Genetics and Biotechnology: Manipulating, Cloning, Analyzing and ...

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20 Φεβ 2013 (πριν από 4 χρόνια και 7 μήνες)

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Diana
Marra

Oram, PhD

Office 9211

doram@umaryland.edu

Please read chapter 20 in your textbook Medical Microbiology


Use of antimicrobials by primitive peoples


There is evidence of successful chemotherapy in
ancient Peru
-

Use of bark from the cinchona tree to
treat malaria.


Many of the salves used by primitive peoples contained
antibacterial and antifungal substances.




1935 Use of protosil
(cleaved in the body to produce sulfanilamide


the first
sulfa drug)

to protect mice from streptococcal infection



Fleming isolated penicillin in 1929 but did not initially appreciate the
magnitude of the discovery


In 1939 Florey and colleagues at Oxford University again isolated penicillin



1944 Waksman isolates streptomycin and subsequently finds agents such
as chloramphenicol, tetracyclines, and erythromycin in soil samples.



1960s Advances in medicinal chemistry permit the synthesis of many new
chemotherapeutic agents by molecular modification of existing
compounds.



Development of new antibacterial agents has moved quickly, but
development of antifungal and antiviral agents has been slow.


Amphotericin B, isolated in the 1950s, remains an effective antifungal agent,
although newer agents such as fluconazole are now widely used.



Nucleoside analogs such as acyclovir have proved effective in the
chemotherapy of selected viral infections.

Antimicrobial Chemotherapy Harold C. Neu
and Thomas D. Gootz


Antibacterial Spectrum (2 types)



Broad


Can inhibit the growth of a wide variety of Gram +
and Gram


bacterial species



Narrow


Is only active against a limited variety of bacteria


Broad spectrum


Tetracycline and carbapenems active against many
Gram
-
positive and Gram negative bacteria


Often used for empirical or “blind” treatment of
infections when the causative agents are unknown (but
likely to be bacterial).


Blind treatments aren’t usually done anymore unless you’re on ur deathbed bc you
can induce resistance. Not saying that this treatment is bad, but these days we want
to understand what’s causing the disease/infection and know that the antibiotic
we’re prescribing will actually treat it.


Narrow spectrum


specific target


Penicillin effective only against Gram
-
positive bacteria


Metronidazole effective against strict anaerobes and
some protozoa.



Bacteriostatic


Inhibits the growth of an organism


Bactericidal


Kills an organism


Antibiotic Combinations


Synergism


Combination of antibiotics that has enhanced antibacterial
activity (
sum

better than individual agents)


Antagonism


Combination of antibiotics in which the activity of one or more
agent interferes with the activity of others (sum worse than
individual agents/inhibitory effect)



Antimicrobial agents can be
directly toxic


Life
-
threatening blood dyscrasia occurs in 1 in 60,000 individuals who
receive
chloramphenicol

-

no longer used. 1/60000 doesn’t seem too high
but it is when we now have better options



Damage to the kidneys can follow the use of aminoglycosides
.



Antimicrobial agents can interact with other drugs to
increase their toxicity



Antimicrobial agents can
alter microbial flora


Can cause what’s called antibiotic induced interpolitis (sp?) by altering
the gastrointestinal flora(killing all of the other natural flora), almost
all antibiotics can cause overgrowth of
Clostridium difficile
, which
produces a toxin that causes diarrhea and even pseudomembranous
colitis.


Alteration of intestinal flora by antibiotics can also result in overgrowth
of
Candida

in the mouth, vagina, or gastrointestinal tract



Allergic reactions
can be caused by antimicrobial agents



Penicillins can produce either immediate, IgE
-
mediated, or delayed
hypersensitivity reactions


Antibiotics must work with the immune system
.
Antibacterial agents are not capable of completely
eliminating an infection in a host with an extremely
compromised immune systems

Reasons:


Antibiotics can not reach all sites in the body


Antibiotics only kill or inhibit sensitive organisms


How does it happen? We do it…


Selection for resistant strains by giving antibiotics


Microbial adaptation and change


Treatment failure and emergence of diseases


Why should we care?


Are we creating “superbugs”? Untreatable infections


How can we stop (or at least slow) it?


Proper use of an antibiotics


Education of the healthcare provider and the patient



Mechanisms by which bacteria can “exchange” DNA


Horizontal
transfer.


It’ll receive foreign DNA to gain resistance via:


Transformation


cells take up DNA


Conjugation


mating through specialized appendages


requires cell to cell contact


Transduction


bacteriophage mediated



Types of mobile elements (ie Vectors)


Transposons


IS elements


Plasmids


Conjugative plasmids


Phage


Conjugaive Transposons


Pathogenicity Islands


Integrating Conjugative Elements (ICEs) and other mobilizable elements




Antibiotic resistance


Appropriate antimicrobial drug use is defined as use that maximizes therapeutic impact
while minimizing toxicity and the development of resistance.


Antibiotic resistance emerges when drugs are prescribed inappropriately


Infections caused by viruses. Ex. Ear infections in kids


most are caused by
viruses. Doctor prescribes antibiotics (not really targeting the exact cause…and
the kid gets better…bc they’ll always get better while the antibiotics didn’t do
anything)


Infections caused by bacteria not susceptible to the antibiotic


this is why we’re
discouraged from blind spectrums antibiotics


Antibiotics used at the wrong dose (mostly too low). If used at wrong dose, you
can induce resistance.


Section for bacteria resistance to the antibiotic


Selection for spread of resistance markers among bacteria

Example she likes to use: you’re actually selecting for resistance for the commensals in a person when you do this
and so what you can create is a person who can never be treated for tetracyline (for example) bc their
commensals become resistant so that every infection they get, those commensals are able to transfer that
resistance to whatever the bacteria is now causing the infection



Appropriate antimicrobial drug use should not be interpreted simply as reduced use
because these drugs offer valuable benefits when used appropriately.


1000

5

1000

100

1

1005

0.49

Sick

901

10

1000

100

2

911

1.09

Sick

704

19

1000

100

3

723

2.62

Better

311

35

1000

100

4

346

10.11

Healthy

0

60

1000

100

5

60

100

Healthy

0

0

1000

100

6

0

100

Healthy

0

0

1000

100

7

0

100

Healthy

0

0

0

100

8

0

100

Cured


Sensitive


Resistant


Killed by
Antibiotic


Killed by
Immune
System


Total
Bacteria


Day


Symptoms


Resistant
killed by
immune
system

1000

5

1000

100

1

1005

0.49

sick

900

10

1000

100

2

911

1.09

sick

704

19

1000

100

3

723

2.62

better

311

35

0

100

4

348

10.05

healthy

544

60

0

100

5

604

9.93

better

1012

110

1000

100

6

1122

9.80

sick

947

210

1000

100

7

1157

18.15

sick

799

402

1000

100

8

1201

33.47

sick

530

771

1000

100

9

1301

59.26

sick

19

1483

1000

100

10

1502

98.73

sick

0

2867

1000

100

11

2867

100

very sick

0

5634

1000

100

12

5634

100

very sick



Sensitive


Resistant


Killed by
Antibiotic


Killed by
Immune
System


Total
Bacteria


Day


Symptoms


Resistant
killed by
immune
system


Antibiotics are not a silver bullet


Antibiotics do not cure all infections


Most infections will resolve without antibiotic
treatment


like common cold


Proper dosage, patient understanding and
adherence to the treatment regiment is essential


Antibiotics are extremely valuable in the treatment
of bacterial infections but must be used properly
ensure that resistance is not induced


Appropriate antimicrobial drug use is defined as use
that maximizes therapeutic impact while minimizing
toxicity and the development of resistance.


Appropriate antimicrobial drug use should not be
interpreted simply as reduced use because these drugs
offer valuable benefits when used appropriately.




Inhibition of Cell Wall Synthesis


Disruption of Membranes


Inhibition of Nucleic Acid Synthesis


Inhibition of Protein Synthesis


Antimetabolites






Mechanism of action


Uses (spectrum)


Mechanisms of resistance


Inhibition of Cell Wall Synthesis


b
-
lactam antibiotics


Penicillins


Cephalosporins


Carbapenems and monbactams


Fosfomycin, Cycloserine and Bacitracin


Glycopeptides


Vancomycin


only talking about this one in particular


Antimycobacterial agents


Isoniazid


only talking about this one as well…there are other
antimycobacterial agents…but this one only targets antimycobacterial
agents


Disruption of Membranes


Polymyxins


Phase 1

In cytoplasm

Phase 2

At the membrane

Phase 3

Outside the cell

Phase 1:
soluble substrates
are activated and assembly
and transport inside the cells


Phase 2:
activated units are
attached and assembled on
the undecaprecol phosphate
membrane pivot at the
membrane.


Phase 3:
The peptidoglycan
units are attached to and
cross
-
linked into the
pepetidoglycan
polysaccharide outside the
cells

Fosfomycin

Cycloserine

b
-
Lactam

Review of Cell Wall Synthesis

Slide courtesy of Dr. Ru
-
ching Hsia

Vancomycin

Bacitrancin


Drugs that inhibit polymerization and attachment
of new peptidoglycan to cell wall


Penicillins


Cephalosporins


Carbapenems and monbactams


Drugs that inhibit biosynthetic enzymes


Fosfomycin and Cycloserine


Drugs that combine with carrier molecules


Bacitracin


Drugs that combine with cell wall substrates


Vancomycin



Slide courtesy of Dr. Ru
-
Ching Hsia


The last step of cell wall synthesis involves polymerization of
peptidoglycan subunits


This is the step of cell wall synthesis inhibited by ß
-
lactam antibiotics


Most ß
-
lactam antibiotics are bactericidal more so than static


ß
-
lactam antibiotics contain a four
-
membered ring, which
undergoes an acylation reaction with the transpeptidases that
cross
-
link the peptidoglycan polymers


not really responsible for structure
but wanted to mention is bc some broad/narrow spectrum are involved with this ring


The enzymes involved in this final process of cell wall formation
are called
penicillin
-
binding proteins (PBPs
) since they were
discovered by labeling with radioactive penicillin G.


The enzymes are different in Gram
-
positive and Gram
-
negative bacteria and
in anaerobic species.


Differences in the penicillin
-
binding proteins are responsible, to some
extent, for the differences in spectrums of the ß
-
lactam antibiotics.

Antibiotics

Spectrum of Activity

Natural penicillins: benzylpenicillin
(penicillin G), phenoxymethyl
penicillin (penicillin V)

Active against all β
-
hemolytic streptococci and
most other species; limited activity against
staphylococci; active against
meningococci

and
most gram
-
positive anaerobes; poor activity
against aerobic and anaerobic gram
-
negative
rods

Penicillinase
-
resistant penicillins:
methicillin, nafcillin, oxacillin,
cloxacillin, dicloxacillin

Similar to the natural penicillins, except
enhanced activity against staphylococci

Broad
-
spectrum penicillins:
aminopenicillins (ampicillin,
amoxicillin, carbenicillin, ticarcillin);
ureidopenicillins (piperacillin)

Activity against gram
-
positive cocci equivalent to
the natural penicillins; active against some gram
-
negative rods with piperacillin the most active

β
-
Lactam with β
-
lactamase inhibitor
(ampicillin
-
sulbactam, amoxicillin
-
clavulanate, ticarcillin
-
clavulanate,
piperacillin
-
tazobactam)

Activity similar to natural β
-
lactams, plus
improved activity against β
-
lactamase producing
staphylococci and selected gram
-
negative rods;
not all β
-
lactamases are inhibited;
piperacillin/tazobactam is the most active


Cephalosporins are
b
-
lactam antibiotics where
the
b
-
lactam ring is fused
with a dihydrothiazine
ring.


Cephamycins are related
to Cephalosporins but are
more resistant to
b
-
lactamases


Both have a
wider
antibacterial spectrum
,
resistance to many
b
-
lactamases and have
longer half
-
lifes improved
pharmacokinetic
properties over penicillins


Carbapenems


Broad spectrum


Resistance has been reported in oxacillin
-
resistant
staphylococci and
Pseudomonas


Monobactams


Narrow spectrum


Effective only against aerobic Gram negative bacteria



Three general mechanisms for resistance to
b
-
lactam
antibiotics


Prevention of the interaction between the
PBP

and the
antibiotic


Only occurs in Gram negative species where changes in porins
result in exclusion of the antibiotic


bc of the outer membrane, we
can’t get to target


Modification of the interaction between the
PBP

and the
antibiotic


Overproduction of the PBP


Acquisition of a new PBP


Modification of an existing PBP


Hydrolysis of the antibiotic by
b
-
lactamases


More than 200 different
b
-
lactamases have been described


Classes A
-
D


Some are specific of particular
b
-
lactam antibiotics while others have
broad spectrum activity against several kinds of
b
-
lactam antibiotics



Mechanism of action


inhibit polymerization and attachment of peptidoglycan
to cell wall (phase 3)


Uses (spectrum)


Range from narrow to broad depending on the particular
compound


Mechanisms of resistance


Exclusion from the target
PBP



Modification of the
PBP


b
-
lactamases






Inhibition of Cell Wall Synthesis


b
-
lactam antibiotics


Penicillins


Cephalosporins


Carbapenems and monbactams


Fosfomycin, Cycloserine and Bacitracin


Glycopeptides


Vancomycin


Antimycobacterial agents


Isoniazid


Cycloserine



Disruption of Membranes


Polymyxins



Fosfomycin and Cycloserine


Fosfomycin inhibits the enzyme UDP
-
GIcNAc
-
3
-
enol
-
pyruvyltransferase that is involved in the
first phase
of cell wall synthesis


Cycloserine is an inhibitor of both alanine racemase
and D
-
alanyl
-
D
-
alanine synthetase preventing the
cross
-
linkage of peptidoglycan that occurs in the
first
phase

of cell wall synthesis


Cycloserine is fairly toxic and is generally only used as a
secondary treatment for tuberculosis


Bacitracin




Bacitracin is a peptide antibiotic that specifically
interacts with the pyrophosphate derivate of the
undecaprenyl alcohol, preventing further transfer of the
muramylpentapeptide from the precursor nucleotide to
the nascent peptidoglycan


This occurs in
Phase II
of Cell Wall Synthesis



Vancomycin is a
glycopeptide.


Vancomycin binds to the
pentapeptide terminus and
inhibits both
transglycosylation and
transpeptidation reactions
during peptidoglycan
assembly



This occurs during
phase II
of cell wall synthesis


Vancomycin


Vancomycin is not effective against Gram
-
negative
bacteria


Because of its large size it can not penetrate the Gram
-
negative outer membrane


Vancomycin is used to treat Gram
-
positive
infections caused by organisms that are resistant
to
b
-
lactams


Vancomycin resistance is mediated by changes in
the pentapeptide terminus (the end where
vancomycin would bind to)


Vancomycin resistance can be encoded on mobile
genetic elements and can be transferred from one
species to another. (in commensals and they are
handing it off…esp in hospitals)


Phase 1

In cytoplasm

Phase 2

At the membrane

Phase 3

Outside the cell

Phase 1:
soluble substrates
are activated and assembly
and transport inside the cells


Phase 2:
activated units are
attached and assembled on
the undecaprecol phosphate
membrane pivot at the
membrane.


Phase 3:
The peptidoglycan
units are attached to and
cross
-
linked into the
pepetidoglycan
polysaccharide outside the
cells

Fosfomycin

Cycloserine

b
-
Lactam

Review of Cell Wall Synthesis

Slide courtesy of Dr. Ru
-
ching Hsia

Vancomycin

Bacitrancin


Isoniazid is bactericidal against actively replicating
Mycobacteria


Isoniazid inhibits synthesis of mycolic acid


which is
why isoniazid is specific for mycobacteria…other
bacteria do not produce mycolic acid


The precise mechanism by which isoniazid acts is not
known


Resistance to isoniazid results both from decreased
uptake of the drug into the cells and by alteration of
the enzymes involved in mycolic acid synthesis


Don’t get it confused…other drugs can work against
mycobacteria…it’s just that isoniazid will work only on
mycobacteria (ie. Useless against e.coli)



Fosfomycin and Cycloserine inhibit the first phase
of cell wall synthesis


Bacitracin inhibits the second phase of cell wall
synthesis


Vancomycin inhibits the second phase of cell wall
synthesis


It is not active against Gram negative bacteria


Resistance is caused by a change of the target enzyme


Isoniazid inhibits synthesis of mycolic acids


Used to treat infections caused by
Mycobacterium


Polymyxins


polymyxin B and colistin (polymyxin E)


high
-
molecular
-
weight octapeptides that inhibit Gram
-
negative bacteria


Not active against Gram positive bacteria


no outer membrane


Interact with the membrane and cause increased cell
permeability therefore cell death


Only used topically
-

bind to various ligands in body tissues
and are potent toxins for the kidney and nervous system

(seen in stuff like neosporin)


Inhibition of DNA Replication


Quinolones


some of the most widely used currently (new class so less
resistance for now…)


Nalidixic acid, ciprofloxacin, gatiflozacin


DNA damaging agents


Nitroimidazoles


Metronidazole


Inhibition of Transcription
(rather than damaging DNA)


Rifamycins


One of the most widely used class of antibiotics


Synthetic

agents that inhibit gyrase (bacterial topoII) or
topoisomerase IV thereby interfering with DNA
replication, recombination and repair


In Gram negative bacteria gyrase is usually the primary target


In Gram positive bacteria topoIV is usually the primary target



Bactericidal



Nalidixic acid was used to treat urinary tract
infections caused by Gram negative bacteria but
resistance to the drug developed rapidly


only
against gyrase


Newer fluoroquinolones such as Ciproflaxacin,
have broader spectrum against both Gram positive
and Gram negative bacteria


against both gyrase
and topoIV


Only

inhibits
anaerobic bacteria
and
protozoa


Nitroimidazols are reduced by an electron transport
protein in anaerobic bacteria. The reduced drug causes
strand breaks in the DNA.
(
They have to be converted to
an active form inside the bacteria. In aerobic…doesn’t
happen bc there’s no reduction)


Mammalian cells and aerobic bacteria (including facultative
bacteria) are unharmed because they lack enzymes to reduce the
nitro group of these agents.


Metroniadazole is one of the most commonly used
nitroimidazols


Bactericidal


Lots of anaerobic bacteria species in the oral cavity
(dentists use them)



Rifamycins (Rifampin, Rifabutin) bind to bacterial RNA
polymerase and inhibit initiation of RNA synthesis



Rifampin is bactericidal for
M. tuberculosis

and active against
many Gram positive bacteria



Gram negative bacteria are intrinsically resistant to rifamycins
because of decreased uptake of the drug


The outer membrane inhibits uptake of this hydrophobic antibiotic



Resistance develops quickly and is the result of a change in the
target


A chromosomal mutation in the gene encoding RNA polymerase


Quinolones


Inhibit gyrase and topoIV


Bactericidal


Widely used new agents have broad spectrum


Nitroimidazoles / Metroniadazole


Inhibits anaerobic bacteria


Bactericidal


Rifamycins


Inhibit bacterial RNA polymerase


Bactericidal


Resistance develops rapidly


Not effective against Gram negative bacteria

Inhibitors of 30S ribosomal subunit components


Aminoglycosides


Streptomycin, kanamycin neomycin, tobramycin, gentamicin,
amikacin


Tetracycines


Tetracycline, doxycycline, minocycline


Inhibitors of the 50S ribosomal subunit


Oxazolidinones and Lincosamde antibiotics


Linezolid


Clindamycin


Chloramphenicol


Macrolides


Erythromycin, Azithromycin, Clarithromycin


Streptogramins


Two
-
components group A and B


Quinupristin
-
dalfopristin (Synercid)


3

1

2

Oxazolindinones

4

5

3

Streptogramins

6


Bactericidal
-

Irreversibly bind to the 30S ribosome


Freeze the 30S initiation complex (30S
-
mRNA
-
tRNA) so that no
further initiation can occur.


Slow down protein synthesis that has already initiated and induce
misreading of the mRNA


Kanamycin, tobramycin, gentamicin



Effective against many Gram
-
negative and some Gram
-
positive bacteria (broad spec)


not useful for anaerobic (oxygen required for uptake of antibiotic) or
intracellular bacteria


Commonly used to treat infections caused by Gram negative rods



Synergize with
b
-
lactam antibiotics


b
-
lactams inhibit cell wall synthesis and thereby increase the uptake
of the aminoglycosides (no wall = easier to entry)



Mutation of the ribosomal binding site


Not common bc to prevent binding requires multiple
mutations


Decreased uptake of the antibiotic


Seen in
Pseudomonas

and anaerobic bacteria


Increased expulsion of the antibiotic (efflux)


Rare

only occurs in Gram negative bacteria


Enzymatic modification of the antibiotic


Most common form of resistance


Actually adding a component to the drug to make it inactive


Phosphotransferases


Adenyltransferases


Acetyltransferases


Bacteriostatic


Reversibly bind to the 30S
ribosomal subunit


Block binding of aminoacyl
-
transfer RNA (tRNA)


thus
very specific function (binding rt where t
-
RNA should
bind)


Broad Spectrum


Used to treat a wide variety of infections caused by:


Chlamydia


Mycoplasma


Rickettsia


And a variety of other Gram positive and Gram negative bacteria



Tetracycline, Doxycycline, Minocycline


Decreased uptake of the antibiotic


Active efflux of the antibiotic


Most common cause of resistance in Gram negative
bacteria by pumping the actual drug out


Alteration of the ribosomal target


Enzymatic modification of the antibiotic



3

1

2

Oxazolindinones

4

5

3

Streptogramins

6


Linezolid most commonly used


Bind the 50S ribosomal subunit


Distorts the binding site for the tRNA and inhibits
formation of the 70S initiation complex


Narrow spectrum


Unique mechanism of action so cross resistance with
other protein inhibitors does not occur


Most commonly used against drug resistant enterococci


Active against Gram positive cocci


Including those resistant to penicillins, vancomycin and
aminoglycosides


Lincomycin and its derivative Clindamycin


Block protein elongation by binding to the 50S subunit of
the ribosome


Spectrum


“relatively broad”


Staphylococci and anaerobic Gram
-
negative rods


Not active against most aerobic Gram negative bacteria


Resistance


Methylation of the 23S RNA


Enzymatic inactivation of the antibiotic


Cross
-
resistance occurs with macrolides since alteration of
23S will affect other drugs


Inhibitors of 30S ribosomal subunit components


Aminoglycosides


Streptomycin, kanamycin neomycin, tobramycin, gentamicin,
amikacin


Tetracycines


Tetracycline, doxycycline, minocycline


Inhibitors of the 50S ribosomal subunit


Oxazolidinones and Lincosamde antibiotics


Linezolid


Clindamycin


Chloramphenicol


Macrolides


Erythromycin, Azithromycin, Clarithromycin


Streptogramins


Two
-
components group A and B


Quinupristin
-
dalfopristin (Synercid)



Bacteriostatic


Binds reversibly to the 50S ribosomal subunit



Broad Spectrum


Similar spectrum to tetracycline


Not commonly used in the US because it can disrupt
protein synthesis in human bone marrow and cause
aplastic anemia (1 in 60,000 patients)



Resistance


Enzymatic inactivation of the antibiotic (acetylation
than phosphorylation)


Changes in the outer membrane of Gram negative
bacteria to reduce uptake


Bacteriostatic


Binds reversibly to the 23S RNA component of the 50S
ribosomal subunit


Erythromycin, Azithromycin, Clarithromycin


Broad Spectrum


Primarily used to treat pulmonary/respiratory infections
caused by Gram+


Mycoplasma, Legionella and Chlamydia species


Most Gram negative bacteria are resistant
(true in that the
level of drug required to combat these are way too high)


Resistance


Methylation of the 23S RNA


Enzymatic inactivation of the antibiotic


Other changes in the 23S RNA and proteins of the 50S
subunit


Cross
-
resistance occurs with Lincosamide antibiotics

3

1

2

Oxazolindinones

4

5

3

Streptogramins

6


Cyclic peptides
administered as a
combination

of
two components


Group A and Group B Streptogramins


The two components act synergistically


The Group A component binds to the 50S ribosomal
subunit and facilitates binding of the Group B
component


The Group B component inhibits chain elongation


Both components NEEDS to work together


The most commonly used Steptogramin is
quinupristin
-
dalfopristin


Spectrum


Most commonly used against staphylococci, streptococci
and
Enterococcus faecium
.
-

primarily against
vancomycin resistant
E. faecium



3

1

2

Oxazolindinones

4

5

3

Streptogramins

6


Inhibition of Cell Wall Synthesis


Disruption of Membranes


Inhibition of Nucleic Acid Synthesis


Inhibition of Protein Synthesis


Antimetabolites





Interference with folate metabolism


Sulfonamides


Inhibitors of pteroic acid synthetase


Trimethoprims


Inhibitors of dihydrofolate reductase


Most bacteria cannot use pre
-
formed folic acid and must synthesize
folic acid


which makes it a key target


Exception: Enterococci can use exogenous thymidine and are
intrinsically resistant inhibitors of this process


In contrast, Human cannot synthesize folic acid and must obtain it
from food consumption so inhibition of folic acid synthesis won’t
negatively affect us


Folic acid is synthesized by joining three components together


Enterococci can use exogenous thymidine and are intrinsically resistant
inhibitors of this process


Sulfonamides


Inhibitors of pteroic acid synthetase


bacteriostatic


Trimethoprims


Inhibitors of dihydrofolate reductase


bacteriostatic


Both are broad range


Used against a variety of Gram positive and Gram
negative bacteria (except enterococci)



Sulfonamides and Trimethoprims are
commonly used together


Synergistic since they act at diff steps of a process


Ex) Dapsone and p
-
aminosalicylic acid



Resistance


Stems from a variety of mechanisms


Permeability barriers


Decreased affinity of dihydrofolate reductase for
trimethoprim by changing enzyme activity


Enterococci can use exogenous thymidine and are
intrinsically resistant


Inhibition of Cell Wall Synthesis


Disruption of Membranes


Inhibition of Nucleic Acid Synthesis


Inhibition of Protein Synthesis


Antimetabolites


Mechanism of action


Uses (spectrum)


Mechanisms of resistance