BUOYANCY‐DEPENDENT IMB·SALMONELLA COMPLEX LOSSES DURING MAGNETIC PHASE SEPARATION1

Abstract

One question associated with the immuno‐magnetic bead (IMB) protocol for pathogen isolation and subsequent concentration is why a significant number of captured cells are lost during each magnetic separation step (MS). For instance, we observe an average IMB‐Salmonella concentration decrease of 4–7% per MS even at extremely low Salmonella Enteritidis cell densities ([S.E.] > 100 mL‐1; [IMB]:[S.E.]< > 1). Such a change in [IMB·S.E.] per separation step could be significant since, in practice, more washing/rinsing steps are necessary when isolating organisms from environmental samples. These apparent losses in pathogen activity are not due to the injury or decease of IMB‐bound bacteria inasmuch as numerous MS steps, accomplished without changing the supernatant, result in no significant diminution in bound Salmonellae. In this manuscript we show that [IMB·S.E.] losses monotonically increase from ca. 5 to 15% per MS with [S.E.]:[IMB]. These observations argue that MS‐dependent pathogen losses are related to changes in the buoyancy of the IMB·S.E. complex since, at elevated pathogen levels ([S.E.]:[IMB]> > 1), we observe as many as 4–6 Salmonella cells binding per IMB which could decrease the IMB·S.E. complex's density by about 8% or 0.1 g cm‐3.