chapter 12 drugs, microbes, host the elements of …
TRANSCRIPT
Chapter 12
Drugs, Microbes, Host – The Elements of
Chemotherapy
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
2
12.1 Principles of Antimicrobial Therapy
• Administer a drug to an infected person that
destroys the infective agent without harming the
host’s cells
• Antimicrobial drugs are produced naturally or
synthetically
4
The Origins of Antimicrobial Drugs
• Antibiotics are common metabolic products of aerobic
bacteria and fungi
– Bacteria in genera Streptomyces and Bacillus
– Molds in genera Penicillium and Cephalosporium
• By inhibiting the other microbes in the same habitat, antibiotic producers have less competition for nutrients and space
Insight 12.1 Sir Alexander
Fleming
5
Interactions Between Drugs and Microbe
• Antimicrobial drugs should be selectively toxic
– drugs should kill or inhibit microbial cells
without simultaneously damaging host tissues
• As the characteristics of the infectious agent
become more similar to the vertebrate host cell,
complete selective toxicity becomes more
difficult to achieve and more side effects are
seen
6
Mechanisms of Drug Action
1. Cell wall inhibitors
Block synthesis and repair
Penicillins
Cephalosporins
Vancomycin
Bacitracin
Monobactams/carbapenems
Fosfomycin
Cycloserine
Isoniazid
2. Cell membrane
Cause loss of selective permeability
Polymyxins
3. DNA/RNA
DNA mRNA
Inhibit replication and transcription
Inhibit gyrase (unwinding enzyme)
Quinolones (ciprofloxacin)
Inhibit RNA polymerase
Rifampin
4. Protein synthesis inhibitors acting
on ribosomes
Site of action
50S subunit
Chloramphenicol
Erythromycin
Clindamycin
Streptogramin (Synercid)
Site of action
30S subunit
Aminoglycosides
Gentamicin
Streptomycin
Tetracyclines
Both 30S
and 50S
Blocks initiation of protein
Synthesis
Linezolid (Zyvox)
5. Metabolic pathways and products
Substrate
Ribosome
Enzyme
Product
Block pathways and inhibit
Metabolism
Sulfonamides (sulfa drugs)
Trimethoprim
Figure 12.2 Major targets of drugs in bacteria
7
The Spectrum of an Antimicrobic Drug
• Spectrum – range of activity of a drug
– Narrow-spectrum: effective on a small
range of microbes
• Target a specific cell component that is found
only in certain microbes
– Broad-spectrum: greatest range of activity
• Target cell components common to most
pathogens (ribosomes)
8
12.2 Survey of Major Antimicrobial Drug Groups
Antimicrobial Drugs That Affect the Bacterial Cell Wall
• Most bacterial cell walls contain peptidoglycan
• Penicillins and cephalosporins block synthesis of peptidoglycan, causing the cell wall to lyse
• Active on young, growing cells
• Penicillins that do not penetrate the outer membrane and are less effective against gram-negative bacteria
• Broad spectrum penicillins and cephalosporins can cross the cell walls of gram-negative bacteria
9
Figure 12.3 Activity of Penicillin
Peptidoglycan
(cell wall)
Exposure to cephalosporin
or penicillin
(a) Normal untreated cells. Left and center views
are Staphylococcus cells; right view is the molecular
structure of peptidoglycan, with chains of NAM
(N-acetyl muramic acid) and NAG (N-acetyl
glucosamine) linked together by cross bridges
(green).
Membrane bulges
out as water diffuse
into cell.
Membrane breaks.
Cell lyses.
Weak points lacking
peptidoglycan
(b) Effects of treatment with penicillin. Left side depicts
the stages in the breakdown of a Staphylococcus cell
exposed to penicillin. Center view shows actual cells
undergoing lysis and cell death. Right is a molecular view
of how penicillin blocks the formation of the cross bridges,
which removes the support that holds the cell wall together.
10,000
Janice Carr/CDC
10,000
© CNRI/Photo Researchers, Inc.
Antibacterial Drugs that Act on the Cell Wall
• Beta-lactam antimicrobials - all contain a highly reactive 3 carbon, 1 nitrogen ring
• Primary mode of action is to interfere with cell wall synthesis
• Greater than ½ of all antimicrobic drugs are beta-lactams
• Penicillins and cephalosporins most prominent beta-lactams
10
Figure 12.7 Penicillin and Its Relatives
• Large diverse group of compounds
• Could be synthesized in the laboratory
• More economical to obtain natural penicillin through microbial fermentation and modify it to semi-synthetic forms
• All consist of 3 parts:
– Thiazolidine ring
– Beta-lactam ring
– Variable side chain dictating microbial activity (R)
Nafcillin
Beta-
lactam
Thiazolidine
CO
R Group
CH3
O COOH
CH3
N
N
S
CH3
O COOH
CH3
N
N
S
CH3
O COOH
CH3
N
N
S
CH3
O COOH
CH3
N
N
S
Nucleus
(Aminopenicillanic acid)
Ticarcillin
CO CH
COONa
Cloxacillin
CO
CH3
Cl
Carbenicillin
CO CH
COONa
S
O N
12
Subgroups and Uses of Penicillins
• Penicillins G and V most important natural forms
• Penicillin is the drug of choice for gram-positive cocci (streptococci) and some gram-negative bacteria (meningococci and syphilis spirochete)
• Semisynthetic penicillins – ampicillin, carbenicillin, and amoxicillin have broader spectra – Gram-negative infections
• Penicillinase-resistant – methicillin, nafcillin, cloxacillin
• Primary problems – allergies and resistant strains of bacteria
13
Cephalosporins
• Account for one-third of all antibiotics administered
• Synthetically altered beta-lactam structure
• Relatively broad-spectrum, resistant to most penicillinases, and cause fewer allergic reactions
• Some are given orally; many must be administered parenterally
• Generic names have root – cef, ceph, or kef
Basic Nucleus
COOH CH2
6 5
4
3 2
CH2
S or O
OC
O
CH2
OCH3
Cefotiam
(second generation)
Cephalothin
(first generation*)
Moxalactam
(third generation)
Cefepime
(fourth generation)
O
C
O
NH2
NH2
N CH2
C CH
COONa
OH
O
N
N
CH2
N
N S CH2 N
CH3
CH3
S CH2
N
N N
N
O
N
S
N
CH3
CH3 CH2
R Group 1 R Group 2
N1 S
S
*New improved versions of drugs are referred to as new “generations.”
N
2nd
3rd
4th
7
Figure 12.8 The structure of
cephalosporins
14
Non Beta-lactam Cell Wall Inhibitors
• Vancomycin – narrow-spectrum, most effective in treatment of Staphylococcal infections in cases of penicillin and methicillin resistance or if patient is allergic to penicillin; toxic and hard to administer; restricted use
• Bacitracin – narrow-spectrum produced by a strain of Bacillus subtilis; used topically in ointment
• Isoniazid (INH) – works by interfering with mycolic acid synthesis; used to treat infections with Mycobacterium tuberculosis
15
Antimicrobial Drugs That Disrupt Cell Membrane Function
• A cell with a damaged membrane dies from disruption in metabolism or lysis
• These drugs have specificity for a particular microbial group, based on differences in types of lipids in their cell membranes
• Polymyxins interact with phospholipids and cause leakage, particularly in gram-negative bacteria
• Amphotericin B and nystatin form complexes with sterols on fungal membranes which causes leakage
16
Figure 12.4 Activity of Polymyxins
Cell
membrane
Polymyxin B
A detergent that
causes openings in the
cell membrane
Antibiotics That Damage Bacterial Cell Membranes
• Polymixins, narrow-spectrum peptide
antibiotics with a unique fatty acid component
– Treat drug resistant Pseudomonas
aeruginosa and severe UTI
17
18
Drugs That Act on DNA or RNA
• May block synthesis of nucleotides, inhibit replication, or
stop transcription
• Chloroquine binds and cross-links the double helix;
quinolones inhibit DNA helicases
• Antiviral drugs that are analogs of purines and
pyrimidines insert in viral nucleic acid, preventing
replication
Drugs that Act on DNA or RNA
• Fluoroquinolones – work by binding to DNA
gyrase and topoisomerase IV
– Broad spectrum effectiveness
• Concerns have arisen regarding the overuse
of quinoline drugs
– CDC is recommending careful monitoring
of their use to prevent ciprofloxacin-
resistant bacteria
19
20
Drugs That Interfere with Protein Synthesis
• Ribosomes of eukaryotes differ in size and structure
from prokaryotes; antimicrobics usually have a selective
action against prokaryotes; can also damage the
eukaryotic mitochondria
• Aminoglycosides (streptomycin, gentamycin) insert on
sites on the 30S subunit and cause misreading of mRNA
• Tetracyclines block attachment of tRNA on the A
acceptor site and stop further synthesis
21
Figure 12.5 Targets on the 70S Ribosome
mRNA
mRNA
Aminoglycosides
50S
30S
aa aa mRNA is
misread,
protein is
incorrect
Chloramphenicol
mRNA
aa aa Formation
of peptide
bonds is
blocked
Erythromycin
mRNA
aa aa 50S
30S
Ribosome
is prevented
from
translocating
Tetracyclines
50S
30S
tRNA is
blocked,
no protein is
synthesized
Oxazolidinones
Prevent
initiation and
block ribosome
assembly
50S
30S
22
Drugs That Interfere with Protein Synthesis
• Aminoglycosides – composed of one or more amino sugars and an aminocyclitol (6C) ring; binds ribosomal subunit
-
NH2
OH
OH
O OH
NH C
NH
NH NH2 C
NH
CHO
O
CH3
O
OH
CH2OH O
OH
OH
6-carbon ring
2 sugars
CH3NH
Figure 12.9 The
structure of an
aminoglycoside
23
Drugs That Interfere with Protein Synthesis
• Aminoglycosides –
• Products of various species of soil actinomycetes in genera Streptomyces and Micromonospora
• Broad-spectrum, inhibit protein synthesis, especially useful against aerobic gram-negative rods and certain gram-positive bacteria – Streptomycin: bubonic plague, tularemia, TB
– Gentamicin: less toxic, used against gram-negative rods
– Newer – tobramycin and amikacin gram-negative bacteria
24
Tetracycline Antibiotics
• Broad-spectrum, block protein synthesis by binding ribosomes
• Treatment for STDs, Rocky Mountain spotted fever, Lyme disease, typhus, acne, and protozoa
• Generic tetracycline is low in cost but limited by its side effects
OH
R
O OH
R R R N
CH3 CH3
OH
OH
COCH2
O
(a) Tetracyclines
Figure 12.10 Structure
of a broad spectrum
antibiotic
25
Chloramphenicol • Potent broad-spectrum drug with unique nitrobenzene
structure
• Blocks peptide bond formation and protein synthesis
• Entirely synthesized through chemical processes
• Very toxic, restricted uses, can cause irreversible damage to bone marrow
• Typhoid fever, brain abscesses, rickettsial, and chlamydial infections
O
Cl2
C NO2
H
OH
C
NH
H
CH2 OH
C CH
(b) Chloramphenicol
Figure 12.10
Structure of a broad
spectrum antibiotic
26
Macrolides and Related Antibiotics
• Erythromycin – large lactone ring with sugars; attaches to ribosomal 50s subunit
• Broad-spectrum, fairly low toxicity
• Taken orally for Mycoplasma pneumonia, legionellosis, Chlamydia, pertussis, diphtheria and as a prophylactic prior to intestinal surgery
• For penicillin-resistant – gonococci, syphilis, acne
• Newer semi-synthetic macrolides – clarithromycin, azithromycin
OH
O O
CH3
CH3
OH
(c) Erythromycin
CH3
CH2
CH3 CH3 OH
CH3
O
O
OH
CH3
O
N(CH3)2
Sugar
O
Sugar
CH3
OH CH3 CH3O
Lactone ring
O Figure 12.10
Structure of a
broad spectrum
antibiotic
27
Drugs that Block Metabolic Pathways
• Sulfonamides and trimethoprim block enzymes required for tetrahydrofolate synthesis needed for DNA and RNA synthesis
Sulfonamides
inhibit enzyme
Only certain bacteria
perform this step
PABA Dihydropteroic
acid
Dihydrofolic
acid
Trimethoprim
inhibits enzyme
Folic acid is coenzyme
required to synthesize
Tetrahydrofolic
acid (folic acid) Pyrimidines
Purines
Amino acids
(a) The metabolic pathway needed to synthesize
tetrahydrofolic acid (THFA) contains two enzymes
that are chemotherapeutic targets. Under normal
circumstances, PABA acts as a substrate for the
enzyme pteridine synthetase, the product of which
is needed for the eventual production of folic acid. Enzyme Enzyme Enzyme
PABA
(substrate)
Sulfa drug
(inhibitor) Sulfa drug
(inhibitor)
Higher levels of
sulfa drug
more likely to
bind to enzyme
Figure 12.6 Competitive
inhibition as a mode of action
28
Drugs that Block Metabolic Pathways
• Competitive inhibition – drug competes with normal substrate for enzyme’s active site
• Synergistic effect – the effects of a combination of antibiotics are greater than the sum of the effects of the individual antibiotics
N
N
H
H H
H
O S O
N H
O HO
H
C
(b)
Sulfanilamide PABA
Structural differences
Figure 12.6 Competitive
inhibition as a mode of
action
29
• Most are synthetic; most important are sulfonamides, or sulfa drugs - first antimicrobic drugs
• Narrow-spectrum; block the synthesis of folic acid by bacteria – Sulfisoxazole: shigellosis, UTI,
protozoan infections
– Silver sulfadiazine: burns, eye infections
– Trimethoprim: given in combination with sulfamethoxazole: UTI, PCP(Pneumocystis carinii pneumonia)
Nucleus R Group
CH3
O
C
CH3
O
CH3
N
N
N
NH2 SO2 NH
(a)
(b)
(c)
Drugs That Block Metabolic Pathways
30
12.3 Drugs to Treat Fungal, Parasitic and Viral Infections
• Fungal cells are eukaryotic; a drug that is toxic to fungal cells also toxic to human cells
• Five antifungal drug groups: – Macrolide polyene
• Amphotericin B – mimic lipids, most versatile and effective, topical and systemic treatments
• Nystatin – topical treatment
(a)
OH
OH O
OH OH
OH
OH OH
OH
O
O
Figure 12.12 an
antifungal drug
31
Antifungal Drugs • Five antifungal drug groups:
– Griseofulvin: stubborn cases of dermatophyte infections, nephrotoxic
– Synthetic azoles: broad-spectrum; ketoconazole, clotrimazole, miconazole
– Flucytosine: analog of cytosine; cutaneous mycoses or in combination with amphotericin B for systemic mycoses
– Echinocandins: damage cell walls; capsofungin
(b) (c) NH2
O
N F
N
H Cl
C
N
N
Figure 12.12 b.
clotrimazole,
inhibits fungal
cell membrane
Figure 12.12 c.
flucytosine,
inhibits DNA and
protein synthesis
32
Antiparasitic Chemotherapy
• Antimalarial drugs: quinine, chloroquinine, primaquine, mefloquine
• Antiprotozoan drugs: metronidazole (Flagyl), quinicrine, sulfonamides, tetracyclines
• Antihelminthic drugs: immobilize, disintegrate, or inhibit metabolism – Mebendazole, thiabendazole: broad-spectrum,
inhibit function of microtubules, interferes with glucose utilization and disables them
– Pyrantel, piperazine: paralyze muscles
– Niclosamide: destroys scolex
33
Antiviral Chemotherapeutic Agents
• Selective toxicity is almost impossible due to obligate intracellular parasitic nature of viruses
• Block penetration into host cell
• Block replication, transcription, or translation of viral genetic material
– Nucleotide analogs • Acyclovir – herpesviruses
• Ribavirin – a guanine analog – RSV, hemorrhagic fevers
• AZT – thymine analog – HIV
• Prevent maturation of viral particles – Protease inhibitors : HIV
34
Drugs for Treating Influenza
• Amantadine, rimantidine – restricted almost exclusively
to influenza A viral infections; prevent fusion of virus with
cell membrane
• Relenza and tamiflu – slightly broader spectrum; blocks
neuraminidase in influenza A and B
Fuzeon
Prevents binding of viral GP-41 receptors to cell
receptor which blocks fusion of virus with cell
Maravoc
Covers up the cell receptors;
HIV cannot adhere and is inert
Amantidine and relatives
Block entry of influenza virus by interfering with
fusion of virus with cell membrane (also release)
Tamiflu, Relenza
Stop the actions of influenza neuraminidase,
required for entry of virus into cell
HIV
virus
Host cell membrane
Drug No Fusion/no entry
Cell
receptors
Influenza
virus Amantidine, Tamiflu
Thought to interfere with
influenza virus assembly or
budding/release
Nucleus
I. Inhibition of Virus Entry or Release
1
2
3
4
1
2
3
4
4
Table 12.6 Antiviral Drugs
35
Antiherpes Drugs
• Many antiviral agents mimic the structure of nucleotides
and compete for sites on replicating DNA
– Acyclovir (Zovirax), Valacyclovir (Valtrex),
Famiciclovir (Famvir), Peniciclovir (Denavir)
• Oral and topical treatments for oral and genital herpes,
chickenpox, and shingles
DNA
Polymerase
Virus DNA
Herpesvirus
Drug
HIV
virus
HIV RNA
RT
no reverse transcription
No HIV
DNA
No Viral DNA
replication
Drug
Acyclovir, other “cyclovirs
Terminates DNA replication in herpesviruses
Nucleoside analog reverse transcriptase (RT) inhibitors
(zidovudine - retrovir)
Stop the action of reverse transcriptase in HIV,
blocking viral DNA production
Non-nucleoside reverse transcriptase inhibitors
(nevirapine)
Attach to HIV RT binding site, stopping its
action
II. Inhibition of Nucleic Acid Synthesis
5
6
7 6
7
5
Table 12.6 Antiviral Drugs
36
Drugs for Treating HIV Infections and AIDS
• Retrovirus offers 2 targets for chemotherapy: – Interference with viral DNA synthesis from viral RNA
using nucleoside reverse transcriptase inhibitors (nucleotide analogs)
– Interference with synthesis of DNA using nonnucleoside reverse transcriptase inhibitors
Azidothymidine (AZT) – first drug aimed at treating AIDS, thymine analog
37
12.4 Interactions between Microbes and Drugs: The Acquisition of Drug
Resistance
• Adaptive response in which microorganisms begin
to tolerate an amount of drug that would ordinarily
be inhibitory; due to genetic versatility or variation;
intrinsic and acquired
38
How Does Drug Resistance Develop?
• Acquired resistance:
– Spontaneous mutations in critical chromosomal genes
– Acquisition of new genes or sets of genes via transfer from another species
• Originates from resistance factors (plasmids) encoded with drug resistance, transposons
Resistance gene
Virus
Plasmid
Resistance
gene
Plasmid donor
DNA fragment
Gene goes to
plasmid or to
chromosome
Transduction
Transfer
by viral
delivery
Transformation
Transfer of
free DNA
Conjugation
Plasmid
transfer
Bacterium
Receiving
Resistance
genes
Dead
bacterium
Figure 12.13 Transfer of drug resistance
39
Specific Mechanisms of Drug Resistance
N
S R
O COOH Penicillinase N
H
R
O C
OH COOH
S CH3
CH3
D C
Cell surface
of microbe Cell surface
of microbe
Cell surface
of microbe Cell surface
of microbe
Drug
Drug
B A C D Product
Drug acts
Active
Drug
pump
Inactive
Drug
pump
Normal
receptor
Different
receptor
X
1. Drug inactivation Result
Active penicillin Inactive penicillin
2. Decreased permeability
3. Activation of drug pumps
4. Change in drug binding site
5. Use of alternate metabolic pathway
1 Inactivation of a drug like penicillin by
penicillinase, an enzyme that cleaves
a portion of the molecule and renders
it inactive.
1.
2
4
3
5
The receptor that transports the drug
is altered, so that the drug cannot
enter the cell.
2.
Specialized membrane proteins are
activated and continually pump the
drug out of the cell.
3.
Binding site on target (ribosome) is
altered so drug has no effect 4.
The drug has blocked the usual
metabolic pathway (green), so the
microbe circumvents it by using an
alternate, unblocked pathway that
achieves the required outcome (red) .
5.
Figure 12.14
Examples of
mechanisms
of drug
resistance
40
12.5 Interactions Between Drugs and Hosts
• Estimate that 5% of all persons taking antimicrobials will
experience a serious adverse reaction to the drug – side
effects
• Major side effects:
– Direct damage to tissue due to toxicity of drug
– Allergic reactions
– Disruption in the balance of normal flora-
superinfections possible
© Kenneth E. Greer/Visuals Unlimited
Figure 12.16
Drug induced
side effects
41
Figure 12.17 Superinfections
(a) (b) (c)
Circulating
drug
Intestine Intestine Intestine
Superinfection Drug
Infection
Pathogen
overgrows
Drug destroys beneficial
flora
Potential pathogen
resistant to drug
but held in check
by other microbes
Normal flora important
to maintain intestinal
balance
(a) A primary
infection in the
throat is treated
with an oral
antibiotic
(b) The drug is
carried to the
intestine and
is absorbed
into the
circulation.
(c) The primary
infection is cured, but
drug-resistant
pathogens have
survived and create
an intestinal
superinfection.
42
12.6 Considerations in Selecting an Antimicrobial Drug
• Identify the microorganism causing the
infection
• Test the microorganism’s susceptibility
(sensitivity) to various drugs in vitro when
indicated
• The overall medical condition of the patient
43
Testing for the Drug Susceptibility of Microorganisms
• Essential for groups of bacteria commonly showing resistance – Kirby-Bauer disk diffusion test
Oxytetracycline 30 µg
(R < 17 mm; S ≥ 22 mm)
Enrofloxacin 5 µg
(R < 17 mm; S ≥ 22 mm) Gentamicin 10 µg
(R < 17 mm; S ≥ 21 mm)
Ampicillin 10 µg
(R < 14 mm; S ≥ 22 mm) Cefotaxime 30 µg
(R < 14 mm; S ≥ 23 mm) Chloramphenicol 30 µg
(R < 21 mm; S ≥ 21 mm)
= Zone of
Inhibition
= Region of
bacterial growth
Disk DifusionT est (schematic).
Example and evaluation of a sensitivity test, agar diffusion methoid
R = resistant, I = intermediate, S = sensitive
OT
30 GN
10
AMP
10 CTX
30
C
30
1 2 3 4 0mm
ENR
5
Kirby-Bauer Disk Diffusion Test*
= Antibiotic carrier (disc) imprinted with abbreviation and concentration
ENR
5
© Prof. Patrice Courvalin, MD, FRCR, Unite desAgentsAntibactcriens, Institut Pasteur, Paris, France
CTX AMC
(a) If the test bacterium is sensitive to a drug, a zone of inhibition develops around
its disk. The larger the size of this zone, the greater is the bacterium’s
sensitivity to the drug. The diameter of each zone is measured in millimeters
and evaluated for susceptibility or resistance by means of a comparative
standard (R, I, or S).
(b) Result for gram negative Klebsiella pneumoniae, with zones
of inhibition indicating sensitivity to cefotaxime (CTX) and
ampicillin and clavulanic acid (AMC). Observe the expanded
zone between these two drugs that indicates their enhanced
action, or synergy, when combined.
(c) Result for gram positive Staphylococcus aureus using
gentamicin (GM, left) and aztreonam (ATM, right), Although the
bacteria are sensitive to both drugs, the GM has a few colonies
growing in the zone of inhibition, indicating some cells that
have developed resistance to that concentration of the drug.
© Kathy Park Talaro
Figure 12.18 Techniques
for preparation and
interpretation of disc
diffusion tests.