Heat resistance and fatty acid composition of Listeria monocytogenes: Effect of pH, acidulant, and growth temperature

The objective of this study was to determine the influence of pH, acidulant, and growth temperature history on the heat resistance and fatty acid composition of Listeria monocytogenes Scott A. Cells were grown to late exponential phase (OD600 = 0.6) at 10, 19, or 37 degrees C in brain heart infusion broth acidified to pH 5.4 or 7 with either acetic or lactic acid. Thermal death times at 60 degrees C subsequently were determined by using a submerged-coil heating apparatus. The surviving cell population was enumerated by spiral-plating heated samples onto tryptic soy agar supplemented with 0.6% yeast extract and 1% sodium pyruvate. The thermal resistance of cells cultured at a particular temperature was significantly lower (P < 0.05) when lactic acid was used to acidify the medium to pH 5.4. Regardless of acid identity, D values significantly decreased (P < 0.05) with increased growth temperature when the pH of the growth medium was 5.4, whereas D values significantly increased (P < 0.05) with increased temperature at pH 7. At pH 5.4 adjusted with lactic acid, D values were 1.30, 1.22, and 1.14 min for cells grown at 10, 19, and 37 degrees C, respectively. At pH 5.4 adjusted with acetic acid, L. monocytogenes failed to grow at 10 degrees C; the D values were 1.32 and 1.22 min when the cells were grown at 19 and 37 degrees C, respectively. At pH 7, the D values were 0.95, 1.12, and 1.28 min with lactic acid and 0.83, 0.93, and 1.11 min with acetic acid at 10, 19, and 37 degrees C, respectively. The most abundant fatty acids (44 to 82%) were branched-chain saturated fatty acids (anteiso- and iso-C15:0 and iso-C17:0) regardless of pH, acidulant, or growth temperature. However, there was an increase in C15:0 isomers at the expense of iso-C17:0 when the growth temperature was lowered from 37 to 10 degrees C. While variable changes in longer-chain fatty acids were found, the percentage of longer-chain (C16 and C18) fatty acids was greatest when L. monocytogenes was grown at 37 degrees C regardless of pH or acidulant. This study demonstrates that the heat resistance of L, monocytogenes depends upon its growth conditions.