Aims: The effect of salt and acid on the growth and surface properties of two strains of Listeria monocytogenes was investigated.
Methods and results: Medium acidification and NaCl supplementation induced a marked increase in the lag and growth times (up to fivefold higher) and a decrease in the maximal optical density. Due to a strong synergic effect of pH and NaCl, growth was only detected after 280 h incubation for Scott A and not detected after 600 h for LO28 at pH 5.0 and 10% NaCl. Furthermore, the addition of NaCl in acidic conditions gave rise to cell filamentation and cell surfaces became strongly hydrophilic.
Conclusions: Some L. monocytogenes strains subjected to high NaCl concentrations in acidic conditions are able to grow but may present altered adhesion properties due to modification of their surface properties.
Significance and impact of the study: This study highlighted that L. monocytogenes do represent a hazard in acid and salted foods, such as soft cheese.