antimicrobial medications chapter 21. history and development of antimicrobial drugs discovery of...
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History and Development ofAntimicrobial Drugs
Discovery of antimicrobial drugs Salvarsan first documented example
of chemical successfully used as antimicrobial
Discovered by Paul Erlich in for treatment of syphilis
Prontosil dye effective against streptococcal infections
No effect on Streptococcus growing in vitro
Enzymes in blood split prontosil into small sulfonamide molecules
Sulfonamide was the first sulfa drug Acts as a competitive inhibitor to para-
aminobenzoic acid
Discovery of antibiotics Alexander Fleming
Discovered penicillin while working with Staphylococcus
Noticed there were no Staphylococcus colonies growing near a mold contaminant
The colonies appeared to be melting Identified mold as Penicillium which was producing a
bactericidal substance that was effective against a wide range of microbes
Fleming unable to purify compound Eventually abandoned his study
History and Development ofAntimicrobial Drugs
Discovery of antibiotics Ernst Chain and Howard Florey successfully purified
penicillin In 1941tested on human subject with life threatening
Staphylococcus aureus infection Treatment effective initially
Supply of penicillin ran out before disease under control Patient dies of infection
Drug tested again with adequate supply Patients recovered fully
Mass production of penicillin during WWII Treatment of wounded soldiers and war workers
Selman Waksman isolated streptomycin from soil bacterium Streptomyces griseus
History and Development ofAntimicrobial Drugs
History and Development ofAntimicrobial Drugs Development of new
generation of drugs In 1960s scientists
alteration of drug structure gave them new properties
Penicillin G altered to created ampicillin
Broadened spectrum of antimicrobial killing
Features of Antimicrobial Drugs
Most modern antibiotics come from organisms living in the soil Includes bacterial species Streptomyces and Bacillus as
well as fungi Penicillium and Cephalosporium To commercially produce antibiotics
Strain is inoculated into broth medium Incubated until maximum antibiotic concentration is
reached Drugs is extracted from broth medium Antibiotic extensively purified In some cases drugs are chemically altered to impart new
characteristics Termed semi-synthetic
Selective toxicity Antibiotics cause greater harm to microorganisms
than to human host Generally by interfering with biological structures or
biochemical processes common to bacteria but not to humans
Toxicity of drug is expressed as therapeutic index Lowest dose toxic to patient divided by dose typically
used for treatment High therapeutic index = less toxic to patient
Features of Antimicrobial Drugs
Features of Antimicrobial Drugs
Antimicrobial action Drugs may kill or inhibit bacterial growth
Inhibit = bacteriostatic Kill = bacteriocidal
Bacteriostatic drugs rely on host immunity to eliminate pathogen
Bacteriocidal drugs are useful in situations when host defenses cannot be relied upon to control pathogen
Spectrum of activity Antimicrobials vary with respect to range of
organisms controlled Narrow spectrum
Work on narrow range of organisms Gram-positive only OR-Gram negative only
Broad spectrum Work on broad range of organisms
Gram-positive AND Gram-negative Disadvantage of broad spectrum is disruption of
normal flora
Features of Antimicrobial Drugs
Tissue distribution, metabolism and excretion Drugs differ in how they are distributed,
metabolized and excreted Important factor for consideration when prescribing
Rate of elimination of drug from body expressed in half-life
Time it takes for the body to eliminate one half the original dose in serum
Half-life dictates frequency of dosage Patients with liver or kidney damage tend to
excrete drugs more slowly
Features of Antimicrobial Drugs
Effects of combinations of antimicrobial drugs Combination some times used to treat infections When action of one drug enhances another effect
is synergistic When action of one drug interferes with another
effect is antagonistic When effect of combination is neither synergistic
or antagonistic effect said to additive
Features of Antimicrobial Drugs
Adverse effects Allergic reactions
Allergies to penicillin Allergies often life threatening
Toxic effects Aplastic anemia
Body cannot make RBC or WBC
Suppression of normal flora Antibiotic associated colitis
Toxic organisms given opportunity to establish themselves
Antimicrobial resistance Microorganisms have innate or adaptive resistance to
antibiotics
Features of Antimicrobial Drugs
Mechanisms of Action of Antibacterial Drugs
Mechanism of action include: Inhibition of cell wall synthesis Inhibition of protein synthesis Inhibition of nucleic acid
synthesis Inhibition of metabolic
pathways Interference with cell
membrane integrity Interference with essential
processes of M. tuberculosis
Mechanisms of Action of Antibacterial Drugs
Inhibition of cell wall synthesis Bacteria cell wall unique in
construction Contains PTG
Antimicrobials interfere with the synthesis of cell walls, and do not interfere with eukaryotic cell
Due to the lack of cell wall in animal cells and differences in cell wall in plant cells
These drugs have very high therapeutic index
Low toxicity with high effectiveness
Antimicrobials of this class include β lactam drugs Vancomycin Bacitracin
Mechanisms of Action of Antibacterial Drugs
Penicillins and cephalosporins Part of group of drugs called β–lactams
Have shared chemical structure called β-lactam ring
Competitively inhibits function of penicillin-binding proteins
Inhibits peptide bridge formation between glycan molecules
Drugs vary in spectrum Some more active against Gram +
Due to access of PTG Some more active against Gram -
Some organisms resist effects through production of β-lactamase enzyme
Enzyme breaks β-lactam ring
Mechanisms of Action of Antibacterial Drugs
The penicillins Each member has common structure
Modified side chains create derivatives
Penicillin drugs include Natural penicillins Penicillinase-resistant penicillin Broad-spectrum penicillins Extended spectrum penicillins Penicillins + β lactamase inhibitor
Mechanisms of Action of Antibacterial Drugs
Natural penicillins Narrow spectrum
Effective against Gram + and some Gram - cocci
Penicillinase-resistant penicillin Developed in laboratory
Side chains prevent inactivation from penicillinase enzymes
Broad-spectrum penicillins Modified side chains give them a more broad
spectrum Effective against Gram + and Gram -
Extended spectrum penicillins Greater effectiveness against Pseudomonas
species Less effective against Gram + organisms
Penicillins + β lactamase inhibitor Combination of penicillin drug and enzyme
inhibitor Augmentin = amoxicillin + clavulanic acid
The cephalosporins Chemical structures make them resistant to
inactivation by certain β-lactamases Tend to have low affinity to penicillin-binding
proteins of Gram + bacteria Chemically modified to produce family of
related compounds First, second, third and fourth generation
cephalosporins
Mechanisms of Action of Antibacterial Drugs
Other β-lactam antibiotics Two groups
Carbapenems and monobactams Very resistant to β-lactamases Carbapenems effective against wide range of
Gram + and Gram - organisms Monobactams primarily effective against
members of family Enterobacteriaceae Can be given to patients with penicillin allergies
Mechanisms of Action of Antibacterial Drugs
Vancomycin Inhibits formation of glycan chains
Inhibits formation of PTG and cell wall construction Does not cross lipid membrane of Gram -
Gram - organisms innately resistant
Important in treating infections caused by penicillin resistant Gram + organisms
Must be given intravenously due to poor absorption from intestinal tract
Acquired resistant most often due to alterations in side chain of NAM molecule
Prevents binding of vancomycin to NAM component of glycan
Mechanisms of Action of Antibacterial Drugs
Bacitracin Interferes with transport of PTG precursors
across cytoplasmic membrane Toxicity limits use to topical applications Common ingredient in non-prescription first-aid
ointments
Mechanisms of Action of Antibacterial Drugs
Inhibition of protein synthesis Structure of prokaryotic ribosome acts as target for many
antimicrobials of this class Differences in prokaryotic and eukaryotic ribosomes
responsible for selective toxicity Drugs of this class include
Aminoglycosides Tetracyclins Macrolids Chloramphenicol Lincosamides Oxazolidinones Streptogramins
Mechanisms of Action of Antibacterial Drugs
Aminoglycosides Irreversibly binds to 30S ribosomal subunit
Causes distortion and malfunction of ribosome Blocks initiation translation
Causes misreading of mRNA Not effect against anaerobes, enterococci and streptococci Often used in synergistic combination with β-lactam drugs
Allows aminoglycosides to enter cells that are often resistant
Examples of aminoglycosides include Gentamicin, streptomycin and tobramycin
Side effects with extended use include Nephrotoxicity Otto toxicity
Mechanisms of Action of Antibacterial Drugs
Tetracyclins Reversibly bind 30S ribosomal subunit
Blocks attachment of tRNA to ribosome Prevents continuation of protein synthesis
Effective against certain Gram + and Gram - Newer tetracyclines such as doxycycline have longer half-
life Allows for less frequent dosing
Resistance due to decreased accumulation by bacterial cells
Can cause discoloration of teeth if taken as young child
Mechanisms of Action of Antibacterial Drugs
Macrolides Reversibly binds to 50S ribosome
Prevents continuation of protein synthesis Effective against variety of Gram + organisms and those
responsible for atypical pneumonia Often drug of choice for patients allergic to penicillin Macrolids include
Erythromycin, clarithromycin and azithromycin Resistance can occur via modification of RNA target
Other mechanisms of resistance include production of enzyme that chemically modifies drug as well as alterations that result in decreased uptake of drug
Mechanisms of Action of Antibacterial Drugs
Chloramphenicol Binds to 50S ribosomal subunit
Prevents peptide bonds from forming and blocking proteins synthesis
Effective against a wide variety of organisms Generally used as drug of last resort for life-
threatening infections Rare but lethal side effect is aplastic anemia
Mechanisms of Action of Antibacterial Drugs
Lincosamides Binds to 50S ribosomal subunit
Prevents continuation of protein synthesis Inhibits variety of Gram + and Gram -
organisms Useful in treating infections from intestinal
perforation Especially effective against Bacterioides fragilis and
Clostridium difficile Most commonly used lincosamide is
clindamycin
Mechanisms of Action of Antibacterial Drugs
Oxazolidinones New class of antimicrobials Binds 50S ribosomal subunit
Interferes with initiation of proteins synthesis Effective against variety of Gram + bacteria
Especially those resistant to β-lactams and vancomycin
Mechanisms of Action of Antibacterial Drugs
Streptogramins Bonds to two different sites on 50S ribosomal
subunit Acts synergistically through the combination of
quinupristin and dalfopristin Medication called Synercid
Effective against variety of Gram + bacteria Especially those resistant to β-lactams and
vancomycin
Mechanisms of Action of Antibacterial Drugs
Inhibition of nucleic acid synthesis These include
Fluoroquinolones Rifamycins
Mechanisms of Action of Antibacterial Drugs
Fluoroquinolones Inhibit action of topoisomerase DNA gyrase
Topoisomerase cuts DNA to allow swiveling during DNA replication.
Effective against Gram + and Gram - Examples include
Ciprofloxacin and ofloxacin Resistance due to alteration of DNA gyrase
Mechanisms of Action of Antibacterial Drugs
Rifamycins Block prokaryotic RNA polymerase
Block initiation of transcription Rifampin most widely used rifamycins Effective against many Gram + and some Gram - as well
as members of genus Mycobacterium Primarily used to treat tuberculosis and Hansen’s disease
as well as preventing meningitis after exposure to N. meningitidis
Resistance due to mutation coding RNA polymerase Resistance develops rapidly
Mechanisms of Action of Antibacterial Drugs
Mechanisms of Action of Antibacterial Drugs Inhibition of metabolic
pathways Relatively few Most useful are folate
inhibitors Mode of actions to
inhibit the production of folic acid
Antimicrobials in this class include
Sulfonamides Trimethoprim
Mechanisms of Action of Antibacterial Drugs
Sulfonamides Group of related compounds
Collectively called sulfa drugs Inhibit growth of Gram + and Gram - organisms
Through competitive inhibition of enzyme that aids in production of folic acid
Structurally similar to para-aminobenzoic acid Substrate in folic acid pathway
Human cells lack specific enzyme in folic acid pathway Basis for selective toxicity
Resistance due to plasmid Plasmid codes for enzyme that has lower affinity to drug
Trimethoprim Inhibits folic acid production
Interferes with activity of enzyme following enzyme inhibited by sulfonamides
Often used synergistically with sulfonamide Most common mechanism of resistance is
plasmid encoded alternative enzyme Genes encoding resistant to sulfonamide and
trimethoprim are often carried on same plasmid
Mechanisms of Action of Antibacterial Drugs
Interference with cell membrane integrity Few damage cell membrane
Polymyxin B most common Common ingredient in first-aid skin ointments
Binds membrane of Gram - cells Alters permeability
Leads to leakage of cell and cell death Also bind eukaryotic cells but to lesser extent
Limits use to topical application
Mechanisms of Action of Antibacterial Drugs
Inference with processes essential to Mycobacterium tuberculosis Limited range of antimicrobials used in treatment of
infections caused by M. tuberculosis Due to numerous factors including chronic nature of
disease, slow growth and waxy lipid in cell wall Waxy cell wall due to mycolic acid is impervious to many drugs
Five medications termed first-line drugs preferentially because of effectiveness with low toxicity
First-line drugs include rifampin, streptomycin and Isoniazid inhibits synthesis of mycolic acid Ethambutol inhibits enzymes for cell wall synthesis Pyrazinamide mechanism of action unknown
Mechanisms of Action of Antibacterial Drugs
Determining Susceptibility of Bacterial to Antimicrobial Drug
Susceptibility of organism to specific antimicrobials is unpredictable
Often drug after drug tried until favorable response was observed If serious infection, several drugs were prescribed at one
time with hope that one was effective Better approach
Determine susceptibility Prescribe drug that acts against offending organism
Best to choose one that effects as few others as possible
Determining MIC MIC = Minimum Inhibitory Concentration
Quantitative test to determine lowest concentration of specific antimicrobial drug needed to prevent growth of specific organism
Determined by examining strain’s ability to growth in broth containing different concentrations of test drug
Determining Susceptibility of Bacterial to Antimicrobial Drug
Determining Susceptibility of Bacterial to Antimicrobial Drug
MIC is determined by producing serial dilutions with decreasing concentrations of test drug
Known concentrations of organism is added to each test tube
Tubes are incubated and examined for growth Growth determined by turbidity
of growth medium Lowest concentration to prevent
growth is MIC
Determining Susceptibility of Bacterial to Antimicrobial Drug
Conventional disc diffusion method Kirby-Bauer disc diffusion
routinely used to qualitatively determine susceptibility
Standard concentration of strain uniformly spread of standard media
Discs impregnated with specific concentration of antibiotic placed on plate and incubated
Clear zone of inhibition around disc reflects susceptibility
Based on size of zone organism can be described as susceptible or resistant
Determining Susceptibility of Bacterial to Antimicrobial Drug
E-test Modification of disc diffusion test Uses strips impregnated with gradient
concentration of antibiotic From highest concentration to
lowest Test organism will grow and form
zone of inhibition Zone is tear-drop shaped Zone will intersect strip at inhibitory
concentration
Resistance to Antimicrobial Drugs
Mechanisms of resistance Drug inactivating enzymes
Some organisms produce enzymes that chemically modify drug
Penicillinase breaks β-lactam ring of penicillin antibiotics
Alteration of target molecule Minor structural changes in
antibiotic target can prevent binding
Changes in ribosomal RNA prevent mecrolides from binding to ribosomal subunits
Determining Susceptibility of Bacterial to Antimicrobial Drug
Mechanisms of resistance Decreased uptake of the drug
Alterations in porin proteins decrease permeability of cells
Prevents certain drugs from entering
Increased elimination of the drug Some organisms produce efflux
pumps Increases overall capacity of
organism to eliminate drug Enables organism to resist
higher concentrations of drug
Tetracycline resistance
Determining Susceptibility of Bacterial to Antimicrobial Drug
Acquisition of resistance Can be due to spontaneous
mutation Alteration of existing genes Spontaneous mutation
called vertical evolution Or acquisition of new genes
Resistance acquired by transfer of new genes called horizontal transfer
Spontaneous mutation Occurs at relatively low rate Such mutations have profound effect of
resistance of bacterial population Example of spontaneous mutation
Resistance to streptomycin is result a change in single base pair encoding protein to which antibiotic binds
When antimicrobial has several different targets it is more difficult for organism to achieve resistance through spontaneous mutation
Determining Susceptibility of Bacterial to Antimicrobial Drug
Acquisition of new genes through gene transfer Most common mechanism of transfer is through
conjugation Transfer of R plasmid Plasmid often carries several different resistance
genes Each gene mediating resistance to a specific antibiotic
Organism acquires resistance to several different drugs simultaneously
Determining Susceptibility of Bacterial to Antimicrobial Drug
Examples of emerging antimicrobial resistance Enterococci
Part of normal intestinal flora Common cause of nosocomial infections Intrinsically resistant to many common antimicrobials Some strains resistant to vancomycin
Termed VRE Vancomycin resistant enterococcus
Many strains achieve resistance via transfer of plasmid
Determining Susceptibility of Bacterial to Antimicrobial Drug
Staphylococcus aureus Common cause of nosocomial infections Becoming increasingly resistant
In past 50 years most strains acquired resistance to penicillin
Due to acquisition of penicillinase genes Until recently most infections could be treated with
methicillin (penicillinase resistant penicillin) Many strains have become resistant
MRSA methicillin resistant Staphylococcus aureus MRSA many of these strains still susceptible to
vancomycin Some hospitals identified VISA
VISA vancomycin intermediate Staphylococcus aureus
Determining Susceptibility of Bacterial to Antimicrobial Drug
Streptococcus pneumoniae Has remained sensitive to penicillin
Some strains have now gained resistance Resistance due to modification in genes coding for
penicillin-binding proteins Changes due to acquisition of chromosomal DNA
from other strains of Streptococcus Generally via DNA mediated transformation
Determining Susceptibility of Bacterial to Antimicrobial Drug
Mycobacterium tuberculosis First-line drugs incur spontaneous mutations
readily Organisms in active infection often resistant to one
of the multiple drugs used to treat Reason for multiple drug therapy required
When organisms become resistant to rifampin and isoniazid organisms termed multi-drug-resistant
Determining Susceptibility of Bacterial to Antimicrobial Drug
Slowing emergence and spread of resistance Responsibilities of physicians and healthcare
workers Increase efforts to prescribe antibiotics for specific
organisms Educate patients on proper use of antibiotics
Responsibilities of patients Follow instructions carefully Complete prescribed course of treatment
Misuse leads to resistance
Determining Susceptibility of Bacterial to Antimicrobial Drug
Slowing emergence and spread of resistance Importance of an educated public
Greater effort made to educate public about appropriateness and limitations of antibiotics
Antibiotics have no effect on viral infections Misuse selects antibiotic resistance in normal flora
Global impacts of the use of antimicrobial drugs Organisms develop resistance in one country can be
transported globally Many antimicrobials are available as non-prescription
basis Use of antimicrobials drugs added to animal feed
Produce larger more economically productive animals Also selects for antimicrobial resistant organisms
Determining Susceptibility of Bacterial to Antimicrobial Drug
Mechanisms of Action of Antiviral Drugs
Available antiviral drugs effective specific type of virus None eliminate latent virus
Targets include Viral uncoating Nucleoside analogs Non-nucleoside polymerase
inhibitors Non-nucleoside reverse
transcriptase inhibitors Protease inhibitors Neuraminidase inhibitors
Viral uncoating Drugs include amantadine and rimantadine Similar in chemical structure and mechanism
of action Mode of action is blocking uncoating of influenza
virus after it enters cell Prevents severity and duration of disease
Resistance develops frequently and may limit effectiveness of drug
Mechanisms of Action of Antiviral Drugs
Nucleoside analogs Similar in structure to nucleoside Analogs phosphorylated and become nucleotide
analogs Incorporation of analog results in termination of
growing nucleotide chain Examples of nucleoside analogs
Zidovudine (AZT) Didanosine (ddI) Lamivudine (3TC)
Mechanisms of Action of Antiviral Drugs
Non-nucleoside polymerase inhibitor Inhibit activation of viral polymerases by binding to
site other than nucleotide binding site Example: foscarnet and acyclovir
Used to treat CMV and HSV
Non-nucleoside reverse transcriptase inhibitor Inhibits activity of reverse transcriptase by binding
to site other than nucleotide binding site Example: nevirapine, delavirdine, efavirenz
Used to in combination to treat HIV
Mechanisms of Action of Antiviral Drugs
Protease inhibitor Inhibit HIV encoded enzyme protease
Enzyme essential for production of viral particles Examples: indinavir and ritonavir
Used in treatment of HIV
Neuraminidase inhibitor Inhibit neuraminidase enzyme of influenza
Enzyme essential for release of virus Examples: zanamivir and oseltamivir
Zanamivir administered via inhalation Oseltamivir administered orally
Mechanisms of Action of Antiviral Drugs
Mechanisms of Action of Antifungal Drugs
Target for most antifungal medications is ergosterol In plasma membrane
Drugs targeting ergosterol include Polyenes Azoles Allylamines
Other targets Cell wall synthesis Cell division Nucleic acid synthesis Ergosterol is to fungi what cholesterol is
to humans: the major membrane sterol.
Polyenes Produced by Streptomyces Disrupts fungal membrane
Causes leakage of cytoplasmic contents leading to cell death
Polyenes very toxic to humans Limited to use in life-threatening disease
Amphotericin B effective against systemic infection Causes severe side effects
Nystatin used topically due to toxicity
Mechanisms of Action of Antifungal Drugs
Azoles Chemically synthesized drugs Includes two classes
Imidazoles and triazoles Both inhibit synthesis of ergosterol Triazoles including fluconazole and itraconazole increased
in use for systemic infections Imidazoles like miconazole used in topical creams and
ointments Allylamines
Inhibit pathway of ergosterol synthesis Administered topically Used for treatment of dermatophyte infections
Mechanisms of Action of Antifungal Drugs
Cell wall synthesis Echinocandins
Family of agents that interfere with synthesis component of fungal cell wall
Cell division Griseofulvin
Exact mechanism unknown Appears to interfere with action of tubulin
Selective toxicity may be due to increased uptake by fungal cells Used to treat skin and nail infections
Nucleic acid synthesis Flucytosine
Synthetic derivative of cytosine Flucytosine converted to 5-fluorouracil
Inhibits enzymes required for nucleic acid synthesis Often combined with amphotericin
Mechanisms of Action of Antifungal Drugs
Mechanisms of Action of Antiprotozoans and Antihelminthics
Many antiparasitic drugs most likely interfere with biosynthetic pathways of protozoan parasites or neuromuscular function of worms
Example of parasitic drugs includes Malarone
Synergistic combination of atovaquone and proguanil HCl
Used to treat malaria Interferes with mitochondrial electron transport and
disruption of folate synthesis