Cronobacter is a Gram negative, facultative anaerobic rod-shaped and motile bacterium and belongs to the
Enterobacteriaceae family. It is closely related to the Enterobacter and Citrobacter species. Cronobacter was first described as yellow-pigmented Enterobacter cloacae (yellow pigment on a tryptic soy agar at 25°C). In the 1980’s researchers used DNA-DNA hybridization to show that these strains were a unique taxonomic group and should be recognized as a separate species ‘Enterobacter sakazakii’ (to honor the Japanese bacteriologist Riichi Sakazaki).
The Cronobacter genus was defined first in 2007 and revised in 2008 based on studies of both partial 16S rDNA and hsp60 gene sequences, which showed that ‘E. sakazakii’ isolates formed at least four distinct genomogroups which could be unique species. Today the genus is composed of C. sakazakii, C. malonaticus, C.turicensis, C. muytjensii, and C. dublinensis, plus an unnamed sixth species.
Cronobacter spp. can grow over a wide temperature range. It starts near refrigeration temperature (5.5°C) and goes up to a growth temperature (44-47°C), depending on the strain.
The organism is very tolerant of drying steps and can survive for two years desiccated in infant formula and then rapidly grow on reconstitution. The organism can cause severe neonatal infections: necrotizing enterocolitis, septicaemia and meningitis. The fatality rate following meningitis and other infections is 50%, with the survivors being neurologically damaged for life. Fortunately, infections are rare in infants but they can occur in all age groups, admittedly with less severe clinical outcomes. Cronobacter spp. has been shown to invade human intestinal cells, replicate in macrophages, and invade the blood-brain barrier.
Peptone and meat extract provides sources of nitrogenous nutrients. Sodium chloride maintains the osmotic balance and the sucrose is a fermentable carbohydrate for Cronobacter. In case of sucrose fermentation the pH will drop and the pH indicator Bromocresol purple change the color