Long Term Survival of Listeria monocytogenes in Stress Conditions: High pH and Salt Concentrations
Author(s): Ehsan Shamloo, Zohreh Abdimoghadam, Kimia Nazari, Seyede Marzieh Hosseini, Hedayat Hosseini*, Masoud Alebouyeh*
Background: Listeria monocytogenes is a bacterium that is transmitted by food. This bacterium is the cause of Listeria disease, which causes severe symptoms such as meningitis, septicemia and abortion. Determination of the effect of different conditions such as pH, salt and temperature on survival of L. monocytogenes in food seems essential.
Methods: A standard strain of L. monocytogenes (ATCC: 19115) was used for this study. The bacteria was added in the concentration of 3 × 108 CFU/ml, to BHI broth medium at pH=4, 5, 6 and 7 and salt concentrations of 0, 7%, 14% and 21%. Temperature was 4°C and 20°C. Samples were collected every 24 hours from the cultured mediums in a special environment of PALCAM agar. This will be done until no colonies grow on the PALCAM agar environment.
Results: The effect of independent variable of storage temperature alone on the survival rate of L. monocytogenes in the level of 5% is not significant. Also, the increase in salt concentrations and pH together, increased on the survival of bacteria at 5% level. The minimum survival time was pH=4 and the salt concentration was 21% at 4°C for 19 days.
Conclusion: The results of this study showed that the effect of three factors (pH=4) and high salt concentration (21%) and temperature can reduce the bacterial survival rate in foods. This means that survival at low pH and salt concentration is strongly dependent on temperature.