Commonly found in the environment and able to survive dessication, opportunistic pathogen Cronobacter sakazakii is known to pose a risk in the production of dried milk products. Since the organism is associated with sporadic cases and outbreaks of meningitis, necrotizing enterocolitis and sepsis, especially in newborns, its reliable detection and monitoring crucial. In developing specific chromogenic media, supported by a selective enrichment broth, the manufacturer says it provides the three key elements which ensure all current C sakazakii testing protocols are covered: CSEB – Cronobacter sakazakii enrichment broth (ISO); CSIM – Cronobacter sakazakii isolation medium (ISO); and CSA-DFI – Cronobacter sakazakii agar – DFI formulation.
CSEB (ISO) is based on lauryl sulphate tryptose broth and is used for the secondary enrichment of C sakazakii. Extra sodium chloride included in the medium delivers enhanced selectivity against competing organisms, while vancomycin inhibits the growth of gram-positive bacteria.
The chromogenic medium CSIM (ISO) is designed for the isolation of C sakazakii from milk and milk products. Alpha-glucosidase, expressed by C sakazakii, hydrolyses the chromogenic substrate in the medium, producing green to blue-green colonies. Other Enterobacteriaceae appear colourless, or purple as a result of crystal violet uptake from the medium. The combination of sodium desoxycholate, crystal violet and an elevated incubation temperature ensure selectivity.
The CSA-DFI also uses alpha-glucosidase activity to differentiate C sakazakii strains from other members of the Enterobacteriaceae. The production of alpha-glucosidase by the pathogen results in hydrolysis of the chromogenic substrate and the formation of green colonies, with other Enterobacteriaceae appearing colourless. Hydrogen sulphide-producing organisms such as Salmonella and Proteus species appear grey, brown or black.