Comparison of Four Enrichment Broths for the Detection of Enterohemorrhagic Escherichia coli from Cheese

Abstract

We compared modified Escherichia coli broth containing novobiocin (mEC+n), modified Tryptic Soy Broth containing novobiocin (mTSB+n), Universal Preenrichment Broth (UPB) and mEC to analyze the enrichment of enterohemorrhagic E. coli (EHEC) serogroups O157, O26, O111 and O103 inoculated into goats' cheese and Gorgonzola cheese samples by the detection with loop-mediated isothermal amplification (LAMP), plating and immunomagnetic separation (IMS) assays. Cheese samples in mEC were incubated at 35°C according to Japanese standard method and those in the other broths were incubated at 42°C. There were significant differences in the detection of low amount of the refrigerate-injured EHEC strains in goats' cheese (1.3-2.6 CFU/25g) between enrichment in UPB and that in mEC (p=0.0016). There were significant declines in the ability to detect low amount of EHEC from Gorgonzola cheese (1.2-2.3 CFU/25g) in mEC compared with in mTSB+n and UPB (p<0.0010). With regard to high inoculation amount (12-23 CFU/25g), there was a significant difference in the detection of refrigerated-injured cells from Gorgonzola cheese samples between enrichment in UPB and that in mEC+n and mEC (p<0.0063). We consider that enrichment of injured EHEC strains in mEC particularly showed low performance because cultivation temperature was 35°C. Enrichment in UPB never showed decline in the detection ability. Non-selective enrichment broth is understandably necessary when a low number of cells or injury to cells is predicted. We expect that UPB can be a leading option among non-selective broths.