Effect of immunomagnetic bead size on recovery of foodborne pathogenic bacteria

Immunomagnetic separation (IMS) as a culture-free enrichment sample preparation technique has gained increasing popularity in the development of rapid detection methods for foodborne pathogens. While the use of magnetic nanoparticles in IMS is on the rise due to substantially larger surface area compared to conventional magnetic microparticles, the effects of immunomagnetic bead (IMB) size on pathogen cell recovery are not fully understood. In this study we used IMBs of different sizes (100, 500, and 1000 nm diameters) to capture Salmonella Enteritidis, a common foodborne pathogen, from buffer solutions as well as food matrices (chicken carcass rinse and liquid egg white). The IMS recovery and non-specific binding rate were compared. The recoveries of Salmonella cells in buffers was highest using the 100 nm IMBs (88-96%), followed by the 500 nm (31-89%) and 1000 nm (4.1-61%) IMBs, demonstrating a significant size effect. The non-specific binding rates of E. coli also increased as IMB size decreased. A 2-72% reduction in Salmonella recovery was observed in chicken carcass rinse and liquid egg white samples compared to in buffers, and this reduction was more significant using 500 and 1000 nm IMBs. However, lower IMS recoveries (10-56%) was found in 100 nm IMBs two months after preparation. Overall, magnetic nanoparticles yielded superior IMS efficiency to micrometer size IMBs and were less subjective to interference from food matrices. Nevertheless, their long term stability remains an obstacle towards successful use in IMS.