Methods: All infants admitted to the unit over a period of one year for whom the parent/guardian gives informed consent will be included. In addition to routine demographic and clinical data for included patients, full patient-level data on antibiotic use within the ward will also be recorded. Carriage of resistant Gram-negative organisms will be determined though rectal swabs, tracheal aspirates (for ventilated patients) and stool samples taken on admission and at twice weekly intervals. Antibiotic-resistant Gram-negative bacteria will be cultured and their resistant patterns determined. Selected environmental surfaces will also be sampled to detect contamination with such organisms. For the most important organisms (Acinetobacter spp. and Klebsiella spp.) whole genome sequencing will be used in conjunction with Bayesian data augmentation techniques to determine transmission pathways and the impact of antibiotic use on the transmission and persistence of such organisms within the hospital ward.