Drugs that Inhibit Cell wall synthesis Beta-lactams Penicillin family Cephalosporin family Carbapenems and Monobactams Β-lactamase inhibitors Vancomycin Bacitracin These drugs are bactericidal Failure of the cell wall results in death

Development of Beta-lactam families Target different species Not all drugs can pass through Gram – OM “Penicillin binding proteins” (PBPs) vary Possession of beta-lactamases varies Beta-lactam ring sensitive to hydrolysis; improved acid stability for oral administration Thus drugs differ In organisms that they affect General pharmacokinetics, administration Type and extent of resistance against

Peptidoglycan Synthesis NAM and peptide with D-ala connected Attached to lipid carrier: bactoprenol-phosphate NAG added (UDP-NAG) to complete unit NAG-NAM-peptide transported through cell membrane to cell wall new NAM-NAG unit attached, autolysins cut old wall crosslinking completed

Beta-Lactam reaction with transpeptidase D-Ala- D-Ala dipeptide http://www.antiinfectieux.org/antiinfectieux/Assets/PLS/Beta-lactames/beta-lactames-mecanisme-action-2-600.gif http://www-organik.chemie.uni-wuerzburg.de/ak_engel/Sebastian/Bilder/diplom5.gif

Consequences of mode of action Enzyme reacts with serine in active site Irreversible binding, inactivates enzyme Also inactivates drug, used up in reaction Target is in cell wall External beta-lactamases destroy drug before target is reached

Vancomycin, a glycopeptide http://www.chemicalforums.com/index.php?page=molecules

Mechanism of vancomycin Binds to peptide with high affinity via 5 hydrogen bonds http://www.ratsteachmicro.com/Assets/Antibiotics_combined/vancomycin.gif

Bacitracin Peptide antibiotic Isolated from Bacillus from a patient named Tracy. With divalent cation, binds to bactoprenol-pyrophosphate, prevents dephosphorylation of carrier, blocks PG biosynthetic pathway. http://smccd.net/accounts/case/biol230/bacitracin/bacitracin2.gif

Resistance to beta-lactams Beta-lactamases Numerous types present among bacteria Found on Gram – as well as Gram + Coded for plasmids or by chromosomal genes Some sensitive to beta-lactamase inhibitors, some not Resistance in Gram - : failure to reach target Passage through OM is through porins Although porins are not highly selective, some drugs cannot pass or the porins become mutated

Resistance to beta-lactams-2 Failure to bind to target Wide variety of bacteria, wide assortment of PBPs Mutations occur in PBP genes About MRSA Staph aureus originally susceptible to penicillin, 1940s; by 1950s, no longer About 40% of Staph aureus now resistant to methicillin and other beta-lactamse resistant drugs Has acquired a gene for a PBP that poorly binds beta-lactams, causing resistance

Resistance to vancomycin A cluster of genes that senses the presence of vancomycin, activates an enzyme that replaces the D-ala-D-ala dipeptide with D-ala-D-lactate. Interesting evolutionary history Gene cluster probably originated with producing streptomyces Known to be present in Enterococci, probably passed by conjugation to Staph aureus. http://www.medscape.com/viewarticle/473156 proposes spread of resistance in animal feed

Pharmacokinetics Beta-lactams differ greatly in Route of administration (oral absorption) Binding to serum proteins Metabolism and extent of renal excretion Tend to be excreted unchanged (good for treatment of urinary tract infections) Benzathine penicillin, im injection Half life of 14 days Mainstay of health clinics for treatment of syphilis

About combinations Beta-lactamase inhibitors Typical example Clavulanate, sulbactam, and tazobactam Some have weak antibiotic activity alone Bind to beta-lactamases and inhibit them Beta-lactamase inhibitors paired with beta-lactam antibiotics which then do the heavy lifting Typical example Clavulanate + amoxicillin = Augmentin

Toxicity Beta-lactams show wide range Diarrhea and other GI problems are most common Problems with upsetting normal ecology Most significant danger: pseudomembranous colitis caused by Clostridium difficile Delayed type hypersensitivity much more likely than immediate type (IgE), fortunately

Toxicity-2 Vancomycin Bacitracin Hypersensitivity reactions with rash and hypotension Ototoxicity, phlebitis Bacitracin Topically administered, few problems Cannot be taken internally because of inhibition of sterol synthesis, nephrotoxicity