Nosocomial infection cause severe morbidity and mortality in pediatric patients. To find the cause of an infectious disease outbreak, epidemiologists need to determine early on whether a single strain of microorganism is responsible for the majority of cases. Phenotypic characteristics have been widely used in epidemiologic studies. However with most systems poor reproducibility, poor sensitivity have been reported and they do not reliably show enough strain-to-strain variation to be sufficiently discriminative. Molecular approaches like the analysis of the restriction fragment length polymorphism of the total DNA and of the rDNA regions and pulsed-field gel electrophoresis typing have now been applied with success to a large number of bacterial species associated with epidemics.