The purpose of this study is to analyze the PK/PD of AvyCaz in critically ill patients in the Intensive Care Unit (12). This study will include medical and post-surgical patients who develop an infection where Avycaz can be utilized. Since these patients will have variable PK parameters, the investigators will also analyze (time-kill) these serum concentrations (ex vivo) against relevant clinical isolates (e.g. GNR with ESBL or KPC) from the ICU to determine microbiologic activity of Avycaz in critically ill patients with variable characteristics. Monte-Carlo simulations will also be conducted against clinical ICU isolates (JMI labs) to help determine appropriate dosing schedules based upon these PK parameters.