Improving the enrichment procedure for the detection of low contamination levels of Shiga Toxin-producing E. coli in sprouts

lntroduction: In 2011 , an outbreak of Shiga Toxin-producing E. coli (STEG) infections in Germany was caused by a strain belonging to serotype 0104:H4. Epidemiological data linked the outbreak to the consumption of sprouts; even so the pathogen could never be detected in sprout samples nor seeds. For STEG detection in sprouts, JSO/TS 13136 prescribes an unselective enrichment step in BPW tor 24 h at 37°G, followed by a PCR detection of stx genes.
Purpose: The purpose of this study was to assess and improve ISO enrichment procedure for the detection of low STEG levels in sprouts.
Methods: First, 10 g of soy sprouts were spiked with 10-100 CFU of stressed STEC and enriched (1:10) overnight in BPW (ISO procedure). The presence of stx1 and stx2 in the enriched samples was tested with a real time multiplex PCR
system. In a second step, six enrichment broths used for the enrichment of Enterobacteriaceae were compared for their ability to promote growth (at 37°G and 42°C) of six stressed and unstressed STEG strains of different serotypes. Based on the results, some of these media were further tested with spiked sprouts.
Results: The experiments with sprouts spiked with low levels of stressed STEC, enriched in BPW, showed that contamination levels below 103 CFU per 10 g could not be reliably detected with PCR. The main reason tor the detection failure is probably the very high levels of the Gram negative background flora (107 CFU/g) in sprouts. The evaluation of selected unstressed STEC strains showed no growth differences between the strains at 37°C and 42°C in EE broth, BGB broth and BPW. Stressed strains, however, showed a longer lag time tor both 37°C and 42°C and a greater strain variation.
Significance: An improved enrichment procedure tor the detection of STEG will significantly strengthen food safety of sprouts.