplasmic
membrane.
Penicillin weakens the cell wall
Beta-lactam ring common with
penicillins and cephalosporins
How organisms degrade penicillins
Antibacterial Drugs That Inhibit Cell Wall Synthesis
§  The Cephalosporins
•  from fungus Acremonium cephalosporium
•  Include closely related group from Streptomyces
•  Structure is resistant to some β-lactamases
•  Some have low affinity for PBPs of Gram-positives
•  Chemical modifications have led to four generations
•  Later generations more effective, resist β-lactamases
§  Other β-Lactam Antibiotics
•  Carbapenems and monobactams resist β-lactamases
•  Carbapenems effective against a wide range
•  Monobactam Aztreonam used against Enterobacteriaceae
Antibacterial Drugs That Inhibit Cell Wall Synthesis
§  Vancomycin blocks peptidoglycan synthesis
•  Binds to peptide side chain of NAM molecules
•  Weakens cell wall, causes lysis
•  Poorly absorbed from intestinal tract, usually administered
via IV except for intestinal infections
•  Important for treating Gram-positives resistant to β-lactam
drugs; also used for severe C. difficile
•  Often drug of last resort; resistance increasingly a problem
–  Change in side chain of NAM prevents binding
•  Does not cross outer membrane of Gram-negatives
§  Bacitracin: toxicity limits to topical applications
•  Interferes with transport of peptidoglycan precursors
across membrane; common in first-aid ointments
20.3. Mechanisms of Action of Antibacterial Drugs
§  Antibacterial Drugs That Inhibit Protein Synthesis
•  All cells synthesize proteins
•  Can exploit differences between prokaryotic and
eukaryotic ribosomes
•  Prokaryotes have 70S, eukaryotes have 80S ribosomes
•  Mitochondria also
Macrolides
Prevent the continuation
of protein synthesis.
have 70S ribosomes Streptogramins
Each interferes with a
distinct step of protein
Chloramphenicol
–  May account for synthesis.
Prevents peptide
bonds from being
some toxicity
formed.
Lincosamides
50S
the
of these drugs Prevent
continuation of
Oxazolidinones
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
protein synthesis.
30S
Tetracyclines and
glycylcyclines
Block the attachment
of tRNA to the ribosome.
Interfere with the
initiation of protein
synthesis.
Aminoglycosides
Block the initiation of
translation and cause the
misreading of mRNA.
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Macrolides
Prevent the continuation
of protein synthesis.
Streptogramins
Each interferes with a
distinct step of protein
synthesis.
Lincosamides
Prevent the
continuation of
protein synthesis.
50S
30S
Tetracyclines and
glycylcyclines
Block the attachment
of tRNA to the ribosome.
Chloramphenicol
Prevents peptide
bonds from being
formed.
Oxazolidinones
Interfere with the
initiation of protein
synthesis.
Aminoglycosides
Block the initiation of
translation and cause the
misreading of mRNA.
Aminoglycosides
§  Bactericidal §  Irreversibly bind to 30S ribosome, cause misreading
of the mRNA
§  Transported into cells that actively respire (not
effective against ananerobes, streptococci,
enterococci)
§  Ex: streptomycin, gentamicin, tobramycin
Tetracyclines §  Bind reversibly to 30S, block attachment of
the tRNA to ribosome
§  Actively transported into bacterial cells
§  Effective against gram positive and gram
negative
§  Resistance: due to decrease in uptake or
increase in excretion
§  Ex: Doxycycline
Macrolides
§  Reversibly bind to the 50S, prevent continuation
of protein synthesis
§  Drug of choice for patients allergic to penicillins
§  Not good for Enterobacteriaceae
§  Ex: Erythromycin, Azithromycin
§  Resistance: enzymes that alter drug, decreased
uptake
Oxazolidinones
§  Reversibly bind to the 50S subunit, interfere with
initiation of protein synthesis
§  Used for treating gram positive infections resistant to
Beta-lactam drugs and Vancomycin
§  Ex: Linezolid
Some drugs target protein synthesis
Antibiotics that inhibit nucleic
acid synthesis
§  Fluoroquinolones
•  Inhibit topoisomerase §  Rifamycins
•  Blocks prokaryotic RNA polymerase
from initiating transcription
Antibacterial Drugs That Inhibit Nucleic Acid Synthesis
§  Metronidazole toxic only in anaerobic organisms
•  Anaerobic metabolism required to convert to active form
•  Binds DNA, interferes with synthesis, causes breaks
Antibacterial Drugs That Interfere with Metabolic
Pathways
§  Few antibacterial drugs interfere with metabolism
•  Folate inhibitors are among most useful
•  Sulfonamides, trimethoprim inhibit different steps in
synthesis of folic acid and coenzyme required for
nucleotide synthesis
–  Animals lack enzymes to
synthesize folic acid;
require in diet
•  Resistance often from plasmid-encoded enzymes
with lower affinity to these drugs; genes for both often
carried on same plasmid
Antibacterial Drugs That Interfere with Metabolic
Pathways
§  Sulfonamides and related are called sulfa drugs
•  Inhibit many Gram-positives and Gram-negatives
•  Structurally similar to PABA, so enzyme binds drug
•  Example of competitive inhibition
•  Human cells lack enzyme
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
§  Trimethoprim inhibits
enzyme in later step
•  Combination of trimethoprim
and sulfonamide has
synergistic effect
Para-aminobenzoic
acid (PABA)
Precursor
#1
PABA
H 2N
COOH
Enzyme
#1
Sulfa drugs
Precursor
#2
Sulfanilamide
H 2N
Glutamate
Enzyme
#2
SO2NH2
Dihydrofolate
(a)
Enzyme
#3
•  Often used to treat UTIs
Trimethoprim
Tetrahydrofolate
Multiple enzymes
and reactions
Thymine, guanine,
and adenine
nucleotides
(b)
Sulfonamides (sulfa drugs)
§  First synthetic drugs to treat microbial infections
§  Used to treat urinary tract infections (UTIs)
§  Combination of trimethoprim and sulfamethoxazole
(TMP-SMZ) example of synergism
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Para-aminobenzoic
acid (PABA)
Precursor
#1
PABA
H 2N
COOH
Enzyme
#1
Sulfa drugs
Precursor
#2
Sulfanilamide
H 2N
Glutamate
Enzyme
#2
SO2NH2
Dihydrofolate
(a)
Enzyme
Trimethoprim
#3
Antibiotics that inhibit
metabolic pathways
Tetrahydrofolate
Multiple enzymes
and reactions
Thymine, guanine,
and adenine
nucleotides
§  Sulfonamides
§  Trimethoprims
(b)
Antibacterial Drugs That Interfere with Cell Membrane
Integrity
§  A few antimicrobials damage bacterial membranes
•  Cause cells to leak, leading to cell death
•  Daptomycin inserts into cytoplasmic membrane
•  Used against Gram-positives resistant to other drugs
•  Ineffective against Gram-negatives
–  Cannot penetrate outer membrane
•  Polymyxin B binds to membranes of Gram-negatives
•  Also bind to eukaryotic cells, though to a lesser extent
•  Limits usefulness to topical applications
Antibacterial Drugs Effective Against Mycobacterium
species
§  Few antimicrobials effective against Mycobacterium
•  Waxy cell prevents entry of many drugs; growth is slow
•  Group of five medications preferred: first-line drugs
•  Most effective, least toxic; given as combination therapy
–  Typically two or more at a time
–  Decrease chance of development of resistant mutants
•  Some target unique cell wall of mycobacteria
–  Isoniazid inhibits mycolic acid synthesis; ethambutol
inhibits enzymes required for synthesis of other cell wall
components
•  Second-line drugs used for resistant strains
–  Less effective or have greater toxicity risk
20.4. Determining Susceptibility of Bacterial Strain
§  Susceptibility of pathogens often unpredictable
§  Minimum Inhibitory and Bactericidal Concentrations
•  MIC is lowest concentration that prevents growth in vitro
•  Microbes with MIC between susceptible and resistant are
termed intermediate
•  MBC is lowest concentration that kills 99.9% of cells in
vitro; determined from plate count from MIC
•  Techniques precise but labor-intense, expensive
Kirby-Bauer tests for sensitivity
Resulting zone of inhibition compared with specially prepared
charts to determine whether strain is susceptible,
intermediate, or resistant
Drug characteristics must be taken into account (e.g.,
molecular weight, stability, amount)
20.4. Determining Susceptibility of Bacterial Strain
§  Minimum Inhibitory Concentration (MIC)
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Decreasing Concentration of the Antimicrobial Drug
Organism A
16 µg/ml
Control
(no bacteria)
8.0
4.0
2.0
1.0
0.5
0.25
0.12
0.06
8.0
4.0
2.0
1.0
0.5
0.25
0.12
0.06
8.0
4.0
2.0
1.0
0.5
0.25
0.12
0.06
0.03
Control
(no drug)
Result: MIC = 0.12 µg/ml
Organism B
Control 16 µg/ml
(no bacteria)
0.03
Control
(no drug)
Result: MIC = 1.0 µg/ml
Organism C
Control 16 µg/ml
(no bacteria)
0.03
Control
(no drug)
Result: MIC =16 µg/ml
E-test for MIC
20.4. Determining Susceptibility of Bacterial Strain
§  Measuring Concentration of Antimicrobial Drug in
Blood or Other Body Fluids
•  Some toxic; levels must be monitored to ensure safety
•  Diffusion bioassay compares known concentrations with
patient samples
•  Can produce
standard curve
•  Comparison yields
concentration
Concentration of drug (µg/ml) (logarithmic scale)
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
6.25
C
1.25
0.25
10
20
D
30
Zone diameter (mm)
Standards and patient’s serum are
added to agar that has been seeded
with susceptible strain of bacteria.
A standard curve that correlates the zone diameter with
the antimicrobial drug concentration is constructed.
The drug concentration in the serum can be read
from the line relating zone size to concentration.
(a)
(b)
20.5. Resistance to Antimicrobial Drugs
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
§  Increasing use, misuse selects
for resistant microorganisms
S
S
S
S
•  Only 3% of Staphylococcus aureus
originally resistant to penicillin; now
more than 90% are resistant
R
S
S
Antimicrobial drug is added;
sensitive organisms are killed
or inhibited.
S
•  Hundreds of tons used each year
•  Antimicrobial resistance alarming
S
S
S
R
S
S
•  Dealing with problem requires
understanding of mechanisms and
spread of resistance
Resistant survivors can
multiply without competition.
R
R
R
R
R
R
R
Emerging Antibiotic Resistance
§  Enterococci
§  Staphylococcus aureus
§  Steptococcus pneumoniae
§  Mycobacterium tuberculosis
20.5. Resistance to Antimicrobial Drugs
§  Examples of Emerging Resistance
•  Enterococci: part of normal intestinal microbiota
•  Common cause of healthcare-associated infections
•  Intrinsically less susceptible to many antimicrobials
–  PBPs have low affinity to many β-lactam drugs
–  Many have R plasmids; some code for resistance to
vancomycin, yield vancomycin-resistant enterococci,
which is transferable to other microbes
–  Vancomycin often drug of last resort
20.5. Resistance to Antimicrobial Drugs
§  Examples of Emerging Resistance (continued…)
•  Staphylococcus aureus: increasingly resistant
•  Common cause of healthcare-associated infections
•  Most strains resistant to penicillin, encode penicillinase
•  New strains recently emerged, have PBPs with low
affinity to all β-lactam drugs including methicillin
–  Methicillin-resistant Staphylococcus aureus (MRSA)
•  Healthcare-associated (HA-MRSA) resistant to wide
range of antibiotics, usually treated with vancomycin
–  Hospitals have reported resistant isolates; strict
guidelines have halted spread of these vancomycinintermediate S. aureus (VISA) and vancomycinresistant S. aureus (VRSA) strains
•  Community acquired (CA-MRSA) currently treatable
20.5. Resistance to Antimicrobial Drugs
§  Examples of Emerging Resistance (continued…)
•  Streptococcus pneumoniae
•  Historically susceptible; some recently acquired penicillin
resistance; produce PBPs with lower affinity, likely via
transformation from other Streptococcus species
•  Enterobacteriaceae intrinsically resistant to many drugs
•  Outer membrane prevents entry
•  Some enterics developed ability to produce β-lactamases
•  Some further developed ability to produce extendedspectrum β-lactamases (ESBLs); resist cephalosporins
and aztreonam in addition to penicillins
•  More recently, carbapenem-resistant (CRE) strains found
–  Resistant to nearly all drugs: carbapenems were last
resort for ESBLs, but enzyme inactivates
20.5. Resistance to Antimicrobial Drugs
§  Examples of Emerging Resistance (continued…)
•  Mycobacterium tuberculosis requires long treatment
•  Can become resistant to first-line drugs via mutation
•  Large numbers of cells found in active infection, so likely
at least one cell has developed resistance
–  Combination therapy is therefore required
•  6 months or more of treatment necessary due to slow
rate of growth; many patients do not comply
–  Multidrug-resistant M. tuberculosis (MDR-TB) resist
two favored first-line drugs: isoniazid and rifampin
–  Directly observed therapy can prevent emergence
–  Extensively drug-resistant M. tuberculosis (XDR-TB)
of even greater concern, additionally resist three or
more second-line drugs
20.5. Resistance to Antimicrobial Drugs
§  Mechanisms of Acquired Resistance
•  Drug-Inactivating Enzymes
•  Penicillinase, chloramphenicol
acetyltransferase
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Non-resistant cell
Target
Drug
•  Alteration in Target Molecule
•  Minor structural changes
can prevent binding
•  E.g., PBPs (β-lactam drugs),
ribosomal RNA (macrolides)
•  Decreased Uptake of the Drug
•  E.g., changes in porin
proteins of outer membrane
of Gram-negatives
Drug binds
target.
Resistant cell
Increased elimination
Drug enters cell but
efflux pump ejects it.
Drug-inactivating enzyme
Enzyme modifies
drug, inactivating it.
Alteration in
target
Decreased uptake
molecule
Porin proteins prevent Drug cannot
entry into the cell.
bind target.
20.5. Resistance to Antimicrobial Drugs
§  Mechanisms of Acquired Resistance (cont…)
•  Increased Elimination of Drug
•  Efflux pumps remove
compounds from cell
•  Increased production of
pumps allows faster removal
•  Structural changes can
influence range of drugs
–  Resistance of this type
particularly worrisome;
might allow resistance
to multiple drugs
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Non-resistant cell
Target
Drug
Drug binds
target.
Resistant cell
Increased elimination
Drug enters cell but
efflux pump ejects it.
Drug-inactivating enzyme
Enzyme modifies
drug, inactivating it.
Decreased uptake
Porin proteins prevent
entry into the cell.
Alteration in
target
molecule
Drug cannot
bind target.
20.5. Resistance to Antimicrobial Drugs
§  Acquisition of Resistance
•  Spontaneous Mutations
•  Occur during replication
•  Happen at low rate but can have significant effect
•  Just a single base-pair change in gene encoding a
ribosomal protein yields resistance to streptomycin
–  In a population of 109 cells, at least one likely has that
mutation; if streptomycin is added, only that cell and
progeny will replicate, yielding resistant population
•  Spontaneous resistance to drugs with several different
targets or multiple binding sites is less likely
•  Combination therapy of multiple drugs is often used;
unlikely cells will simultaneously develop resistance
20.5. Resistance to Antimicrobial Drugs
§  Acquisition of Resistance (continued…)
•  Gene Transfer
•  Genes encoding resistance can spread to different
strains, species, even genera
–  Most commonly through conjugative transfer of R
plasmids, which often carry several resistance genes
•  Resistance can originate through spontaneous mutations
–  May also originate from the soil microbes that
naturally produce the antibiotic
–  Gene coding for enzyme that modifies
aminoglycoside likely originated from the
Streptomyces species that produces the drug
20.5. Resistance to Antimicrobial Drugs
§  Slowing Emergence and Spread of Resistance
•  cooperation needed
•  Physicians and Healthcare Workers
•  identify cause of infection before prescribing
•  Use suitable antimicrobials
•  Educate patients about proper use of drugs
•  Responsibilities of Patients
•  Follow instructions even if inconvenient
•  skipping a dose may reduce levels in the blood, allowing
less-sensitive microbes a chance to grow and spread
•  Failure to complete treatment may not kill least-sensitive,
20.5. Resistance to Antimicrobial Drugs
§  Slowing Emergence and Spread of Resistance
•  Educate Public
•  Antibiotics ineffective against viruses
–  Cannot cure common cold!
•  Misuse selects for antibiotic-resistant bacteria in normal
microbiota; they can eventually transfer to pathogens
•  Global Impacts of the Use of Antimicrobial Drugs
•  Overuse is a worldwide concern
•  Antimicrobial drugs available without prescription in many
parts of the world, may allow improper use
•  Antimicrobial drugs used in animal feeds selects for drugresistant microbes
–  Resistant Salmonella strains linked to animals
20.6. Mechanisms of Action of Antiviral Drugs
§  Viruses difficult to target selectively
•  Rely on host cell’s metabolic machinery; lack cell walls,
ribosomes, other structures targeted by antibiotics
•  Many encode polymerases;
represent potential targets
•  Scientists trying to develop
antiviral drugs that interfere
with viral replication
•  Current options effective
only against specific type
of virus; none eliminate
latent infections
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Entry inhibitors
Effective against HIV
Eukaryotic
host cell
Entry
Uncoating
Nucleic acid synthesis
(viral enzyme directed)
Integrase
inhibitors
Effective
against HIV
Viral particle
production
Exit
Viral uncoating
Effective against
influenza A virus
Amantadine
Rimantadine
Nucleic acid synthesis
Effective against herpesviruses
Nucleoside analogs
Non-nucleoside polymerase
inhibitors
Effective against HIV
Nucleoside analogs
Non-nucleoside reverse
transcriptase inhibitors
Assembly and release
of viral particles
Effective against HIV
Protease inhibitors
Effective against influenza viruses
Neuraminidase inhibitors
20.6. Mechanisms of Action of Antiviral Drugs
20.6. Mechanisms of Action of Antiviral Drugs
§  Entry Inhibitors
•  New group of drugs prevent viral entry into host cell
•  Enfuvirtide binds to an HIV protein that promotes fusion
of viral envelope with cell membrane
•  Maraviroc blocks HIV co-receptor CCR5
§  Viral Uncoating
•  Nucleic acid must separate from protein coat
•  Two drugs target this step—amantadine, rimantadine
•  Block influenza A viruses, prevent or reduce severity
•  Viral strains easily develop resistance; usefulness limited
20.6. Mechanisms of Action of Antiviral Drugs
§  Nucleic Acid Synthesis
•  Nucleoside Analogs: structure similar to nucleosides
•  Phosphorylated in vivo by virally encoded or normal
cellular enzyme to form nucleotide analog
•  Incorporation into nucleotide chain can stop nucleotides
from being added or alter base-pairing properties
•  Selective toxicity since virally encoded enzymes more
likely than host cell polymerases to incorporate
–  More damage done to rapidly replicating viral genome
–  But only effective against replicating viruses
•  Most reserved for severe infections; significant side effects
–  Acyclovir is exception; treats herpesvirus with little
harm to uninfected cells since only converted by virally
encoded enzymes present only in infected cells
20.6. Mechanisms of Action of Antiviral Drugs
§  Nucleic Acid Synthesis
•  Nucleoside Analogs (continued…)
•  Ganciclovir used to treat life- or sight-threatening
cytomegalovirus (CMV) infections in immunocompromised
•  Ribavirin used to treat respiratory syncytial virus infections
(RSV) in newborns
•  Nucleoside reverse transcriptase inhibitors (NRTIs) used
to treat HIV; virus rapidly develops mutational resistance
–  Often used in combination with other anti-HIV drugs
–  NRTIs include zidovudine (AZT), didanosine (ddI), and
lamivudine (3TC); two often used in combination
20.6. Mechanisms of Action of Antiviral Drugs
§  Nucleic Acid Synthesis
•  Non-Nucleoside Polymerase Inhibitors
•  Inhibit viral polymerases by binding to site other than
nucleotide-binding site
•  Foscarnet used to treat ganciclovir-resistant CMV and
acyclovir-resistant HSV
•  Non-Nucleoside Reverse Transcriptase Inhibitors
(NNRTIs) inhibit reverse transcriptase by binding to site
other than nucleotide-binding site
•  Often used with nucleoside analogs to treat HIV infections
•  Include nevirapine, delavirdine, efavirenz
20.6. Mechanisms of Action of Antiviral Drugs
§  Integrase Inhibitors
•  Offer new option for treating HIV infections
•  Inhibit HIV-encoded enzyme integrase; prevent virus from
inserting DNA copy of genome into host cell
•  Raltegravir is first approved drug of this class
§  Assembly and Release of Viral Particles
•  Virally encoded enzymes required for assembly, release
•  Protease Inhibitors
•  During replication of HIV, several proteins translated as a
polyprotein that must be cleaved
•  Includes indinavir, ritonavir, saquinavir, nelfinavir
•  Neuraminidase Inhibitors
•  Enzyme encoded by influenza viruses, needed for release
20.7. Mechanisms of Action of Antifungal Drugs
§  Eukaryotic pathogens difficult to target
•  More closely resemble human cells than bacteria
§  Plasma Membrane Synthesis and Function
•  Most antifungal drugs target ergosterol; humans lack
•  Polyenes produced by Streptomyces, bind to ergosterol,
cause membrane to leak; toxic to humans
•  Azoles inhibit ergosterol Cell division
Griseofulvin
synthesis, cause
Plasma membrane
synthesis/function
membrane to leak
Polyenes
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•  Many have side effects
•  Allylamines inhibit
enzyme in ergosterol
synthesis pathway
Azoles
Allylamines
Nucleic acid
synthesis
Flucytosine
Cell wall synthesis
Echinocandins
Nucleus
20.7. Mechanisms of Action of Antifungal Drugs
§  Cell Wall Synthesis
•  Fungal cell walls have some components animals lack
•  Echinocandins interfere with synthesis of β-1, 3 glucan
•  Causes cells to lyse; caspofungin is first to be approved
§  Cell Division
•  Griseofulvin targets cell division, interferes with tubulin
•  Found in all eukaryotic cells; selective toxicity may be due
to greater uptake by fungal cells
§  Nucleic Acid Synthesis
•  Common to all eukaryotes, generally poor drug target
•  Flucytosine taken up by yeast cells, converted to active
form, inhibits nucleic acid synthesis
20.7. Mechanisms of Action of Antifungal Drugs
§  Characteristics of Antifungal Drugs
20.8. Mechanisms of Action of Antiprotozoan and
Antihelminthic Drugs
20.8. Mechanisms of Action of Antiprotozoan and
Antihelminthic Drugs
§  Relatively little research and development
•  Most parasitic diseases concentrated in poorer areas of
the world; medications unaffordable
•  Most drugs interfere with biosynthetic pathways of
protozoan parasites or neuromuscular function of worms