Effect of overexpression or deletion of sigH on viability of L. sakei in mixed cultures with WT strain. Each pair of mutant and WT strains has been mixed after separate growth until an OD600 of 0.3, in MCD medium at 30°C in microaerobiosis. Enumeration on appropriate agar plates allowed to distinguish WT from mutant strains. sigH(nul) mutant (black triangles) was mixed with WT strain 23 K (empty triangles). sigH(hy)* overexpression mutant (black circles) was mixed with sigH(wt)* strain (empty circles), and 30 μM CuSO4 was added to the culture. Curves are the mean of two independent experiments.