Antibiotics - method of action

7/27/2019 Antibiotics - method of action

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ANTIBIOTICS ANTIBIOTICS

HOW DO THEY WORK?HOW DO THEY WORK?


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DefinitionsDefinitions Antibiotic Antibiotic -- antimicrobial agents producedantimicrobial agents produced

by microby micro --organisms that kill or inhibitorganisms that kill or inhibitother microother micro --organisms.organisms. Chemotherapeutic agentsChemotherapeutic agents --

antimicrobial agents of synthetic originantimicrobial agents of synthetic originuseful in the treatment of microbial oruseful in the treatment of microbial orviral disease.viral disease.

eg. sulfonilamides, isoniazid, ethambutol,eg. sulfonilamides, isoniazid, ethambutol, AZT, chloramphenicol. AZT, chloramphenicol.


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Antibiotics that Inhibit Cell Antibiotics that Inhibit Cell

Wall SynthesisWall Synthesis


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Cell Wall SynthesisCell Wall Synthesis

Prokaryotes differ from eukaryotes by having aProkaryotes differ from eukaryotes by having acell wall that includes a peptidoglycan layer.cell wall that includes a peptidoglycan layer.

GramGram veve s have a thin layer covered by thes have a thin layer covered by thelipopolysaccharide (endotoxin). Gram +velipopolysaccharide (endotoxin). Gram +ve ss

have thick layer with no covering.have thick layer with no covering.


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Cell Wall SynthesisCell Wall Synthesis Peptidoglycan consists of multiple aminoPeptidoglycan consists of multiple amino --sugarssugars

that alternate Nthat alternate N --acetylglucosamine and Nacetylglucosamine and N --acetylmuramic acid, which are cross linked toacetylmuramic acid, which are cross linked toform a lattice.form a lattice.

XX--linking is essential to risk the high internallinking is essential to risk the high internalosmotic pressures that are generated.osmotic pressures that are generated.

Peptidoglycan components manufacturedPeptidoglycan components manufactured

intracellularly and transported across the cellintracellularly and transported across the cellmembrane, where they link bymembrane, where they link bytranspeptidation.transpeptidation.


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Cell Wall SynthesisCell Wall Synthesis


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BetaBeta -- Lactam AntibioticsLactam Antibiotics Penicillins, Cephalosporins, Carbapenems andPenicillins, Cephalosporins, Carbapenems and

monobactams.monobactams. In 1928, Alexander Fleming observed a fungalIn 1928, Alexander Fleming observed a fungal

contamination of a plate of contamination of a plate of Staph. Aureus Staph. Aureus ..

The fungus was identifiedas Penicillium notatum ,and penicillin was

discovered. Florey and Chain were the

first to mass produceenicillin.


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PenicillinsPenicillins

The beta-lactam ring isderived from the amino-acids valine andcysteine. A secondaryamino group (RNH)determines theindividual properties of these antibiotics.


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CephalosporinsCephalosporins

Cephalosporium Cephalosporium fungus from sewer outlet infungus from sewer outlet in

SardiniaSardinia

--

shown to inhibitshown to inhibit

Staph. aureus Staph. aureus

growth.growth. 3 types of cephalosporins, N,C and P.3 types of cephalosporins, N,C and P.

The type C, is the basis for most of theThe type C, is the basis for most of thecephalosporin family by addition of variouscephalosporin family by addition of varioussidechains to coresidechains to core --lactam nucleus.lactam nucleus.

Action same as penicillins Action same as penicillins -- interferes withinterferes withpeptidoglycan synthesis.peptidoglycan synthesis.

Resistant toResistant to --lactamases, acid stable.lactamases, acid stable.


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CephalosporinsCephalosporins

Some are oral, but most are given parentally.Some are oral, but most are given parentally. Wide distribution after absorptionWide distribution after absorption -- pleura,pleura,

pericardium, joint fluid and across placenta.pericardium, joint fluid and across placenta.Cefuroxime, cefotaxime and ceftriaxone also cross theCefuroxime, cefotaxime and ceftriaxone also cross theblood brain barrier.blood brain barrier.

Elimination via renal tubular secretion and glomerularElimination via renal tubular secretion and glomerularfiltration, some are eliminated in bile.filtration, some are eliminated in bile.

SideSide --effects include penicillin type hypersensitivity,effects include penicillin type hypersensitivity,10% cross10% cross --reactivity. Nephrotoxicity and alcoholreactivity. Nephrotoxicity and alcoholintolerance reported, but uncommon. Diarrhoea withintolerance reported, but uncommon. Diarrhoea withoral route.oral route.

Broadspectrum, active against most Gram +veBroadspectrum, active against most Gram +ve s, somes, someGramGram ve activity, especially 2ve activity, especially 2 ndnd and 3and 3 rdrd generation.generation.


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OtherOther -- Lactam AntibioticsLactam Antibiotics

Carbapenems (eg Imipenen)Carbapenems (eg Imipenen) -- resistant to mostresistant to most--lactamases, wide spectrum against gramlactamases, wide spectrum against gram veveand +ve aerobic and anaerobic organisms.and +ve aerobic and anaerobic organisms.

Monobactams (aztreonam)Monobactams (aztreonam) -- derived from thederived from thebacteriabacteria Chromobacterium violaceum Chromobacterium violaceum ..


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Antimicrobial agents affecting Antimicrobial agents affecting

Bacterial Protein SynthesisBacterial Protein Synthesis


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Protein synthesisProtein synthesis Ribosomes are the basic units of machinery for proteinRibosomes are the basic units of machinery for protein

synthesis.synthesis.

Bacterial ribosomes have 50S and 30S subunits,Bacterial ribosomes have 50S and 30S subunits,mammalian ribosomes have 60S and 40S subunits.mammalian ribosomes have 60S and 40S subunits. mRNA attaches to 30S of ribosome with readsmRNA attaches to 30S of ribosome with reads

successive mRNA from the right.successive mRNA from the right. The next tRNA and attached aa, attach byThe next tRNA and attached aa, attach bycomplementary basecomplementary base --pairing to the mRNA, A site.pairing to the mRNA, A site.

The P site contains tRNA with its bound peptide chain.The P site contains tRNA with its bound peptide chain.The aa on the tRNA in the A site binds to the peptideThe aa on the tRNA in the A site binds to the peptidechain at the P site by transpeptidation, the tRNA atchain at the P site by transpeptidation, the tRNA atthe P site is ejected and the tRNA at the A sitethe P site is ejected and the tRNA at the A sitetranslocates to the P site.translocates to the P site.


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Protein synthesisProtein synthesis


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TetracyclinesTetracyclines Broad spectrum, original derived fromBroad spectrum, original derived from Streptomyces Streptomyces ..

More recent compounds are semiMore recent compounds are semi --synthetic orsynthetic or

synthetic (doxycycline, tetracycline).synthetic (doxycycline, tetracycline). They block protein synthesis by competing with tRNA They block protein synthesis by competing with tRNA for the A site of the ribosome/ mRNA complex.for the A site of the ribosome/ mRNA complex.


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TetracyclinesTetracyclines They are bacteriostatic, active against gram +ve/They are bacteriostatic, active against gram +ve/ --veveand some protazoa, problems with resitance.and some protazoa, problems with resitance.

Absorbed orally, excreted via bile and glomerular Absorbed orally, excreted via bile and glomerularfiltration, except doxycyclinefiltration, except doxycycline -- GI excretion.GI excretion.

Chelate metal ions, bone/teeth/ milk, GI upset.Chelate metal ions, bone/teeth/ milk, GI upset.


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ChloramphenicolChloramphenicol Originally fromOriginally from Streptomyces,Streptomyces, but nowbut now

synthetic.synthetic.

Binds to the 50S subunit at same site asBinds to the 50S subunit at same site aserythromycin, and inhibits transpeptidation.erythromycin, and inhibits transpeptidation.Drugs therefore competative.Drugs therefore competative.


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ChloramphenicolChloramphenicol

Broad spectrum and bacteriostatic, crossesBroad spectrum and bacteriostatic, crossesbloodblood --brain barrier. Problems with resistancebrain barrier. Problems with resistancedue to chloramphenicol acetyldue to chloramphenicol acetyl --transferase.transferase.

Aplastic anaemia (1 in 50,000). Grey Aplastic anaemia (1 in 50,000). Grey --babybabysyndrome (40% mortality).syndrome (40% mortality).


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ErythromycinErythromycin

Inhibits translocation of ribosome along mRNA Inhibits translocation of ribosome along mRNA chain.chain.


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ErythromycinErythromycin Spectrum similiar to penicillins, used inSpectrum similiar to penicillins, used in

penicillin allergies.penicillin allergies. Bacteriostatic or bacteriocidal, oral orBacteriostatic or bacteriocidal, oral or

parenteral, good prostate penetration, poorparenteral, good prostate penetration, poorsynovial penetration and does not cross BBsynovial penetration and does not cross BBbarrier.barrier.

SE rare, can cause hepatitis with prolongedSE rare, can cause hepatitis with prolongeduse.use.


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Aminoglycosides Aminoglycosides

Bind to 30S subunit and prevent complexing of Bind to 30S subunit and prevent complexing of the 50S subunit.the 50S subunit.

Bacteriocidal and effects enhanced by agentsBacteriocidal and effects enhanced by agentsthat interfere with cell wall synthesis.that interfere with cell wall synthesis.


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Aminoglycosides Aminoglycosides Not absorbed from GI tract. Do not cross BBNot absorbed from GI tract. Do not cross BB

barrier.barrier.

Plasma half Plasma half --life of 2life of 2 --3 hours, eliminated3 hours, eliminatedentirely by glomerular filtration.entirely by glomerular filtration. Good for gramGood for gram ve and some gram +veve and some gram +ve

organisms, no effect on anaerobes.organisms, no effect on anaerobes. Gentamicin commonest.Gentamicin commonest. Some resistance due to microbial inactivatingSome resistance due to microbial inactivating

enzymes. Amikacin is resistant to these.enzymes. Amikacin is resistant to these. Nephrotoxicity and ototoxicity, very rarelyNephrotoxicity and ototoxicity, very rarely --

neuromuscular blockade.neuromuscular blockade.


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Other Antibacterial agentsOther Antibacterial agents


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FluoroFluoro -- QuinolonesQuinolones Synthetic antibiotics which act by inhibition of bacterialSynthetic antibiotics which act by inhibition of bacterial

DNA gyrase (topoisomerase II).DNA gyrase (topoisomerase II).

This is responsible for supercoiling of the DNA helix.This is responsible for supercoiling of the DNA helix.Prokaryotic DNA gyrase is structurally different fromProkaryotic DNA gyrase is structurally different fromeukaryotic.eukaryotic.

Bacteriocidal and early drugs (nalidixic acid) wereBacteriocidal and early drugs (nalidixic acid) weremainly active against grammainly active against gram veve s.s. Newer forms (ciprofloxacin, ofloxacin and norfloxacin)Newer forms (ciprofloxacin, ofloxacin and norfloxacin)

have wider activity.have wider activity. Absorbed orally with a half Absorbed orally with a half --life of 4 hours.life of 4 hours. GI disturbances, skin rashes, rarely CNS disturbancesGI disturbances, skin rashes, rarely CNS disturbances

(confusion, drowsiness, agitation), blood dyscrasias.(confusion, drowsiness, agitation), blood dyscrasias.


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SulphonamidesSulphonamides

Discovered in 1930. Prontosil, a dye, could protectDiscovered in 1930. Prontosil, a dye, could protectmice from lethal doses of haemolytic streptococci.mice from lethal doses of haemolytic streptococci.

Found to be proFound to be pro --drug of sulphanilamide.drug of sulphanilamide. Landmark discovery as chemical modification of Landmark discovery as chemical modification of

sulphonamide structure has produced thiazides,sulphonamide structure has produced thiazides,

sulphones and oral hypoglycaemics.sulphones and oral hypoglycaemics. Folate synthesised from pABA in bacteria byFolate synthesised from pABA in bacteria by

dihydropteronate synthetase.dihydropteronate synthetase.

Bacteriostatic, problems with resistance, liverBacteriostatic, problems with resistance, livermetabolised.metabolised. Hypersensitivity, bone marrow depression, crystalluriaHypersensitivity, bone marrow depression, crystalluria

and methaemaglobinaemia.and methaemaglobinaemia.


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TrimethoprimTrimethoprim A folate antagonist. A folate antagonist. Bacteria must synthesise folate. Folate is converted toBacteria must synthesise folate. Folate is converted to

tetrahydrofolate by dihydrofolate reductase.tetrahydrofolate by dihydrofolate reductase.Trimethoprim inhibits this enzyme.Trimethoprim inhibits this enzyme.

There is differential sensitivity to this in humans andThere is differential sensitivity to this in humans andbacteria. The ICbacteria. The IC 5050 (( mol/l)mol/l) is 0.005 and 260, foris 0.005 and 260, forbacteria and humans.bacteria and humans.

Bacteriostatic, good against gramBacteriostatic, good against gram veve s, best given ins, best given incombination with sulphonamide (sulphamethoxazolecombination with sulphonamide (sulphamethoxazole --cotrimoxazole). However this does not seem to be thecotrimoxazole). However this does not seem to be thecase with UTI.case with UTI.

Can cause rashes, GI upset and folate deficiency.Can cause rashes, GI upset and folate deficiency.


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Vancomycin Vancomycin

Glycopeptide antibiotic that inhibits cell wallGlycopeptide antibiotic that inhibits cell wallsynthesis, by preventing new disaccharidesynthesis, by preventing new disaccharide --pentapeptide releasing from carrier lipid,pentapeptide releasing from carrier lipid,preventing attachment to peptidoglycan layer.preventing attachment to peptidoglycan layer.

Active against gram +ve, including methicillin Active against gram +ve, including methicillinresistantresistant Staph. aureus Staph. aureus ..

Resistance is rare.Resistance is rare.

Not absorbed from GI tract, excreted byNot absorbed from GI tract, excreted byglomerular filtration.glomerular filtration. Ototoxicity and nephrotoxicity have beenOtotoxicity and nephrotoxicity have been

reported.reported.


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NitrofurantoinNitrofurantoin Synthetic compound with broad spectrumSynthetic compound with broad spectrum

across gramacross gram ve and +ve.ve and +ve. Mechanism of action unknown.Mechanism of action unknown. Rapid excretion in high urine concentrations, soRapid excretion in high urine concentrations, so

only used for UTI.only used for UTI. Works better in acid urine.Works better in acid urine. Side effects rare and mainly GI upset orSide effects rare and mainly GI upset or

hypersensitivity skin rash. Bone marrowhypersensitivity skin rash. Bone marrowsupression, hepatotoxicity and peripheralsupression, hepatotoxicity and peripheralneuropathy have been reported.neuropathy have been reported.


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OthersOthers

PolymixinsPolymixins -- Not absorbed from GI tract,Not absorbed from GI tract,extremely toxic systemically, topical or gutextremely toxic systemically, topical or gutuse only. Interrupt bacterial cell wall.use only. Interrupt bacterial cell wall.

BacitracinBacitracin -- inhibits cell wall synthesis. Notinhibits cell wall synthesis. Notabsorbed, systemically toxic, topical useabsorbed, systemically toxic, topical useonly.only.


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Summary of Antibiotic ActionsSummary of Antibiotic Actions

Inhibition of cell wall synthesisInhibition of cell wall synthesis Penicillins, Cephalosporins, Carbapenems, VancomycinPenicillins, Cephalosporins, Carbapenems, Vancomycin

Inhibition of protein synthesisInhibition of protein synthesis Aminoglycosides, Tetracycline, Chlorampenicol, Erythromycin Aminoglycosides, Tetracycline, Chlorampenicol, Erythromycin

FluoroquinolonesFluoroquinolones Prevent coiling of bacterial DNA Prevent coiling of bacterial DNA

TrimethoprimTrimethoprim Inhibits bacterial folate synthesisInhibits bacterial folate synthesis

Nitrofurantoin Mechanism of action unknown Metronidazole

?Cleavage of bacterial DNA


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Minimum Inhibitory ConcentrationMinimum Inhibitory Concentration

The minimum inhibitory concentrationThe minimum inhibitory concentration(MIC) of an antibacterial is defined as the(MIC) of an antibacterial is defined as themaximum dilution of the product that willmaximum dilution of the product that will

still inhibit the growth of a teststill inhibit the growth of a testmicroorganism.microorganism.


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Bactericidal vsbacteriostatic

Minimum Inhibitory ConcentrationMinimum Inhibitory Concentration


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Bacterial ResistanceBacterial Resistance

Resistance to antibiotics develops byResistance to antibiotics develops byseveral different mechanisms.several different mechanisms. IntrinsicIntrinsic

Natural ResistanceNatural Resistance Acquired Acquired

Resistance via mutationsResistance via mutations Transferable resistanceTransferable resistance


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Natural ResistanceNatural Resistance Some species of bacteria have inherentSome species of bacteria have inherent

resistance to antibiotics.resistance to antibiotics. Increased use of certain antibioticsIncreased use of certain antibiotics naturallynaturally

selectsselects species with inherent resistance.species with inherent resistance.

Proteus Proteus is always resistant to Nitrofurantoin.is always resistant to Nitrofurantoin.

E. Faecalis E. Faecalis is resistant to cephalosporins andis resistant to cephalosporins andfluoroquinolones.fluoroquinolones.


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Resistance by MutationResistance by Mutation

DNA replicationDNA replication 1010 --99 to 10to 10 --1010 uncorrecteduncorrected

base substitutionsbase substitutions leading to geneleading to genealteration.alteration. Drug inactivating enzymes eg penicillase.Drug inactivating enzymes eg penicillase.

Alteration in drug target molecule. Alteration in drug target molecule. Decreased uptake eg porins.Decreased uptake eg porins. Increased elimination eg eflux pumps.Increased elimination eg eflux pumps.

PrePre --existing mutations, not newly aquired.existing mutations, not newly aquired.


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Transferable ResistanceTransferable Resistance

Usually via plasmids (R Usually via plasmids (R --factor).factor).

Commonest form of bacterial resistance.Commonest form of bacterial resistance. Multiple resistant strains.Multiple resistant strains.

Very common in Gram +ve Very common in Gram +ve s, esp. faecals, esp. faecalflora.flora.


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Common Resistant BacteriaCommon Resistant Bacteria

MRSA (methicillin resistant Staph. Aureus),MRSA (methicillin resistant Staph. Aureus),11--3% of Staph. Aureus infections in 19893% of Staph. Aureus infections in 1989 --1993, 42% by 2000.1993, 42% by 2000.

VRE (vancomycin resistant enterococcus). VRE (vancomycin resistant enterococcus). ESBLESBL s (Extended spectrum betas (Extended spectrum beta --lactamaselactamase

gramgram --ve bacteria). Esp. K. pneumonia andve bacteria). Esp. K. pneumonia and

E. coli.E. coli.


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Anti Anti -- Mycobacterial AgentsMycobacterial Agents


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IsoniazidIsoniazid Synthetic compound, very important in TB treatment.Synthetic compound, very important in TB treatment. It is taken up by mycobacterium, and can penetrateIt is taken up by mycobacterium, and can penetrate

mammalian cells.mammalian cells. Mechanism not fully understood, but inhibits mycolicMechanism not fully understood, but inhibits mycolic

acids, a component of the cell wall specific toacids, a component of the cell wall specific tomycobacteria.mycobacteria.

PO or IV, widely distibuted.PO or IV, widely distibuted. Inactivated partly by acetylation, and excreted in urineInactivated partly by acetylation, and excreted in urine

either unchanged or acetylated.either unchanged or acetylated. Allergic skin eruptions commonest, can cause Allergic skin eruptions commonest, can cause

hepatotoxicity, vasculitis and arthritic symptoms.hepatotoxicity, vasculitis and arthritic symptoms.


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RifampicinRifampicin

SemiSemi --synthetic that inhibits RNA polymerasesynthetic that inhibits RNA polymerasein prokaryotes only.in prokaryotes only. Also active against gram Also active against gram ve/+ve bacteria.ve/+ve bacteria. Oral, metabolised by liver.Oral, metabolised by liver. Excreted in many body fluidsExcreted in many body fluids -- orange tinge.orange tinge.

Very few side effects Very few side effects


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EthambutolEthambutol

Only effects mycobacteria.Only effects mycobacteria.

Mechanism unknown.Mechanism unknown. Has to be used in combination.Has to be used in combination.

Taken orallyTaken orally SE rare but can cause optic neuritis andSE rare but can cause optic neuritis anddisturbances in reddisturbances in red --green colour perception.green colour perception.


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PyrazinamidePyrazinamide

Only works at acid pH, and is thereforeOnly works at acid pH, and is thereforeeffective against the mycobacteria ineffective against the mycobacteria inmacrophage phagosomes, which are lowmacrophage phagosomes, which are low

pH.pH. Oral, excreted by glomerular filtration.Oral, excreted by glomerular filtration. Arthalgia, GI upset, decreased uric acid Arthalgia, GI upset, decreased uric acid

secretion.secretion.


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Anti Anti -- Fungal AntibioticsFungal Antibiotics

Amphoteracin Amphoteracin

Amphoteric polyene Ab. Amphoteric polyene Ab. Binds to cell membrane causing pore formationBinds to cell membrane causing pore formation

and thus loss of K and thus loss of K ++ ..

High affinity to membranes of fungi and someHigh affinity to membranes of fungi and someprotazoa due to ergosterol.protazoa due to ergosterol.

Poorly absorbed from GI tract therefore IV.Poorly absorbed from GI tract therefore IV.

NystatinNystatin As above, only for skin and GI tract infections. As above, only for skin and GI tract infections.


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Synthetic AntiSynthetic Anti -- FungalsFungals

Azoles Azoles

Ketoconazole, miconazole, clotrimazole,Ketoconazole, miconazole, clotrimazole,fluconazole, etc.fluconazole, etc. Made from azole rings!!Made from azole rings!!

Block synthesis of ergosterol.Block synthesis of ergosterol.


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Anti Anti -- Viral Drugs Viral Drugs

ViViruses


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Viruses Viruses

Nucleic acid core enclosed in aNucleic acid core enclosed in aprotein coat (capsid)protein coat (capsid)

Some have lipoprotein envelopeSome have lipoprotein envelopecontaining viral glycoproteins andcontaining viral glycoproteins andalso acquired host phospholipidsalso acquired host phospholipids

DNA virusesDNA viruses Pox virus, herpes virusesPox virus, herpes viruses

(chickenpox, shingles, herpes,(chickenpox, shingles, herpes,EBV), adeonviruses andEBV), adeonviruses andpapilloma virus.papilloma virus.

RNA virusesRNA viruses Orthomyxoviruses (influenza),Orthomyxoviruses (influenza),

paramyxoviruses (measles,paramyxoviruses (measles,mumps), rhabdoviruses (rabies),mumps), rhabdoviruses (rabies),retroviruses and hepadnaviruses.retroviruses and hepadnaviruses.


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A iA ti Vi l AVi l A t


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Anti Anti -- Viral Agents Viral Agents

Inhibition of attachment or penetration of host.Inhibition of attachment or penetration of host. Amantidine Amantidine -- InfluenzaInfluenza Gamma globulinGamma globulin hepatitis, polio, rabies, measles.hepatitis, polio, rabies, measles.

Inhibition of nucleic acid synthesisInhibition of nucleic acid synthesis Acyclovir Acyclovir mainly against herpes virusesmainly against herpes viruses Zidovudine (AZT) inhibits viral RT.Zidovudine (AZT) inhibits viral RT. IndinavirIndinavir proteases inhibitor, prevents HIV formingproteases inhibitor, prevents HIV forming

protein coat.protein coat.

Antibiotics - method of action

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