Discovery of antimicrobial drugs› Paul Erlich (1909)found the first

pharmaceutical effective for treatment of syphilis: Salvarsan Arsphenamine highly toxic

› Sulfonamide was the first sulfa drug In vitro derivative of Prontosil dye effective against streptococcal infections Bayer Labs, 1939 Nobel prize in Medicine

Discovery of antibiotics› Penicillin discovered by Alexander Fleming

Identified mold Penicillium that produced a bactericidal substance that was effective against a wide range of gram + microbes

Inhibits cell wall synthesis Mass production of penicillin during WWII

› Streptomycin (1943) isolated from soil bacterium Streptomyces griseus by Selman Waksman Bacteriostatic Inhibits protein synthesis by binding to ribosome

Development of new generation of drugs› In 1960s scientists

alteration of drug structure gave them new properties Penicillin G altered to

create ampicillin Broadened spectrum

of antimicrobial killing

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 grown until maximum antibiotic

concentration is reached› Drug is extracted from broth medium› Extensively purified› May be chemically altered

Termed semi-synthetic

Selective toxicity› Antibiotics cause greater harm to

microorganisms than to human host› 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 Narrow therapeutic index = more toxic, monitor

closely

Antimicrobial action› Bacteriostatic drugs

Inhibit bacterial growth rely on host immunity

› Bacteriocidal drugs Kill bacteria Most useful in situations when host defenses

cannot 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

Advantage: effects pathogen only Disadvantage: requires identification of pathogen

Broad spectrum Advantage: Work on broad range of organisms Disadvantage : disruption of normal flora

Effects of combinations of antimicrobial drugs› Combination sometimes used to treat

infections Synergistic: whole is > sum Antagonistic: whole is < sum Additive: whole is the sum

Tissue distribution, metabolism and excretion› Drugs differ in how they are distributed,

metabolized and excreted› Half-life: Rate of elimination of drug from

body 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

Adverse effects› Allergic reactions› Toxic effects› Suppression of normal flora› Antimicrobial resistance

Mechanism of action include:› Inhibition of cell wall synthesis

Penicillins, Cephalosporins, Vancomycin, Bacitracin

› Inhibition of protein synthesis Aminoglycosides, tetracyclines, macrolides, chloramphenicol,

lincosamides

› Inhibition of nucleic acid synthesis Fluoroquinolones, rifamycins

› Inhibition of metabolic pathways Sulfonamides, trimethoprim

› Interference with cell membrane integrity Polymyxin

Inhibition of cell wall synthesis› Antimicrobials that interfere

with the synthesis of peptidoglycan

› These drugs have very high therapeutic index

› Antimicrobials of this class include β lactam drugs (penicillin,

cephalosporin) Vancomycin Bacitracin

Drugs vary in spectrum Some more active against Gram (+) Some more active against Gram (-)

› Resistance through production of β-lactamase enzyme

› Penicillins + β lactamase inhibitor Augmentin = amoxicillin + clavulanic

acid

Vancomycin› Inhibits formation of glycan

chains Does not cross lipid

membrane of Gram (-)› Important in treating

infections caused by penicillin resistant Gram (+) organisms

› Given intravenously due to poor GI absorption

› Acquired resistance 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› Drugs of this class include

Aminoglycosides Tetracyclins Macrolids Chloramphenicol Lincosamides Oxazolidinones Streptogramins

Aminoglycosides› Irreversibly binds to 30S

ribosomal subunit Blocks initiation

translation Causes misreading of

mRNA› Not effective against

anaerobes, enterococci and streptococci

› Often used in synergistic combination with β-lactam drugs

› Examples 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

› Narrow range: Effective against certain Gram (+) and Gram (-)

Macrolids› Reversibly binds to 50S

ribosome Prevents continuation of

protein synthesis› Effective against variety

of Gram (+) organisms› Often drug of choice for

patients allergic to penicillin

› Macrolids include Erythromycin,

clarithromycin and azithromycin

› Resistance can occur via modification of RNA target

Chloramphenicol› Binds to 50S

ribosomal subunit Prevents peptide bond

formation

› Wide spectrum› Drug of last resort› Rare but lethal side

effect is aplastic anemia

Lincosamides: clindamycin› 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

New class effective against β-lactams and vancomycin resistant Gram (+) forms› Oxazolidinones

Binds 50S ribosomal subunit Interferes with initiation

› Streptogramins Bonds to two different sites on 50S

ribosomal subunit

Fluoroquinolones› Inhibit action of topoisomerase DNA gyrase

Topoisomerase maintains supercoiling of DNA

› Broad-Spectrum: Effective against Gram (+) and Gram (-)

› Examples include Ciprofloxacin and ofloxacin

› Resistance due to alteration of DNA gyrase

Rifamycins› Block prokaryotic RNA polymerase

initiation of transcription

› Rifampin most widely used rifamycins› Broad-spectrum: Effective against many Gram

(+) and some Gram (-) as well as Mycobacterium

› Treatment of › Tuberculosis › Hansen’s disease › N. meningitidis meningitis

› Resistance develops rapidly

Folate inhibitors Mode of actions to

inhibit the production of folic acid

Mimic PABA

› Antimicrobials in this class include Sulfonamides Trimethoprim

› Human cells lack specific enzyme in folic acid pathway

› Resistance due to plasmid

› Polymixn B most common

Common ingredient in first-aid skin ointments

› Binds membrane of Gram (-) cells Alters permeability Also binds eukaryotic

cells Limits use to topical

application

Susceptibility of organism to specific antimicrobials is unpredictable

Often drug after drug tried until favorable response was observed

Better approach› Determine susceptibility› Prescribe drug that acts against offending

organism Best to choose one that affects as few others as

possible

MIC = Minimum Inhibitory Concentration

Quantitative test to determine lowest concentration of specific antimicrobial drug needed to prevent growth of specific organism

Kirby-Bauer disc diffusion method› qualitative

determination of susceptibility

› Discs impregnated with specific concentration of antibiotic placed on plate and incubated

› Clear zone of inhibition around disc reflects susceptibility

size of clearing zone indicates if susceptible or resistant

E-test› Uses strips

impregnated with gradient concentration of antibiotic

› Test organism will grow and form zone of inhibition Zone is tear-drop

shaped Zone will intersect

strip at inhibitory concentration

Mechanisms of resistance› Drug inactivating enzymes

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 macrolids from binding to ribosomal subunits

Mechanisms of resistance› Decreased uptake of the

drug Alterations in porin proteins

decrease permeability of cells

› Increased elimination of the drug Some organisms produce

efflux pumps Tetracycline resistance

Acquisition of resistance› Can be due to spontaneous mutation

vertical evolution

› Or acquisition of new genes horizontal transfer Plasmid mediated

Spontaneous mutation› 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 Organism acquires resistance to several different

drugs simultaneously

Examples of emerging antimicrobial resistance› Enterococci

Intrinsically resistant to many common antimicrobials

Some strains resistant to vancomycin VRE: Vancomycin resistant enterococcus

Many strains achieve resistance via transfer of plasmid

Staphylococcus aureus› Common cause of nosocomial infections› Becoming increasingly resistant

Most strains acquired resistance to penicillin Until recently most infections could be treated

with methicillin MRSA methicillin resistant Staphylococcus aureus

many of these strains still susceptible to vancomycin 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

Acquisition via DNA mediated transformation

Slowing emergence and spread of resistance› Responsibilities of physicians and healthcare

workers 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 which develop resistance in one country can

be transported globally Many antimicrobials are available as non-prescription

basis Use of antimicrobial drugs added to animal feed

Produce larger more economically productive animals Also selects for antimicrobial resistant organisms

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› 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› 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

Non-nucleoside reverse transcriptase inhibitor› Inhibits activity of reverse transcriptase by

binding to site other than nucleotide binding site Example = nevirapine, delavirdine, efavirenz

Used in combination to treat HIV

Protease inhibitor› Inhibit HIV encoded enzyme protease

Enzyme essential for production of viral particles

Examples = indinavir and ritonavir Neuraminidase inhibitor

› Inhibit neuraminidase enzyme of influenza Enzyme essential for release of virus Examples = zanamivir and oseltamivir

Target for most antifungal medications is plasma membrane› Ergosterol

Include› Polyenes› Azoles› Allylamines

Other targets› Cell wall synthesis› Cell division› Nucleic acid synthesis

Cell wall synthesis› Echinocandins

interfere with synthesis 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

Inhibits enzymes required for nucleic acid synthesis Flucytosine converted to 5-fluorouricil

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

Interferes with mitochondrial electron transport and disruption of folate synthesis