Survival of Listeria monocytogenes in Low pH Model Broth Systems ¹

Four strains of Listeria monocytogenes were studied for pH tolerance in pH-adjusted tryptic soy broth supplemented with yeast extract. After incubation at 30°C, all strains grew at pH 4.5 and above. Survival of strain F5069(4b) was further investigated in sterile orange serum in which pH was adjusted from 3.6 to 5.0 in 0.2 unit increments. At 4°C, viable cells were reduced from 10⁶ cfu/ml to less than 25 cfu/ml in 25 d at pH 3.6, 43 d at pH 4.0, and 81 d at pH 4.6. Viable cell populations remained at levels about 10² after 90 d in orange serum at pH 4.8 and 5.0. At 30°C, the same reductions were observed at 5 d at pH 3.6 and 4.0 and 8 d at pH 5.0. Growth in orange serum was observed prior to the reduction in viable cell numbers at pH 4.8 and 5.0 for both storage temperatures.