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

2 Key Words Sterilization/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

3 STERILIZATION All killed All killed non-selective non-selective

4 Sterilization autoclavingautoclaving –121 o C (heat/pressure) *Heat resistant materials ethylene oxideethylene oxide –non heat resistant – Usually equipment –being replaced e.g. H 2 O 2 /free radicals ultra-violet lightultra-violet light –surfaces (e.g. operating rooms) not totally effective gamma radiationgamma radiation – mainly food industry

5 Membrane filters pores bacteria

6 Disinfection Liquids that kill bacteriaLiquids that kill bacteria –e.g. phenol based –too toxic for skin surfaces

7 Antiseptics Topical (e.g. skin) Topical (e.g. skin) – e.g. iodine or 70% alcohol – “reduce” bacterial load

8 ANTIBIOTICS Selectively toxic for bacteria – bactericidal (killing) – bacteriostatic (growth inhibition) minimize harm to patient minimize harm to patient

9 Antibiotics damage structuresdamage structures – present in bacteria – not present in host

10 Antibiotics work together with the immune system

11 Minimal inhibitory concentration lowest level stopping growthlowest level stopping growth e. g. zone of inhibition around a disk impregnated with antibiotice. g. zone of inhibition around a disk impregnated with antibiotic

12 Antibiotics that inhibit cell wallAntibiotics that inhibit cell wall biosynthesis are bactericidal Without cell wall, osmotic pressureWithout cell wall, osmotic pressure causes bacteria to burst

13 Cross-linking of peptidoglycan Old New

14 Peptidoglycan synthesis Cytoplasm Cell wall undecaprenol sugar aminoacid Cell Membrane

15 Cycloserine X Cytoplasm sugar aminoacid X X X 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

16 Bacitracin Inhibits dephosphorylationInhibits dephosphorylation

17 TRANSPORT OF PEPTIGOGLYCAN SUBUNIT ACROSS MEMBRANE Cell membrane undecaprenol P P Cell wall

18 TRANSPORT OF PEPTIGOGLYCAN SUBUNIT ACROSS MEMBRANE Cell membrane undecaprenol PP Cell wall

19 TRANSPORT OF PEPTIGOGLYCAN SUBUNIT ACROSS MEMBRANE Cell membrane undecaprenol PP Cell wall

20 TRANSPORT OF PEPTIGOGLYCAN SUBUNIT ACROSS MEMBRANE Cell membrane undecaprenol PP Cell wall

21 TRANSPORT OF PEPTIGOGLYCAN SUBUNIT ACROSS MEMBRANE Cell membrane undecaprenol P P Cell wall

22 MINUS BACITRACIN Cell membrane undecaprenol P Cell wall TRANSPORT OF PEPTIGOGLYCAN SUBUNIT ACROSS MEMBRANE

23 PLUS Bacitracin Cell membrane undecaprenol PP Cell wall TRANSPORT OF PEPTIGOGLYCAN SUBUNIT ACROSS MEMBRANE

24 Vancomycin Cell wall binds to D-ala-D-ala binds to D-ala-D-ala inhibits cross-linking inhibits cross-linking Vancomycin

25 Beta lactam antibiotics penicillinspenicillins cephalosporins/cephamycinscephalosporins/cephamycins monobactamsmonobactams

26 lactams β lactams Cell wall Penicillin binding protein inhibit penicillin binding proteinsinhibit penicillin binding proteins stop cross-linkingstop cross-linking β lactam Active site of enzyme

27 CNHCH C O OCN CH 3 COOH S Site of penicillinase action. Breakage of the β lactam ring. STRUCTURE OF PENICILLIN

28 Attached to lactam ring penicillinspenicillins –5 membered ring Cephalosporins/cephamycinsCephalosporins/cephamycins –6 membered ring monobactamsmonobactams –no second ring

29 Chemical modifications change biological activity Early lactam antibioticsEarly lactam antibiotics – inactive against Gram negative bacteria no penetration of outer membraneno penetration of outer membrane

30 Resistance mechansims Produce lactamase (penicillinase)Produce β lactamase (penicillinase) –destroys antibiotic modified penicillin binding proteinsmodified penicillin binding proteins – don’t bind antibiotic modified porinsmodified porins –no internalization of antibiotic

31 lactam β lactam binds strongly to lactamases binds strongly to β lactamases – inhibits activity limited potency as antibiotic limited potency as antibiotic used in conjunction with other lactams used in conjunction with other β lactams Clavulinic acid

32 Polymyxin B binds binds – lipid A – phospholipids disrupts outer membrane, Gram negative bacteria disrupts outer membrane, Gram negative bacteria toxic to human cells toxic to human cells

Daptomycin 33 Causes depolarization of bacterial cell membrane

34 Isoniazid and Ethionamide - chemically related - blocks mycolic acid synthesis Ethambutol -blocks arabinogalactan synthesis Cycloserine Anti-tuberculosis drugs