Comparison of selective direct plating media for enumeration and recovery of L. monocytogenes from cold-process (smoked) fish

Three selective media were evaluated for direct plating recovery and enumeration of Listeria monocytogenes in the presence of high levels of a variety of microorganisms occurring on cold-process (smoked) salmon products. Sliced salmon was brined to contain either no added salt, 3, or 5% water-phase NaCl, or 3 or 5% NaCl plus 140 ppm NaNO2. The slices were packaged in oxygen-permeable film or sealed under vacuum in oxygen-impermeable film, and stored at 10 degrees C or 5 degrees C until total microbial loads reached 10(6) to 10(9) CFU/g. Oxford formulation of Listeria selective agar and Lee's modification of Listeria selective agar achieved quantitative recovery of 10(2) cells per ml of L. monocytogenes strain Scott A in the presence of diluted slurries of these fish containing 10(4) to 10(8) CFU/ml of background organisms. A modification of lithium chloride-phenylethanol-moxalactam agar containing an iron-esculin indicator system sometimes failed because of interfering growth by the background microflora