Antimicrobial Medications Chapter 21. History and Development of Antimicrobial Drugs Discovery of antimicrobial drugs Salvarsan first documented example

Antimicrobial Medications

Chapter 21

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


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 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



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


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


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


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


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


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


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


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


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


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


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


Bacitracin Interferes with transport of PTG precursors

across cytoplasmic membrane Toxicity limits use to topical applications Common ingredient in non-prescription first-aid

ointments


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


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


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


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


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


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


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


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


Inhibition of nucleic acid synthesis These include

Fluoroquinolones Rifamycins


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


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 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


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


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


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


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



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


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


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


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


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


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


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


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


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


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


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


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


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


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)


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


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 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


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


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 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

end

Documents

Antimicrobial Medications Chapter 21. History and Development of Antimicrobial Drugs Discovery of antimicrobial drugs Salvarsan first documented example