"Powdered infant formula" (PIF) is a term used to cover both breast milk fortifi ers and breast milk substitutes. Both are given to neonates who are immunedefi cient. Although their low water activity stops bacterial growth during storage, bacteria are able to grow rapidly following reconstitution. Such organisms may be present through intrinsic contamination or through extrinsic contamination from the water, preparation equipment or carer. This chapter focuses on Cronobacter spp. and Salmonella serovars due the substantiated cases of neonatal infections through the ingestion of contaminated PIF, as emphasized by the three FAO-WHO risk assessments on the microbiological safety of PIF.The taxonomic re-evaluations of the Cronobacter genus are presented, emphasizing the improved understanding using Next Generation Sequencing, along with multilocus sequence typing to reveal clonal lineages. One particular lineage, C. sakazakii clonal complex 4, is particularly associated with neonatal meningitis cases. It is also one of the dominant sequence types that has been isolated from PIF manufacturing plants and PIF. The organism can survive spray-drying and persist for long periods in the dried state. Salmonella is another organism of concern for neonatal infections, and although better understood than Cronobacter there have been rare outbreaks due to atypical, lactose-fermenting strains.