Major Action Modes of
Antimicrobial Drugs – Foundation Figure 20.2 Bacterial Resistance to
Antibiotics – Foundation Figure 20.20
The
goal of antimicrobial treatment is to kill or inhibit the growth of microbes
without damaging the host. This is referred to as selective toxicity. To
achieve selective toxicity, antimicrobial drugs generally target bacterial cell
structures, enzymes, or processes that are unique to the microbe and not found
in host cells. In seeking targets that are unique to the microbes, developers
of antimicrobial drugs must consider the differences between prokaryotic and
eukaryotic cells. Those structures that are exclusive to or distinctly
different in prokaryotic cells are likely targets for antibacterial drugs.
Figure 20.2
1.
Inhibition of cell wall
synthesis:
penicillins, cephalosporins, bacitracin, vancomycin
2.
Inhibition of protein synthesis: chloramphenicol, eruthromycin,
tetracyclines, streptomycin
3.
Inhibition of nucleic acid
replication and transcription:
quinolones, rifampin
4.
Injury to plasma membrane: polymyxin B
5.
Inhibition of essential
metabolite synthesis:
sulfanilamide, trimethroprim
Note:
Antimicrobial drugs target
certain essential functions of the microbe. Mechanisms of action include
inhibiting cell wall synthesis, inhibiting protein synthesis, inhibiting
nucleic acid synthesis, injuring the plasma membrane, or inhibiting synthesis
of essential metabolites. The antimicrobial drug mut not interfere with
essential functions of the microbe’s host.
Figure 20.20
1. Blocking
entry
2. Inactivation
by enzymes
3. Alteration
of target molecule
4.
Efflux of antibiotic
Note:
·
There
are only a few mechanisms of microbial resistance to antimicrobial agents:
blocking the drug’s entry into the cell, inactivation of the drug by enzymes,
alteration of the drug’s target site, efflux of the drug from the cell, or
alteration of the metabolic pathways of the host.
·
The
mechanisms of bacterial resistance to antibiotics are limited. Knowledge of
these mechanisms is critical for understanding the limitations of antibiotic
use.
1.
Potential Targets of Antibacterial Drugs
Identify
potential targets of antimicrobial drugs.
Top
to bottom; Left to right
Inhibition
of cell wall synthesis
Interference
with DNA replication
Interference
with transcription
Interference
with translation
Inhibition
of synthesis of essential metabolites
Injury
to plasma membrane
**You
have correctly identified the major targets of antimicrobial drugs. These
include inhibition of cell wall synthesis and interference with cell membrane
permeability. Some antimicrobial drugs interfere with enzymes necessary for the
synthesis of various metabolites. There are antimicrobials that interfere with
DNA replication, as well as those that interfere with transcription (synthesis
of RNA using the DNA template). Lastly, there are a number of antibiotics that
interfere with some aspect of protein synthesis (translation).
Some
of the broader targets of antimicrobials (such as cell wall synthesis or
protein synthesis) are made up of a series of steps or multiple separate
components that are eventually assembled to form the final product. For
example, think about the bacterial cell wall and the variety of substances that
must be synthesized to form the wall. Cell wall synthesis presents multiple
potential targets for antimicrobial activity. The same is true of protein
synthesis.
2.
How Do Antimicrobial Drugs Inhibit Bacterial Growth?
Identify
the general mechanism of inhibition for the antimicrobial actions.
·
inhibiting
cell wall synthesis
o
inhibiting
synthesis of peptide cross-links
o
inhibiting
bonding of N-acetyl glucosamine to N-acetylmuramic acid
o
inhibiting
lipopolysaccharide synthesis
o
inhibiting
mycolic acid synthesis
·
injuring
the plasma membrane
o
inhibiting
fatty acid synthesis
·
interfering
with DNA replication
o
interfering
with DNA polymerase
o
inhibiting
DNA gyrase
·
interfering
with RNA synthesis (transcription)
o
interfering
with RNA polymerase
·
interfering
with protein synthesis
o
interfering
with activity of 30s ribosomal subunit
o
interfering
with activity of 50s ribosomal subunit
o
interfering
with attachment of tRNA to mRNA
o
interfering
with peptide bond formation, catalyzed by the ribosome
**
You have correctly associated each of the inhibitory mechanisms with their
broader activity.
Most
people have heard of penicillin, the first major antibiotic discovered and
developed. There are actually over 50 different antibiotics in the penicillin,
or “cillin,” group of antibiotics. All of the members of this group share a
structural feature referred to as the common nucleus, within which is a
chemical group referred to as the beta-lactam ring. The “cillins” differ in the
side chain attached to the common nucleus. The different side chains alter the
spectrum as well as the stability of the antibiotic. All of the “cillins”
interfere with the peptide cross-linking that stabilizes peptidoglycan cell
walls. Note the four different “cillins” illustrated here.
3.
The Penicillins
All
penicillins, or “cillins,” share several structural features but differ in
others. This activity asks that you identify several features common to all
“cillin” antibiotics.
**You
have correctly identified the different parts of the “cillin” molecule. Any
antibiotic that has this common nucleus is a type of penicillin. The
beta-lactam ring portion of the molecule is the target of beta-lactamase
enzymes (also known as penicillinase) that are made by some bacteria. These
bacteria are able to break the beta-lactam ring and inactivate the antibiotic.
4.
Antibiotics That Inhibit Protein Synthesis
There
are a large number of antibiotics that inhibit protein synthesis at 70s
ribosomes found in bacterial cells but do not interfere with protein synthesis
at the 80s ribosomes found in eukaryotic cells. Some of these antibiotics bind
to the smaller ribosomal subunit and interfere with the reading of the mRNA
code, whereas others bind to the larger ribosomal subunit and inhibit the
formation of peptide bonds. Unfortunately, some of the antibiotics that inhibit
protein synthesis in bacteria exhibit some toxicity to the eukaryotic host
cells as well. What is the most likely reason for this toxicity to the host
cell?
These
antibiotics interfere with protein synthesis within eukaryotic mitochondria
**Eukaryotic
mitochondria have 70s ribosomes, composed of 50s and 30s subunits, which are
very similar to the ribosomes of bacterial cells. Some of the antibiotics that
target bacterial ribosomes will cause some toxicity in eukaryotic cells because
of their effects on the mitochondrial ribosomes.
A
substantial amount of progress has been made in the development of
antibacterial drugs. The search for new antibiotics is critical, however, as
bacteria continue to develop resistance to currently used antibiotics.
Resistance typically arises by mutation, and once microorganisms have become resistant,
the continued use of antibiotics enables those that have developed resistance
to survive at the expense of those that are susceptible. Over time, this
selection of the resistant strains gives rise to their predominance in the
bacterial population. Antibiotic resistance can spread among bacterial
populations both vertically (by binary fission) and horizontally (via genetic
transfer mechanisms)
5.
Mechanisms of Antibiotic Resistance, Part 1
Consider
the different mechanisms through which antibiotics inhibit microbial growth,
and consider what changes in the microbe might enable it to resist the
inhibitory effects of antibiotics.
Altered
porins in cell wall, which block passage of antibiotic through cell wall.
Production
of an enzyme that destroys the antibiotic.
Microbe
develops transport mechanism in plasma membrane that rapidly pumps antibiotic
out of the bacterial cell.
Modified
target site, such that an antibiotic is unable to bind to its target.
6.
Mechanisms of Antibiotic Resistance, Part 2
Top
to bottom.
Entry
of antibiotic into the cell is blocked.
Cellular
enzyme inactivates an antibiotic.
Target
site to which antibiotic binds is altered.
Efflux
mechanism pumps antibiotic out of the cell.
**The
figure illustrates the four basic mechanisms that enable microorganisms to
resist the inhibitory or lethal effects of antibiotics.
Antibiotic
resistance has become a serious problem in health care that continues to
worsen. Organisms such as MRSA (methicillin-resistant Staphylococcus aureus),
VRE (vancomycin-resistant Enterococcus), and XDR-TB (extensively
drug-resistant tuberculosis) are in the news almost every week as their
frequency increases in both hospitals and the community. The names MRSA and VRE
really don’t tell the entire story. These two bacterial strains are resistant
to more than methicillin (MRSA) or vancomycin (VRE); they exhibit resistance to
numerous different classes of antibiotics, which greatly complicates treatment
of infections.
7.
Examples of Antibiotic Resistance
There
are numerous examples of microorganisms displaying each of the four major
resistance mechanisms. In this activity, you are asked to identify the
mechanism that each example best illustrates.
·
Enzymatic
inactivation of the antibiotic
o
Many
strains of Neisseria gonorrhoeae are
resistant to penicillin because of the
production of beta-lactamases.
·
Prevention
of penetration to the target site within the microbe
o
Resistance
to tetracycline may result from modified pore proteins in the outer membrane
that form a porin too small for the tetracycline to pass through
·
Alteration
of the drug’s target site
o
Resistance
to clindamycin develops when mutations in bacteria alter the ribosomal binding
site to which clindamycin would normally bind
o
MRSA
(methicillin-resistant Staphylococcus aureus)
is resistant to all beta-lactam drugs because of a mutation in its
penicillin-binding protein (PBP)
·
Rapid
efflux of the antibiotic
o
Pseudomonas aruginosa has membrane pumps that export a number of different
antibiotics from its cells
8.
How Do Microorganisms Acquire Antibiotic Resistance?
Identify
the statements below that accurately describe the mechanisms through which
organisms acquire antibiotic resistance.
Antibiotic-resistance
genes can be passed horizontally via transduction.
Antibiotic
resistance is readily transmitted to the next generation during binary fission.
Mutations
are the ultimate source of antibiotic-resistance genes.
Antibiotic-resistance
genes can be passed horizontally via bacterial conjugation.
Antibiotics
select for those microbes that have developed resistance, increasing their
frequency in the bacterial population.
Antibiotic-resistance
genes can be passed from one bacterium to another by bacterial transformation.
**It
is important to understand that resistance typically originates through
spontaneous mutations and then can spread horizontally through bacterial
transformation, conjugation, and transduction and vertically through binary
fission. Antibiotic use provides the selective pressure that reduces the number
of antibiotic-susceptible bacteria, resulting in an increase in the number of
antibiotic-resistant strains. Antibiotics don’t cause the DNA changes that
bring about the resistance, but rather provide a selective environment in which
only those microbes that are resistant can proliferate.
Microbiology Animation:
Chemotherapeutic Agents: Modes of Action
1.
What is meant by selective toxicity?
Chemotherapeutic
agents should act against the pathogen and not the host.
2.
Why are chemotherapeutic agents that work on the peptidoglycan
cell wall of bacteria a good choice of drug?
Humans and other animal hosts lack peptidoglycan
cell walls.
3.
Why is polymyxin only used on the skin?
It
can also damage living human cell membranes, but the drug is safely used on the
skin, where the outer layers of cells are dead.
4.
Quinolones and fluoroquinolones act against what bacterial
target?
DNA
gyrase
5.
Why is it difficult to find good chemotherapeutic agents
against viruses?
Viruses depend on the host cell's machinery, so it
is hard to find a viral target that would leave the host cell unaffected.
Microbiology Animation:
Antibiotic Resistance: Forms of Resistance
1.
Which antibiotic is overcome by beta-lactamases?
Penicillin
2. How might efflux pumps increase
antibiotic resistance in bacteria?
Resistant
bacteria can have more efflux pumps, and can have less specific efflux pumps.
3.
Bacteria that are resistant to sulfonamide have enzymes that
have a greater affinity for what?
PABA
4. Why would an efflux pump for
penicillin located on a bacterial cell membrane not be effective at
providing resistance to the drug?
Penicillin disrupts the cell wall, which is located
outside of the cell membrane
5.
Membrane transport proteins are required for which mode(s)
of antibiotic resistance?
Efflux pumps, beta-lactamases, and modification of
porins all utilize membrane transport proteins.
Chapter 20 Reading Questions
1.
Which of the following is an antiprotozoan drug that
interferes with anaerobic metabolism?
Metronidazole
2.
Why is it more difficult to treat viral infections than it
is to treat bacterial infections?
viruses use the host cell's processes to carry out
their own reproduction
3.
Consider a Kirby-Bauer disk-diffusion assay. If you put
penicillin and streptomycin disks adjacent to one another, the zone of
inhibition is greater than that obtained by either disk alone. This is an
example of __________.
Synergism
Chapter 20
1. A drug that inhibits mitosis, such
as griseofulvin, would be more effective against
Fungi
2.
Most of the available antimicrobial agents are effective
against
Bacteria
3.
The following data were obtained from a broth dilution test.
|
Concentration of Antibiotic X |
Growth |
Growth
in Subculture
|
|
2
μg/ml
|
+
|
+
|
|
10
μg/ml
|
-
|
+
|
|
15
μg/ml
|
-
|
-
|
|
25
μg/ml
|
-
|
-
|
In
the table, the minimal bactericidal concentration of antibiotic X is
15
ug/ml
4.
The following data were obtained from a broth dilution test.
|
Concentration
of Antibiotic X
|
Growth
|
Growth
in
Subculture |
|
2
μg/ml
|
+
|
+
|
|
10
μg/ml
|
-
|
+
|
|
15
μg/ml
|
-
|
-
|
|
25
μg/ml
|
-
|
-
|
In
the table, the minimal inhibitory concentration of antibiotic X is
10
ug/ml
5.
Which of the following antibiotics does NOT interfere with
cell wall synthesis?
Macrolides
6.
Protozoan and helminthic diseases are difficult to treat
because
their
cells are structurally and functionally similar to human cells.
7.
The following results were obtained from a disk-diffusion test for microbial
susceptibility to antibiotics. Staphylococcus aureus was the test organism.
susceptibility to antibiotics. Staphylococcus aureus was the test organism.
|
Antibiotic
|
Zone
of Inhibition
|
|
A
|
3
mm
|
|
B
|
7
mm
|
|
C
|
0
mm
|
|
D
|
10
mm
|
In
the table, the antibiotic that exhibited bactericidal action was
The
answer cannot be determined based on the information provided.
8.
The following results were obtained from a disk-diffusion test for microbial
susceptibility to antibiotics. Staphylococcus aureus was the test organism.
susceptibility to antibiotics. Staphylococcus aureus was the test organism.
|
Antibiotic
|
Zone
of Inhibition
|
|
A
|
3
mm
|
|
B
|
7
mm
|
|
C
|
0
mm
|
|
D
|
10
mm
|
In
the table, which antibiotic would be most useful for treating a Salmonella
infection?
The
answer cannot be determined based on the information provided.
9.
The following data were obtained from a broth dilution test:
Concentration of Antibiotic X
Growth
|
2.0 μg/ml
|
-
|
|
1.0 μg/ml
|
-
|
|
0.5 μg/ml
|
-
|
|
0.25 μg/ml
|
+
|
|
0.125 μg/ml
|
+
|
|
0
|
+
|
Bacteria
from the 0.25 μg/ml tube were transferred to new growth media
containing antibiotic X with the following results:
containing antibiotic X with the following results:
Concentration of Antibiotic X
Growth
|
2.0 μg/ml
|
-
|
|
1.0 μg/ml
|
+
|
|
0.5 μg/ml
|
+
|
|
0.25 μg/ml
|
+
|
The
data in the table show that these bacteria
developed
resistance to antibiotics.
10.
The structures of the influenza drug Tamiflu and sialic acid, the substrate for
influenza viruss neuramidase, are shown in the figure. What is the method of
action of Tamiflu?
Competitive
inhibition
11.
An antibiotic that attacks the LPS layer would be expected
to have a narrow spectrum of activity.
TRUE
12.
Both trimethoprim and sulfamethoxazole inhibit reactions
along the same metabolic pathway.
TRUE
Russia, You Take My Breath Away
Caleb
Bakersfield, a 42-year-old real estate agent, had just returned from a vacation
to Russia. His childhood had been rough because of an alcoholic and abusive
father, and Caleb had started his own drug addiction in his early teens. By his
early twenties, he was addicted to heroin, lived on the streets, and frequently
used dirty needles. In his thirties, Caleb joined a program to beat his
addiction and to turn his life around. The trip to Russia was to celebrate a
decade of being clean.
Less
than two months after his trip, Caleb started having respiratory complications,
including a frequent cough and shortness of breath. He figured it was most
likely a respiratory infection and made an appointment with his physician.
After
listening to Caleb’s lungs, Dr. Bell determines that Caleb most likely has a
lower respiratory infection and prescribes the antibiotic azithromycin. Dr.
Bell reminds Caleb that it is important to complete his entire course of
antibiotics, even if he feels better before he finishes all of the medicine.
Dr. Bell also collects a sputum sample (mucus coughed up from the lower
respiratory tract) and sends it to the laboratory for evaluation.
1.
Why does the physician start Caleb on the antibiotic
azithromycin before laboratory results come back?
Antibiotic therapy is started with a broad-spectrum
antibiotic because broad-spectrum antibiotics are effective against many
gram-positive and many gram-negative bacteria.
**
Azithromycin is a semisynthetic broad-spectrum antibiotic that can be used as
an alternative to penicillin, is broader in range than erythromycin, and has
better tissue penetration. Dr. Bell most likely assumes that the bacterium
causing Caleb’s infection will be susceptible to azithromycin and that this
drug should clear the infection.
2.
Which of the following choices correctly matches the class
of antibiotic and its mode of action?
Aminoglycosides and tetracyclines are inhibitors of
protein synthesis.
Sulfonamides inhibit the synthesis of essential metabolites.
Sulfonamides inhibit the synthesis of essential metabolites.
**
Antibacterial drugs are often categorized by their mode of action against the
target microbe. The following make up a short list of antibacterial drugs that
are commonly used to treat infections. Penicillins (natural and semisynthetic)
and cephalosporins are commonly used to inhibit synthesis of the cell wall.
Chloramphenicol, aminoglycosides, tetracyclines, and macrolides are common
inhibitors of protein synthesis. Polymyxin B and lipopeptides cause damage to
the plasma membrane. Rifamycins and quinolones inhibit nucleic acid synthesis.
Sulfonamides inhibit metabolic pathways. Combinations of these drugs can also
be used to increase efficacy.
The
azithromycin does not clear Caleb’s respiratory infection. In fact, his cough
is getting worse, and on several occasions his sputum has contained blood.
Caleb schedules another appointment with Dr. Bell. This time, the two
thoroughly discuss his recent travel to Russia, his medical history, and his
time spent as an IV (intravenous) drug user. Dr. Bell requests that Caleb
provide samples of sputum and blood and that he undergo a tuberculin skin test.
Image A shows what Caleb’s sputum sample looks like on the microscope.
Dr.
Bell confirms his diagnosis of tuberculosis using an X-ray image of Caleb’s
lungs and a rapid diagnostic test (Xpert MTB/RIF), which uses automated PCR to
detect M. tuberculosis in 90 minutes. Dr. Bell also receives the results
of Caleb’s blood test, which confirms that he is also infected with HIV, most
likely contracted from a dirty needle. The HIV has weakened Caleb’s immune
system, rendering it unable to fight off the M. tuberculosis pathogen.
His X-ray film shows the presence of a walled-off lesion of bacterial cells (a
tubercle) in the lungs.
Dr.
Bell meets with Caleb to discuss treatment options for his infections. Given
that he is infected with HIV, treatment for the tuberculosis is imperative but
depends greatly on the susceptibility of the M. tuberculosis to the
available antibiotics. Dr. Bell also starts Caleb on HIV therapy.
3.
If Caleb’s strain of M. tuberculosis is sensitive to antibiotic
treatment, which of the following could be used to treat his infection?
streptomycin
isoniazid and ethambutol
rifampin
isoniazid and ethambutol
rifampin
**
Treatment of susceptible strains of M. tuberculosis typically includes a
6-month regimen of isoniazid, ethambutol, pyrasinamide, and rifampin. These are
considered first-line drugs for the treatment of tuberculosis. If resistance
develops, second-line alternatives, such as aminoglycosides, fluoroquinolones,
and para-aminosalicylic acid (PAS) can be added to the regimen. In
resistant strains, treatment is more difficult.
4.
Why does Dr. Bell start Caleb on HIV therapy in addition to the
antibiotics used to treat the tuberculosis?
Dr. Bell prescribes Caleb HIV therapy because the
virus is not affected by the antibiotics used to treat tuberculosis.
**
HIV infections are caused by the human immunodeficiency virus, a RNA virus.
Viruses are not susceptible to antibiotics; thus the treatment for Caleb’s
tuberculosis would be effective only against the bacterium, M. tuberculosis.
Dr. Bell prescribes the HIV therapy and the antibiotics in order to help
Caleb’s immune system battle both infections. HIV therapy includes the use of
antiretroviral drugs, nucleoside analogs, and nucleotide analogs such as HAART,
zidovudine, and tenofovir, respectively. Newer HIV drugs are being created to
inhibit viral entry into the cell and to prevent integration of the genome into
host DNA. Often, HIV-infected individuals are given a “cocktail” of medications
to treat the infection and to help combat antiviral resistance.
Tuberculosis
strains that are drug resistant can be defined as multi-drug resistant (MDR) or
extensively drug resistant (XDR). Multi-drug-resistant strains are defined as
being resistant to the two most effective first-line drugs, rifampin and
isoniazid. Extensively drug-resistant strains are defined as being resistant to
rifampin, isoniazid, to the most effective second-line drugs (fluoroquinolone),
and to at least one of three injectable second-line drugs. Unfortunately,
Caleb’s strain of M. tuberculosis is an XDR strain that is untreatable.
The tuberculosis infection, in addition to complications from his HIV, leads to
his death 6 months after the appearance of his initial symptoms.
5.
Which of the following contribute to drug resistance in M.
tuberculosis?
Many individuals fail to complete their entire
regimen of antibiotics.
Some physicians prescribe the wrong medication, the wrong dosage, or the wrong length of time for treating tuberculosis.
In many areas, tuberculosis antibiotics are unavailable or of poor quality.
Some physicians prescribe the wrong medication, the wrong dosage, or the wrong length of time for treating tuberculosis.
In many areas, tuberculosis antibiotics are unavailable or of poor quality.
**
One of the major reasons for drug resistance in M. tuberculosis is the
length of antibiotic treatment. It is hard enough to get people to completely
finish a course of antibiotics that lasts 7 to 10 days, let alone one that
lasts 6 months. Individuals who begin treatment but do not finish only
contribute to the resistance problem because the organisms they are harboring
and transmitting to others are the ones resistant to treatment. Other factors
that contribute to resistance include improper diagnosis and treatment, lack of
drug availability, and poor quality of the antibiotics. With the increase in
the number of organisms exhibiting resistance, researchers are now developing
newer antimicrobials and revisiting ideas from before the antibiotic era.
Chapter 20
1.
The following results were obtained from a disk-diffusion test for microbial
susceptibility to antibiotics. Staphylococcus aureus was the test organism.
susceptibility to antibiotics. Staphylococcus aureus was the test organism.
|
Antibiotic
|
Zone
of Inhibition
|
|
A
|
3
mm
|
|
B
|
7
mm
|
|
C
|
0
mm
|
|
D
|
10
mm
|
In
the table, the most effective antibiotic tested appears to be
Letter
D
You are just the best <3
ReplyDeleteawesome!
ReplyDelete