Antibiotics; Inhibitors of Cell Wall Synthesis LECTURE 10: Microbiology and Virology; 3 Credit hours Atta-ur-Rahman School of Applied Biosciences (ASAB) National University of Sciences and Technology (NUST)

Antibiotics Antibiotics are agents that are "selectively" toxic for bacteria (either killing them [bactericidal] or inhibiting their growth [bacteriostatic]) without harm to the patient. Antibiotics work most efficiently in conjunction with an active immune system to kill infecting bacteria in the host. The minimal inhibitory concentration (MIC) refers to the lowest concentration of an antibiotic that stops visible growth.

Components of Cell Wall

Peptidoglycan Peptidoglycan consists of polysaccharide backbone consisting of N-acetyl muramic acid and N-acetyl glucosamine with peptide side chains containing D- and L- amino acids and in some instances diaminopimelic acid. The side chains are cross-linked by peptide bridges by Penicillin binding protein (PBP).

Antibiotics; Mechanism

1- Cycloserine; Inhibition of peptidoglycan synthesis The terminal two amino acids of a peptide side chain of peptidoglycan are unusual amino acids (D-alanine as opposed to its isomer L- alanine). The antibiotic cycloserine is an analog of D-alanine and interferes with enzymatic conversion of L-alanine to D- alanine in the cytoplasm. Thus, subsequent synthesis of peptidoglycan cannot occur.

Cycloserine; Antibiotic

2- β-Lactam; Inhibition of peptidoglycan synthesis The beta lactam antibiotics include penicillins (e.g. ampicillin), cephalosporins and monobactams and carbapenems. They bind to and inhibit enzymes (penicillin binding proteins) involved in the transpeptidation (cross-linking) of peptidoglycan. These antibiotics have in common the four membered lactam ring. Attached to the lactam, penicillins have an additional five membered ring and cephalosporins a six membered ring. Monobactams consist of the lactam ring alone and display antibiotic activity. Structure of a lipopolysaccharide

β-Lactam Antibiotics Penicillin nucleus Benzyl penicillin Monobactam nucleus Ampicillin Aztreonm Carbapenem nucleus Imipenem

3- Bacitracin; Inhibition of peptidoglycan synthesis The peptidoglycan subunit is passed across the cytoplasmic membrane attached to undecaprenol diphosphate. After the nascent peptidoglycan monomer leaves the carrier on reaching the cell wall, the undecaprenol diphosphate is dephosphorylated to its monophosphate form. Bacitracin inhibits the dephosphorylation reaction and in the absence of monophosphorylated carrier peptidoglycan subunit synthesis stops.

Lipopolysaccharides (LPS), lipoglycans and endotoxin, are large molecules consisting of a lipid and a polysaccharide composed of O-antigen, outer core and inner core joined by a covalent bond; found in the outer membrane of Gram negative bacteria and elicit strong immune responses in animals. lipooligosaccharide ("LOS") a low molecular weight form of bacterial lipopolysaccharides Structure of a lipopolysaccharide

4- Polymyxin B ; Inhibition of peptidoglycan synthesis It is derived from the bacterium Bacillus polymyxa. Polymyxin B binds to the lipid A portion of lipopolysaccharide and also to phospholipids. However, it binds preferentially to lipid A. This disrupts the outer membrane of Gram negative bacteria. Since the cell membrane is not exposed in Gram positive bacteria polymyxin has little activity against them. This drug is toxic to human cells, since it can also lyze eukaryotic membranes; this explains its limited clinical use.