The use of immuno-magnetic separation (IMS) as a tool in a sample preparation method for direct detection of L-monocytogenes in cheese

A sample preparation procedure was developed for direct detection of L.. mo nocytogenes in cheese. The sample preparation protocol consisted of a 10-fo ld dilution and homogenization, a centrifugation step to precipitate large food particles, passage of the supernatant over a sieve and through a separ atory funnel to further eliminate food particles and fat, a centrifugation step to recover the bacterial pellet and finally enzymatic digestion of the suspension to degrade the remaining small food particles. Recovery of L. m onocytogenes was confirmed by plating on Oxford medium and confirmation of suspected colonies. This protocol enabled direct detection (without prior e nrichment) of low numbers of L, monocytogenes (0.5-1.5 cfu/g cheese) from d ifferent types of cheese. The performance of Dynabeads(R) Anti-Listeria (Dy nal, Oslo, Norway) for selective recovery of L. monocytogenes and their app licability in the above mentioned procedure for direct detection of low num bers of L. monocytogenes from cheese was evaluated. IMS could not separate and recover L. monocytogenes from the food particles in the concentrated su spension. The use of IMS after a 24 h enrichment procedure las recommended by the manufacturer) allowed for the detection of low numbers of L.. monocy togenes ( < 10 cfu/g). However, experiments in broth cultures showed that a lthough the detection limit of IMS with Dynabeads(R) Anti-Listeria was 40-1 00 cfu/ml, the ratio of L. monocytogenes to non-Listeria flora was not incr eased. Thus, selective enrichment or concentration of L, monocytogenes was not obtained. (C) 2000 Elsevier Science B.V. All rights reserved.