Evaluation of ALOA plating medium for its suitability to recover high pressure-injured Listeria monocytogenes from ground chicken meat

Abstract

To evaluate a chromogenic plating medium for the isolation of sublethally injured cells of Listeria monocytogenes from processed foods. The inactivation of L. monocytogenes at pressures up to 400 MPa and 12 degrees C in ground chicken meat was employed to examine the recovery of high-pressure injured cells. Before and after different repair incubation periods at 30 degrees C in a nonselective broth, samples were plated onto a selective and differential agar [Agar Listeria according to Ottaviani and Agosti (ALOA)] and in the same medium supplemented with 4% sodium chloride (ALOA-S), and incubated at 37 degrees C. Sublethally injured cells were able to grow when directly plated onto the ALOA medium, without a previous repair incubation period. However, only uninjured cells grew on the ALOA-S medium. Sublethally injured cells of L. monocytogenes can be quantified by subtracting counts on ALOA-S medium from counts on ALOA medium. Possible applications include direct enumeration on ALOA of stressed cells of L. monocytogenes in foods with more than 100 colony forming units per gram.