1 Dr. Alvin Fox Antibiotics: cell envelope, Lecture 28

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Dr. Alvin Fox

Antibiotics: cell envelope,Lecture 28

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Key WordsKey WordsSterilization/disinfection/antisepsis Beta lactam

Antibiotic Penicillins

Selective toxicity Cephalosporins/cephamycins

Bactericidal Monobactam

Bacteriostatic Clavulinic acid

Minimal inhibitory concentration (MIC) Penicillinase/beta lactamase

Susceptibility testing Polymyxin B

Penicillin binding proteins Daptomycin

Autolysins Isoniazid

Cycloserine Ethioniamide

Bacitracin Ethambutol

Vancomycin Resistance

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STERILIZATIONSTERILIZATION

• All killedAll killed• non-selective non-selective

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SterilizationSterilization• autoclavingautoclaving

– 121121ooC (heat/pressure)C (heat/pressure)* Heat resistant materialsHeat resistant materials

• ethylene oxideethylene oxide– non heat resistantnon heat resistant– Usually equipmentUsually equipment– being replaced e.g. Hbeing replaced e.g. H22OO22/free radicals/free radicals

• ultra-violet lightultra-violet light– surfaces (e.g. operating rooms)surfaces (e.g. operating rooms)not totally effectivenot totally effective

• gamma radiationgamma radiation– mainly food industrymainly food industry

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Membrane filters

porespores

bacteriabacteria

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DisinfectionDisinfection

• Liquids that kill bacteriaLiquids that kill bacteria– e.g. phenol basede.g. phenol based– too toxic for skin surfacestoo toxic for skin surfaces

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AntisepticsAntiseptics

• Topical (e.g. skin) Topical (e.g. skin) – e.g. iodine or 70% alcohole.g. iodine or 70% alcohol– “ “reduce” bacterial loadreduce” bacterial load

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ANTIBIOTICSANTIBIOTICS

• Selectively toxic for bacteriaSelectively toxic for bacteria– bactericidal (killing) bactericidal (killing) – bacteriostatic (growth inhibition)bacteriostatic (growth inhibition)

• minimize harm to patientminimize harm to patient

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AntibioticsAntibiotics

• damage structuresdamage structures– present in bacteriapresent in bacteria

– not present in hostnot present in host

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Antibiotics work togetherAntibiotics work togetherwith the immune systemwith the immune system

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Minimal inhibitory Minimal inhibitory concentrationconcentration

• lowest level stopping growthlowest level stopping growth

• e. g. zone of inhibition around a disk e. g. zone of inhibition around a disk impregnated with antibioticimpregnated with antibiotic

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• Antibiotics that inhibit cell wall Antibiotics that inhibit cell wall biosynthesis are bactericidalbiosynthesis are bactericidal

• Without cell wall, osmotic pressure Without cell wall, osmotic pressure causes bacteria to burstcauses bacteria to burst

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Cross-linking of peptidoglycanOld New

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Peptidoglycan synthesisPeptidoglycan synthesis

CytoplasmCytoplasm Cell wallCell wall

undecaprenolundecaprenol

sugarsugar

aminoaminoacidacid

Cell MembraneCell Membrane

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CycloserineCycloserine

XX

CytoplasmCytoplasm

sugarsugar

aminoaminoacidacid

XX

XX

XX

alanine (ala) analog alanine (ala) analog - inhibits conversion L-ala to D-ala - inhibits conversion L-ala to D-ala - inhibits formation of D-ala-D-ala- inhibits formation of D-ala-D-ala

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BacitracinBacitracin

• Inhibits dephosphorylationInhibits dephosphorylation

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TRANSPORT OF TRANSPORT OF PEPTIGOGLYCAN SUBUNIT PEPTIGOGLYCAN SUBUNIT

ACROSS MEMBRANEACROSS MEMBRANECell membraneCell membrane


PPPP

Cell wallCell wall

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PP

PP

Cell wallCell wall

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PPPP

Cell wallCell wall

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PPPP

Cell wallCell wall

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PPPP

Cell wallCell wall

22MINUSMINUS BACITRACINBACITRACIN

CellCell membranemembrane


PP

Cell wallCell wall

TRANSPORT OF TRANSPORT OF PEPTIGOGLYCAN SUBUNIT PEPTIGOGLYCAN SUBUNIT ACROSS MEMBRANEACROSS MEMBRANE

23PLUSPLUS BacitracinBacitracin

Cell membraneCell membrane


PPPP

Cell wallCell wall

TRANSPORT OF TRANSPORT OF PEPTIGOGLYCAN SUBUNIT PEPTIGOGLYCAN SUBUNIT ACROSS MEMBRANEACROSS MEMBRANE

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VancomycinVancomycin

Cell wallCell wall

• binds to D-ala-D-alabinds to D-ala-D-ala• inhibits cross-linkinginhibits cross-linking

Vancomycin

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Beta lactam antibioticsBeta lactam antibiotics

• penicillins penicillins

• cephalosporins/cephamycins cephalosporins/cephamycins

• monobactamsmonobactams

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β lactamslactams

Cell wallCell wall

Penicillin binding proteinPenicillin binding protein

•inhibit penicillin inhibit penicillin binding proteinsbinding proteins

•stop cross-linkingstop cross-linking

β lactam

Active site of enzyme

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C NH CH CH C

O

O C N CH

CH3

CH3

COOH

S

Site of penicillinase action.Breakage of the β lactam ring.

STRUCTURE OF PENICILLIN

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Attached to lactam ringAttached to lactam ring

• penicillins penicillins – 5 membered ring 5 membered ring

• Cephalosporins/cephamycins Cephalosporins/cephamycins – 6 membered ring6 membered ring

• monobactams monobactams – no second ringno second ring

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Chemical modifications change Chemical modifications change biological activitybiological activity

• Early lactam antibiotics Early lactam antibiotics

– inactive against Gram negative inactive against Gram negative bacteriabacteria

• no penetration of outer membraneno penetration of outer membrane

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Resistance mechansimsResistance mechansims

• Produce Produce β lactamase (penicillinase)lactamase (penicillinase)

– destroys antibioticdestroys antibiotic

• modified penicillin binding proteins modified penicillin binding proteins

– don’t bind antibioticdon’t bind antibiotic

• modified porinsmodified porins– no internalization of antibioticno internalization of antibiotic

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• β lactam lactam • binds strongly to binds strongly to β lactamases lactamases

– inhibits activityinhibits activity• limited potency as antibioticlimited potency as antibiotic• used in conjunction with other used in conjunction with other β lactams lactams

Clavulinic acidClavulinic acid

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Polymyxin BPolymyxin B

• bindsbinds– lipid A lipid A – phospholipidsphospholipids

• disrupts outer membrane, Gram negative bacteria disrupts outer membrane, Gram negative bacteria • toxic to human cells toxic to human cells

Daptomycin

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• Causes depolarization of bacterial cell membrane

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Isoniazid and Ethionamide - chemically related- blocks mycolic acid synthesis

Ethambutol-blocks arabinogalactan synthesis

Cycloserine

Anti-tuberculosis drugs

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1 Dr. Alvin Fox Antibiotics: cell envelope, Lecture 28