Evaluation of the technique of immunomagnetic separation for recovery of Escherichia coli O157:H7 in milk creams.

The utility of immunomagnetic separation (IMS) for the recovery of Escherichia coli O157:H7 in milk creams was evaluated. The efficiency of different pre-enrichments broths previous to ISM was determined and different agars were analyzed (differential and/ or selective) for plating of the inmunobeads. Portions of 25 g from milk creams were inoculated with pathogen low levels (50, 10 and 1 cell/g) and then, were enriched in buffered peptone water with vancomycin (BPW-V), buffered peptone water with vancomycin and cefixime (BPW-VC) and triptone soya broth with vancomycin, cefixime and tellurite (TSB-VCT) being incubated 35 degrees C/18 hours. Then, IMS was applied and immunobeads were plated onto sorbitol MacConkey agar (SMA), sorbitol MacConkey agar with cefixime and tellurite (SMA-CT) and chromogenic agar with tellurite and cefixime (CRGA-TC). It was observed significant differences (p < 0,05) in the recovery percentages according to the pre-enrichment broth used, being obtained the biggest recovery rates with the use of TSB-VCT for the three inocula levels. The recovery rates did not vary significantly (p < 0,05) among the utilized agars, while the number of inoculated cells impacts the recovery percentages, being bigger as it increases the inocula. In conclusion, IMS is a sensitive method for the recovery of Escherichia coli O157:H7 in milk creams, recommending for isolation of this pathogen, the enrichment in TSB-VCT, application of IMS, and plating of immunobeads onto nonselective agar (SMA) and selective agar (SMA-TC or CRGA-TC).