Real-time monitoring of luciferase-tagged Cronobacter sakazakii in reconstituted infant milk formula

The aim of this study was to examine the potential of using a lux-tagged Cronobacter sakazakii strain to monitor growth of the bacterium in various liquids. C. sakazakii was transformed with plasmid p16S lux, and integration of the plasmid at the desired site on the chromosome was confirmed by PCR. The growth of the lux-tagged strain was similar to that of the non-lux-tagged strain, and the integrated plasmid was stable when cells were cultured in the absence of antibiotic. Growth of the lux-tagged strain was monitored in real time in Luria-Bertani broth, skim milk, and infant milk formula by using both the Luminoskan luminometer and the Xenogen IVIS imager. Bioluminescence could be detected when the lux-tagged strain was cocultured with other bacteria. The effect of monocaprylin and nisin on the growth of C. sakazakii in milk was monitored by measuring bioluminescence. In conclusion, growth of a lux-tagged C. sakazakii can be monitored in real time in both clear and opaque liquids by measuring bioluminescence. lux-tagged C. sakazakii strains could be potentially used in high-throughput assays to monitor the effects of various infant milk formula compositions on growth of the bacterium.

[Enterobacter sakazakii in powdered infant formula]

Under inadequate hygienic conditions, opportunistic bacteria may multiply in powdered infant formula (PIF) and cause severe, often fatal neonatal infections. Enterobacter sakazakii has obtained Public Health relevance causing neonatal meningitis (often fatal), bacteremia and necrotizing enterocolitis. At highest risk are neonates up to two months of age. The new genus designation Cronobacter spp. nov. has been proposed to replace Enterobacter sakazakii. Enterobacter sakazakii is relatively resistant to osmotic and dry stress and may survive in PIF more than 2 years. (Inter)national organisations (EFSA, FAO, WHO, ESPGHAN, DGKJ, OGKJ, ISO) published their opinions recently. Manufacturers can minimize the risk of contamination of PIF by continuously improving technologies and by microbiological surveillance. Institutional and private consumers may reduce the risk of infection by using appropriate hygienic procedures.

Enterobacter sakazakii in food and beverages (other than infant formula and milk powder)

The ubiqitous microorganism Enterobacter sakazakii is a rare contaminant of infant formula and may cause severe systemic infection in neonates. So far, other food is not known to cause E. sakazakii-infections. The scarce information about the ecology of E. sakazakii and the uncertainty concerning the source of infection in children and adults warrant a summary of the current knowledge about the presence of this opportunistic microorganism in food other than infant formula. This review systematizes publications on the presence of E. sakazakii in food and beverages until June 2006. Food other than infant formula has been rarely investigated for the presence of E. sakazakii. Nevertheless, this microorganism could be isolated from a wide spectrum of food and food ingredients. E. sakazakii was isolated from plant food and food ingredients like cereal, fruit and vegetables, legume products, herbs and spices as well as from animal food sources like milk, meat and fish and products made from these foods. The spectrum of E. sakazakii-contaminated food covers both raw and processed food. The kind of processing of E. sakazakii-contaminated food was not restricted to dry products. Fresh, frozen, ready-to-eat, fermented and cooked food products as well as beverages and water suitable for the preparation of food, were found to be contaminated by E. sakazakii. Although E. sakazakii-contaminated food do not have general public health significance, measures for prevention should consider the presence of E. sakazakii in food, food ingredients, their processing and preparation as possible source of contamination, colonization or infection.