 Assignment to Therapy Groups Infants meeting the eligibility will receive piperacillin-tazobactam. Concomitant use of an aminoglycoside is suggested. Enrollment and study drug dose will be stratified by GA and PNA. After enrollment information for the eligible infant is provided, the dosage assignment and subject number will be allocated. If an infant is assigned a study drug and a number, but does not receive study drug, the subject number will not be used again. The reason for not dosing the subject will be noted on the case report form CRF. 