Salmonella and Enterobacter sakazakii are the microorganisms of greatest concern in infant formula. This Opinion concentrates on infant formula and E. sakazakii, rather than Salmonella, for which much information is available elsewhere (e.g. SCVPH Opinion on Salmonellae in Foodstuffs, adopted 14-15 April 2003). Contamination of powdered infant formula with E. sakazakii and with salmonellae has been the cause of infection in infants, sometimes with serious sequelae or death. Although E. sakazakii has caused illness in all age groups of neonates (up to ca 4-6 weeks of age), pre-term or low birth weight infants and those immunocompromised are at greatest risk. There are no experimental or epidemiological studies on the specific dose/response relationships of E. sakazakii infections in humans. As is the case for other microorganisms, the dose/response relationship may vary according to the characteristics and physiological state of the organism, the state of the host and the food matrix. The widespread distribution of E. sakazakii suggests that consumption of low numbers in infant formula and follow-on formula by healthy infants and children does not lead to illness. Salmonella and E. sakazakii do not survive the pasteurization processes used during manufacture but recontamination of the powdered infant formula during handling and filling processes may occur. E. sakazakii, due to its ubiquitous character, seems to be more difficult to control in the processing environment than Salmonella. Control measures at the manufacturing process for E. sakazakii include: the microbiological quality of ingredients; reducing the level of Enterobacteriaceae in the production environment; and avoiding recontamination of the final product. The presence of E. sakazakii in the processing environment can be minimized by strict hygiene measures including control of movement of personnel; the separation of wet and dry processes; and avoiding condensation and water ingress in dry areas. Environmental microbiological testing in the processing area is necessary to monitor the effectiveness of the hygiene measures. Testing the processing environment for Enterobacteriaceae is the most effective method of monitoring the efficacy of processing and hygiene since Enterobacteriaceae are more often present than Salmonella and E. sakazakii. Salmonella and E. sakazakii can grow in the reconstituted product if stored above 5 ºC for a sufficient time and multiply very rapidly at room temperatures. Good Hygienic Practices at reconstitution, storage and feeding are essential to avoid recontamination and/or multiplication of the pathogens in the reconstituted formula. The most effective control measure to minimise risks of Salmonella and E. sakazakii in high-risk infants (pre-term, underweight, immunocompromised), would be to use commercial sterile liquid formula. It is recommended that a Performance Objective (PO) for powdered infant formula and follow-on formula, aiming at very low levels of Salmonella and E. sakazakii (e.g. absence in 1, 10 or 100 kg) is introduced and that verification of compliance with the PO is confirmed by testing for Enterobacteriaceae in the environment and in the product. In addition it is recommended that guidelines for preparation, handling, storage and use of infant formula in the home and in hospitals are developed.  For citation purposes: Opinion of the Scientific Panel on Biological Hazards on a request from the Commission related to the microbiological risks in infant formulae and follow-on formulae.