Impact of growth conditions and role of sigB on Listeria monocytogenes fitness in single and mixed biofilms cultured with Lactobacillus plantarum

• Temperature and medium affect L. monocytogenes biofilm formation with L. plantarum.
• MnSO4 promotes L. monocytogenes biofilm stress management.
• Poor survival of L. monocytogenes in mixed biofilms in glucose-rich medium.
• Salt enhances L. monocytogenes mixed biofilm formation in a sigB-dependent manner.

The role of sigB, a major transcriptional regulator of stress response genes, was assessed in formation of single and mixed species biofilms of Listeria monocytogenes EGD-e and Lactobacillus plantarum WCFS1 as secondary species at 20 °C and 30 °C using different medium compositions (nutrient-rich medium with and without supplementary manganese, glucose and salt). L. monocytogenes showed significant biofilm development at both temperatures and in all media tested although less biofilm was formed when glucose was supplemented only. The contribution of L. monocytogenes to the mixed species biofilm declined especially at higher temperature in glucose-rich medium in the absence and presence of manganese, due to lactic acid formation with concomitant decrease in culture pH below the pHmin of L. monocytogenes. Using an in-frame sigB deletion mutant and a complementation mutant we showed that sigB contributed to survival under these acid stress conditions. Notably, the additional presence of salt protected L. monocytogenes in the acidic mixed species biofilms resulting in an increase of around 2–3 log10 cfu/ml and this phenomenon showed to be sigB-dependent.