Zwitterionic 7-methoxyimino cephalosporins (cefpirome, cefepime, cefclidin, DQ2556, FKO37 and SCE2787) possess a variable substitution at C3 which contains a quarernary nitrogen. These cephalosporins display low affinities for Class I /7-lactamase and rapid penetration through the outer membrane of Gram-negative bacilli, so that an increased number of periplasmic β-lactam molecules interact with PBP's per unit of time. As a consequence, the new zitterionic compounds remain active against some, but not all, ceftazidime-resistant Enterobacteriaceae producing high levels of Class Iβlactamase or Bush type 2bβlactamases. Antipseudomonas activities are generally similar to that of ceftazidime except that cefclidin is more active. The new zwitterionic compounds, especially cefpirome and FK037, express greater antistaphylococcal potency than does ceftazidime. A variety of animal models including meningitis and endocarditis have confirmed the potential of these compounds in-vivo. On the basis of structural and antibacterial characteristics, the expression ‘forth generation' is acceptable to describe the zwitterionic 7-methoxyimino cephalosporin
