Reservoir and routes of transmission of Enterobacter sakazakii (Cronobacter spp.) in a milk powder-producing plant

Several outbreaks of Cronobacter spp. (Enterobacter sakazakii) have been described as food-borne illness in neonates and infants. Powdered infant formula has been identified as a source of infection, especially in hospital nurseries, where a bulk of formula nutrient is prepared for the whole day and instructions for preparation are not always followed correctly. Neonates who are underweight or immunosuppressed are especially at risk for an E. sakazakii infection. Considering that milk powder is the main ingredient of powdered infant formula, we analyzed the incidence and distribution of E. sakazakii in a milk powder-producing plant. We looked specifically at the spray-drying towers and roller dryers. Selected isolates from samples taken from the environment and final product were typed by pulsed-field gel electrophoresis to investigate the epidemiology of the organism within the production area of the plant. Seven pulsed-field gel electrophoresis types were detected in the spray-drying area, which presumably entered the plant through an aperture for process air and an improperly controlled roller shutter. Furthermore, textile filters for exhaust air of both the spray-drying towers were identified as internal reservoirs of the pathogen. For economic reasons, powder from the textile filters is reintroduced into the product flow; this can contaminate the final product. For the production of milk powder to be used as an ingredient of powdered infant formula, it was suggested to terminate the process of reintroducing the filtered powder into the product flow. A second transmission route was identified in the roller dryer section of the factory. It could be shown that contaminated milk concentrate could pass the process unheated, thus leading to a contamination of the product with E. sakazakii.