Simple method to observe the adaptive response of Listeria monocytogenes in food

A simple, novel method for determining stress‐adaptive response of Listeria monocytogenes in food systems is presented. The method involves plating samples on Listeria‐selective agar (LSA) acidified to pH 5·25 with incubation at 36 °C for 60 h to detect acid adaptation and plating on LSA with 70 gl−1 NaCl and incubation at 7 °C for 7 d to detect cold‐osmotic adaptation. Adapted cells produced larger colonies (>1 mm) under these conditions than unadapted cells. Scot A (97%) and Brie‐1 (100%) cells incubated in milk at pH 5 for 3 h manifested the acid‐adapted colony type compared with 6% and 21% of viable cells in the unstressed control population. After a 5‐d adaptation period at 4 °C in milk with 80 gl−1 salt, 29% of Scot A and 91% of Brie‐1 viable cells exhibited the adapted colony type compared with <1% of the unstressed control population. Stress‐adapted L. monocytogenes were isolated from soft cheese held for 42 d at 10 C.

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