• Journal of Microbiology and Biotechnology
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• v.18 no.3
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• pp.579-584
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• 2008

A commercial chromogenic agar medium (DFI) was supplemented with glucose (mDFI) to enhance the specificity of Enterobacter sakazakii (E. sakazakit) detection. Escherichia vulneris (E. vulneris), a putative false-positive strain on the DFI medium, produces ${\alpha}$-glucosidase. The enzyme ${\alpha}$-glucosidase hydrolyzes a substrate, 5-bromo-4-chloro-3-indolyl-${\alpha}$, D-glucopyranoside $(X{\alpha}Glc)$, producing green colonies. E. sakazakii strains produced green colonies on both DFI and mDFI agar, whereas E. vulneris produced green colonies on DFI agar but small white colonies on mDFI agar. E. sakazakii and E. vulneris were also readily differentiated by colony color when the mixed culture of the two strains was plated on mDFI agar and incubated for 24 h at $37^{\circ}C$. The results indicate that the selectivity of the commercial chromogenic agar medium could be improved by a simple supplementation with glucose.

• Journal of Food Hygiene and Safety
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• v.31 no.6
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• pp.439-444
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• 2016

In the present study, the performance of culture methods using two selective agars and real-time PCR were compared for selective isolation of Cronobacter in powdered infant formula and dried pumpkin. Two food samples were spiked with the pathogen and then preenriched in distilled water. A small portion of preenrichment (10 mL) was incubated in Enterobacteriaceae enrichment both, followed by inoculation onto Druggan-Forsythe-Iversen agar (DFI agar) and Cronobacter sakazakii chromogenic plating agar (R&F agar). The preenrichment and enrichment (1 mL each) was used in real-time PCR assay. In powdered infant formula (PIF), no statistical difference was observed between both culture methods and real-time PCR with preenrichemt (p > 0.05). However, the number of positives obtained by R&F agar and real-time PCR was much higher than that of culture method using DFI agar in dried pumpkin (p < 0.05). In particular, R&F agar yielded a significantly greater selectivity than DFI agar in dried pumpkin (p < 0.05). Real-time PCR and R&F agar, which are currently recommended by US FDA, could be used as an alternative detection tools for the isolation of Cronobacter in PIF and ingredient of child foods such as dried pumpkin that has high number of competing natural microflora.