<prompt> All have beta lactam ring EXCEPT -PenicillinLinezolidCefotaximeImipenem  <response>: Ans. is b i.e., Linezolid beta-lactam antibiotics contain beta-lactam ring. These are penicillins, cephalosporins e.g. cefotoxime, monobactams aztreonam and carbapenems imipenem, meropenem. 