sigma(B) Affects Biofilm Formation under the Dual Stress Conditions Imposed by Adding Salt and Low Temperature in Listeria monocytogenes

Abstract

The food-borne pathogenic bacteria Listeria monocytogenes can form biofilms on various surfaces including food-processing equipment. Biofilms offer survival benefits to the organisms entrapped against environmental insults. Moreover, the sigma(B) transcription factor of L. monocytogenes plays an important role in its survival under various stress conditions. In this study, we evaluated whether sigma(B) contributes to biofilm formation when L. monocytogenes is grown under various temperatures and media. When the wild-type strain was grown under static biofilm culture below ambient temperature (15 degrees C) for 72 h, the difference in viable cell number (in both planktonic and biofilm cells) between the wild-type and Delta sigB mutant increased by adding NaCl to BHI broth (9% salt BM > 6% salt BHI > BM, w/v), and the specific activity of beta-galactosidase was highly induced in the wild-type strain grown in 6% salt containing BHI broth. Furthermore, we measured surface-adhered biofilm forming ability using the crystal violet staining method. The wild-type strain formed a four times larger biofilm than that of the Delta sigB mutant in 6% salt-BHI medium at 15 degrees C over a 72 h incubation and also showed the highest level of beta-galactosidase specific activity. However, both the wild-type and Delta sigB mutant L. monocytogenes were defective for forming a biofilm in 9% salt-BHI medium at 15 degrees C. Our results suggest that sigma(B) plays an enhanced role in surface-adhered biofilm formation when L. monocytogenes encounters dual stress conditions, such as 6% NaCl and low temperature.