A prospective randomized two-centre antibiotic regimen switch study will be conducted in the NICU-s of Tartu University Clinics and of Tallinn Children's Hospital. Initially all patients who need empiric treatment for early onset neonatal sepsis (as defined by Schrag et al. 2002) in Tartu will be treated with penicillin/gentamicin and those in Tallinn with ampicillin/gentamicin. When half of the needed subjects have been recruited, departmental antibiotic regimen will be switched so that ampicillin is used in Tartu and penicillin in Tallinn. Based on the present patient population and hospitalization rate, about 120-150 babies, eligible for the study will be admitted to either units every a year.

In all subjects predefined pre- and intranatal risk factors of infection will be registered. During the NICU stay laboratory and clinical signs of infection, need for respiratory support and vasoactive therapy, enteral and parenteral nutrition will be recorded.

Blood, CSF and urine cultures will be taken according to the routine of the ward but certainly before every change in antibacterial treatment. For colonization studies nasopharyngeal or tracheal and anal swabs will be collected from all neonates admitted during the study period on admission and thereafter biweekly until discharge from the NICU or until the 60th day of treatment. A separate protocol will be followed for microbiological investigations.

The endpoints:

The primary endpoint is the need for a change in antibacterial treatment within 72 hours. In discussions with clinical experts in both wards the following criteria for the change in antibacterial treatment were defined:

1. proven or suspected meningitis or abdominal infection
2. isolation from a relevant site of the mother or an infant of a microorganism, resistant to initial empiric treatment regimen in babies with early onset neonatal sepsis or septic shock
3. deterioration of the clinical status on initial antibiotic regimen and suspected/proven neonatal sepsis
4. suspected/proven late onset sepsis or nosocomial infection (defined as the development of clinical/ laboratory signs of infection at postnatal age of 72 hours or more)
5. other situations, where the treating physician considers change in antibiotic regimen necessary - the reasons will be recorded in the case report form Patients, who die before 72 hours or in whom the antibacterial therapy is changed for other than the above-mentioned reasons, will be handled as treatment failures.

Secondary endpoints will be the following:

• incidence rate and etiology of early and late onset neonatal sepsis, susceptibility pattern of causative microorganisms
• incidence rate and etiology of nosocomial sepsis, susceptibility pattern of causative microorganisms
• mortality rate within 60 days
• duration of hospitalization in NICU stay
• duration of artificial ventilation
• colonization pattern and susceptibility of colonizing bacteria (including resistance to empiric antibiotic regimen).