Bacteria that cause most of the hospital-acquired infections make use of class C beta-lactamase (CBL) among other enzymes to resist a wide spectrum of modern antibiotics and pose a major public health concern. Other than the general features, details of the defensive mechanism by CBL, leading to the hydrolysis of drug molecules, remain a matter of debate, in particular the identification of the general base and role of the active site residues and substrate. In an attempt to unravel the detailed molecular mechanism, we carried out extensive hybrid quantum mechanical/molecular mechanical Car-Parrinello molecular dynamics simulation of the reaction with the aid of the metadynamics technique. On this basis, we report here the mechanism of the formation of the acyl enzyme complex from the Henry Michaelis complex formed by beta-lactam antibiotics and CBL. We considered two beta-lactam antibiotics, namely, cephalothin and aztreonam, belonging to two different subfamilies. A general mechanism for the formation of a beta-lactam antibiotic CBL acyl enzyme complex is elicited, and the individual roles of the active site residues and substrate are probed. The general base in the acylation step has been identified as Lys(67), while Tyr(150) aids the protonation of the beta-lactam nitrogen through either the substrate carboxylate group or a water molecule.