A chromogenic medium for the isolation of Cronobacter sakazakii from

milk and milk products as described in ISO/TS 22964:2006(E).

Cronobacter sakazakii (formerly Enterobacter sakazakii) is a member of the Enterobacteriaceae family and has been associated with serious outbreak infections in neonates (premature infants) which have been fed on infant formula milk. Although rarely causing infections in immunocompetent adults, C. sakazakii has been implicated in sepsis, meningitis and necrotising enterocolitis with a high death rate in neonates. This opportunistic pathogen is common in the environment and its ability to survive desiccation presents a significant risk for post pasteurisation contamination and survival in spray dried milk products.

C. sakazakii appears to constitutively express high levels of α-glucosidase. This enzyme hydrolyses the chromogenic substrate 5-bromo-4-chloro-3-indolyl-α-D-glucopyranoside present in the medium, producing green to blue-green coloured colonies. Other Enterobacteriaceae such as E. coli do not express strong α-glucosidase activity and appear colourless or purple due to the uptake of crystal violet.

The combination of sodium desoxycholate, crystal violet and elevated incubation temperature produce a very selective and specific medium. Non-Enterobacteriaceae may appear colourless or violet coloured (due to their inability to hydrolyse the chromogenic substrate) or are inhibited by the selective components and incubation temperature.

This media formulation is currently recommended as part of the isolation protocol under ISO/TS 22964:2006(E) for the isolation of Enterobacter sakazakii from milk and milk products.