Rapid detection of low levels of Listeria in foods and next-day confirmation of L-monocytogenes

Outbreaks of foodborne listeriosis caused by Listeria monocytogenes in rece nt years, and the high mortality rate associated with listeriosis, have rai sed the need for reliable and rapid detection of the pathogen. A simple, au tomated method was developed fur the detection of Listeria organisms in foo ds. It consists of a 6-h pre-enrichment step followed by overnight incubati on in selective broth at 35 degrees C. Changes in light transmittance in th e selective broth are registered continuously by an optical sensor of the B ioSys instrument (MicroSys, Ann Arbor, MI), and recorded in the computer. E sculin hydrolysis by listeriae results in black coloration of the media tha t causes a sharp drop in light transmittance, whereas negative samples rema in colorless. Confirmation of L. monocytogenes is carried out only on escul in-positive samples and is completed within 6 h. Detection of 10-50 cells o f Listeria inoculated into 25 g of food was confirmed in shell eggs, milk a nd ground beef. Naturally contaminated raw and ready-to-eat foods were furt her screened to validate the procedure. (C) 2000 Elsevier Science B.V. All rights reserved.