Rapid separation and immunoassay for low levels of Salmonella in foods using magnetosome-antibody complex and real-time fluorescence quantitative PCR

A rapid and economical method for detecting Salmonella was developed, based on a novel complex for immunomagnetic separation, which was composed of anti-Salmonella polyclonal antibody (Ab) and magnetosome (bacterial magnetic particle, BMP) produced by the bacterium Magnetospirillum gryphiswaldense MSR-1. BMP-Ab complex was used to capture Salmonella from pure suspensions of S. dublin, S. enteritidis, S. aesch, S. agona, S. abony and S. bareily, from mixed suspensions of S. dublin and Vibrio parahaemolyticus, and from artificially contaminated food samples. Captured Salmonella were then detected by plate count, or real-time fluorescence quantitative PCR. Capture efficiencies, calculated from plate count, were >80% for the pure Salmonella suspensions of all six strains, and >70% for the mixed suspension. Samples of six food products, with artificial contamination by 6000, 600, 60, or 0.6 cfu/mL S. dublin, were captured by complex and detected by real-time fluorescence quantitative PCR. Threshold cycle values varied depending on type of food. The lower limit of detectability was 60 cfu/mL without pre-enrichment, and <0.6 cfu/mL after 3-h pre-enrichment. The method described here, based on capture pathogens by BMP-Ab complex, is sensitive, rapid, and considerably simpler than traditional methods for Salmonella detection. It can be extended to other pathogens by the use of appropriate antibodies.