Conductivity and pH Dual Detection of Growth Prfile of Healthy and Stressed Listeria monocytogenes

In this study, growthofListeriamonocytogenes in a low conductivity growth medium (LCGM) was simultaneously monitored by conductivity and pH measurements. Detection times obtained from the conductivity and pH growth curves were inversely related to the initial concentration of L.monocytogenes in themedium. Linear responses were found by plotting detection times obtained from both conductivity and pH growth curves as a function of initial cell concentration in the range of 10 to 10 cfu/mL. The detection time was approximately 12 and 2 h for 10 and 10 cfu/mL of viable L. monocytogenes, respectively, using the conductivity growth curves, whereas it was approximately 1 h less using the pH growth curves. This dual detection system was used for evaluating the growth of acid-, temperature-, and salttreated L. monocytogenes in the medium. Acid stress at pH2and3 for 3h causedapproximately 12 and4hdelay in the detection time on pH growth curves, while stress at pH 5 for 3 h did not cause a significant delay in detection time. Delay in detection times was also observed for L.monocytogenes cells exposed to 458C formore than 1 h (2 and 6 h). Exposure to 10% NaCl for 3 h did not cause visible delay in the detection time. These observations on detection times for stressed L. monocytogenes had a consistent trend with the cell number decrease determined by surface plating method. ! 2005 Wiley Periodicals, Inc.