Beta lactam These are antibiotics having a beta-lacatam ring. The two major groups are penicillins and cephalosporins. Monobactams and carbapenems are relatively newer additions. Mechanism of action All beta-lactam antibiotics interfere with the synthesis of bacterial cell wall. The P-lactam antibiotics inhibit the transpepti,iases so that cross linking (which maintains the -:lose knit structure of the cell wall) does not take rlace. These enzymes and related proteins constitute the penicillin binding proteins (PBPs)which have been located in the bacterial cell membrane. Each organism has several PBPs and PBPs obtained from different species differ in their affinity towards different P-lactam antibiotics. This fact probably explains their differing sensitivity to the various P-lactam antibiotics. When susceptible bacteria divide in the ,presence of a P-lactam antibiotic-cell wall deficient (CWD) forms are produced. Because the interior of the bacterium is hyperosmotic, the CWD forms swell and burst causing bacterial lysis. This is how P-lactam antibiotics exe bactericidal action. Cnder ceain conditions and in case of ceain organisms, bizarre shaped or filamentous forms, which are incapable of multiplying, result. Grown in hyperosmotic medium, globular' giant' forms or protoplasts are produced. Lytic effect of these antibiotics may also be due to derepression of some bacterial autolysins which normally function during cell division. Rapid cell wall synthesis occurs when the organisms are actively multiplying; P-lactam antibiotics are more lethal in this phase. The peptidoglycan cell wall is unique to bacteria. Essentials of medical pharmacology K D Tripathi Sixth edition Pg no 694,695