A prospective two-center antibiotic regimen switch study will be conducted to compare the clinical efficacy of two antibiotic regimens - penicillin/gentamicin versus ampicillin/gentamicin - in the empirical treatment of early onset neonatal sepsis. The influence of either regimen on bowel colonization pattern and on the development of antibiotic resistance of gut microflora will also be assessed. The primary endpoint is the need for a change in antibacterial treatment within 72 hours of therapy, based on pre-defined criteria. Secondary endpoints will be the incidence rate and etiology of early and late onset neonatal sepsis and susceptibility pattern of causative microorganisms; mortality rate within 60 days; duration of hospitalization in NICU; duration of artificial ventilation; colonization pattern and susceptibility of colonizing bacteria (including resistance to empiric antibiotic regimen).