Strain variability in growth and thermal inactivation characteristics of Listeria monocytogenes strains after acid adaptation
Given the importance of strain variability to predictive microbiology and risk assessment, the present study aimed to quantify the magnitude of strain variability in growth and thermal inactivation kinetics behaviors after acid adaptation. Thirty-three Listeria monocytogenes strains were exposed to acid-adapted tryptic soy broth with yeast extract and nonacid-adapted TSB-YE (pH 7.0) for 20 hours. Then, the growth parameters of these adapted and non-adapted strains that grew in non-buffered TSB-YE at 25℃ were estimated. The tested strains were inactivated at 60°C in non-buffered broth to obtain the heat resistance parameters. The results revealed that strain variability was present in the growth and thermal inactivation characteristics. The maximum specific growth rate ( μ max ) ranged within 0.21-0.44 and 0.20-0.45 h -1 after acid and non-acid adaptation, respectively. The lag times ( λ ) were 0.69-2.56 and 0.24-3.36 hours for acid-adapted and non-acid adapted cells, respectively. The apparent D -values at 60°C ( D 60 -values) of the pathogen ranged within 0.56-3.93 and 0.52-3.63 minutes for the presence and absence of acid adaptation condition, respectively. Acid adaptation increased the magnitude of strain variability in the thermal inactivation characteristics of the organism ( P <0.05), with the coefficient of variation (CV) increasing to 0.17, while acid adaptation did not significantly influence the variabilities in the growth parameters of the tested strains ( P ≥0.05). Furthermore, the subsequent growth behaviors of all strains did not exhibit significant changes ( P >0.05) after exposure to acidic broth. However, the thermal resistance of most of the tested strains (25/33) increased ( P <0.05) after growing in acid-adapted broth. The relevant data generated in the present study can be used to describe the strain variability in predictive microbiology, and deeply understand the behavior responses of different strains to acid adaptation.