Lactococcus lactis strains with improved preservation characteristics, and improved acid and bile salt tolerance are disclosed. More specifically, a L. lactis strain having a heterologous trehalose-6-phosphate synthase gene and/or a trehalose-6-phosphate phosphatase gene, results in an accumulation of trehalose in the cytoplasm and/or in the cytoplasmic membrane. A L. lactis strain, having an internal trehalose concentration of at least 10 mg per gram cells (w/w) is disclosed.