MECHANISMS OF ACTION OF ANTIBIOTICS

BACTERIOSTATIC AGENTS Sulfonamides Drugs inhibiting protein synthesis except aminoglycosides (macrolides, chloramphenicol, tetracyclines etc).

BACTERICIDAL AGENTS Beta lactams (penicillins, cephalosporins, imipenem) Trimethoprim/sulfamethoxazole Vancomycin Fluoroquinolones Aminoglycosides

MECHANISMS OF ACTION Inhibitors of cell wall synthesis Drugs altering cell membranes Inhibitors of protein synthesis Antimetabolites Inhibitors of nucleic acid synthesis.

DRUGS INHIBITING CELL WALL SYNTHESIS Penicillins Cephalosporins Imipenem Vancomycin Fosfomycin β-lactams

Plus penicillin Spheroplast Emerging Spheroplast Dividing Bacteria Division Growth site Growth

Mur NAc X Glycopeptide Polymer X Mur NAc Glycopeptide Polymer X Glycopeptide Polymer D-Alanine Transpeptidase

Penicillin Binding Proteins Transpeptidases Carboxypeptidases Endopeptidases Penicillin

AUTOLYSINS

All Beta lactam antibiotics act by the same mechanism

PENICILLINS ACTIVE VS GRAM - BACTERIA

S C C C C C NCOOH O CH 3 NC O R Penicillinase S C C C CNCOOH CH 3 N C O R OH O Penicilloic Acid (β-Lactamase)

COMBINATIONS WITH BETA LACTAMASE INHIBITORS Penicillin plus a beta lactamase inhibitor.

CEPHALOSPORINS AND IMIPENEM Same mechanism of action as penicillins but bind to different binding proteins.

FOSFOMYCIN Inhibits peptidoglycan synthesis at an earlier stage than where the beta lactams act.

Fosfomycin UDP-GNAc-pyruvate enol ether UDP-NAc-muramyl- L -Ala- D -Glu- L -Lys- D -Ala- D -Ala UTP + N-acetylglucosamine-1-P UDP-GNAc PP Phosphoenolpyruvate (Biel pp 24-26)

VANCOMYCIN

E nz y m e NAG-NAM D-ALA L-GLU LYS D-ALA Transpeptidase PENICILLINS X

NAG-NAM D-ALA L-GLU LYS D-ALA E nz y m e VAN Transglycosylase

RESISTANCE TO BETA LACTAMS Penicillinase Beta lactamases

RESISTANCE Increased production of beta- lactamase (penicillinase) enzymes.

S C C C C C NCOOH O CH 3 NC O R Penicillinase S C C C CNCOOH CH 3 N C O R OH O Penicilloic Acid

METHICILLIN RESISTANCE Altered PBP’s.

RESISTANCE TO OTHER BETA LACTAM ANTIBIOTICS Most prevalent mechanism is hydrolysis by beta lactamases. Cephalosporins have variable susceptibility to βlactamases. Some even induce formation of the enzymes.

RESISTANCE TO VANCOMYCIN

ANTIBIOTICS AFFECTING CELL MEMBRANES Polymyxins Daptomycin

POLYMYXINS Surface active amphipathic agents. Interact strongly with phospholipids and disrupt the structure of cell membranes.

DAPTOMYCIN Depolarizes the cell membrane

ANTIBIOTICS INHIBITING PROTEIN SYNTHESIS Macrolides Clindamycin Linezolid Streptogramins Chloramphenicol Tetracyclines Aminoglycosides

50S 30S Procaryotic Ribosome 70S-- M.W.2,500,000 60S 40S Eucaryotic Ribosome 80S--M.W. 4,200,000

Antibiotics binding to the 50S ribosomal subunit and inhibiting protein synthesis Erythromycin and other macrolides Chloramphenicol Linezolid Streptogramins

Antibiotics binding to the 30S ribosomal subunit and inhibiting protein synthesis Aminoglycosides Tetracyclines

CLE an S AT

Macrolides (Erythromycin, Azithromycin and Clarithromycin)

aa A 50S 30S mRNA template Transferase site P Nascent polypeptide chain MACROLIDES TRANSLOCATION

CHLORAMPHENICOL

aa A 50S 30S mRNA template Transferase site P Nascent polypeptide chain Chloramphenicol Mechanism of action of Chloramphenicol

INITIATION

STREPTOGRAMINS Quinupristin/Dalfopristin (30:70)

aa A 50S 30S mRNA template Transferase site P Nascent polypeptide chain QUINUPRISTIN (MACROLIDE) DALFOPRISTIN

MECHANISM OF ACTION Act synergistically to inhibit bacterial protein synthesis. They bind to separate sites on the 50 S ribosomal subunit and form a ternary complex with the ribosome.

MECHANISM OF ACTION Quinupristin binds at the same site as the macrolides and has a similar effect. Dalfopristin directly blocks peptide bond formation by inhibiting peptidyl transferase. Dalfopristin results in a conformational change in the 50S ribosome subunit.

INITIATION

AMINOGLYCOSIDES Bind irreversibly to the 30S subunit. Exact mechanism of cell death is unknown. Postantibiotic effect.

A 50S 30S mRNA template Transferase site P Nascent polypeptide chain Tetracycline aa

INHIBITION OF MITOCHONDRIAL PROTEIN SYNTHESIS Mitochondrial ribosome resembles bacterial ribosome. May account for some toxic effects (e.g. chloramphenicol, linezolid).

RESISTANCE Alterations in ribosomal proteins (e.g. macrolides). Decreased permeability to the antibiotic.

Tetracycline ATP TETRACYCLINE RESISTANCE

ANTIBIOTICS ACTING AS ANTIMETABOLITES Sulfonamides Trimethoprim plus sulfamethoxazole

2 HNCOOH DIHYDROPTERIDINE PYROPHOSPHATE DERIVATIVE DIHYDROPTEROIC ACID DIHYDROFOLIC ACID FOLIC ACID BIOSYNTHESIS Glutamic Acid 2 ATP 2 HN SO 2 NH 2 Dihydropteroate Synthetase

TRIMETHOPRIM- SULFAMETHOXAZOLE 2 HNCH 2 OCH 3 80 mg TRIMETHOPRIM O 2 HN SO 2 NH NCH mg SULFAMETHOXAZOLE

PABA DIHYDROPTEROIC ACID DIHYROFOLIC ACID TETRAHYDROFOLIC ACID + Pteridine SULFONAMIDE TRIMETHOPRIM Dihydrofolate Reductase Dihydrofolate Synthetase Dihydropteroate Synthetase

Advantages of sulfonamide- trimethoprim combination

Results from multiple mechanisms. Altered dihydropteroate synthetase. Cross-resistance among all sulfonamides. SULFONAMIDE- RESISTANCE

ANTIBIOTICS AFFECTING NUCLEIC ACID SYNTHESIS. Fluoroquinolones Metronidazole Rifampin

FLUOROQUINOLONES

Gyrase (Topoisomerase I)-older quinolones Topoisomerase IV-3 rd and 4 th gen quinolones.

FQ RESISTANCE Changes in gyrase and topoisomerase Increased efflux

Inactive End Products Metronidazole Metronidazole Short lived intermediates Inactive end products products DNA RNA Protein Other targets Mechanism of action of metronidazole on an anaerobic organism Ferredoxin reduced

RIFAMPIN