The genomic DNA of the bacteriophage BPS13 was prepared by phenol extraction (Manfioletti & Schneider, 1988). The 834-bp-long putative endolysin gene was amplified using the following Ku-0059436 nmr primers: BPS13ORF194_F (5′-GATGATTCACATATGAATATCAATACA-3′) and BPS13ORF194_R (5′-AACCCCGAAGGATCCTCTTAAT-3′). The

resultant polymerase chain reaction (PCR) product was digested with NdeI and BamHI, followed by ligation into the expression vector pET15b (Novagen, Germany) containing a His-Tag at the N-terminus. Plasmid-expressing E. coli BL21 Star™ (DE3) cells were grown until the optical density at 600 nm (OD600 nm) reached 0.5. Then, 1 mM isopropyl-β-d-thio-galactoside (IPTG) was added, followed by further incubation for 5 h at 30 °C. Cells were harvested, resuspended in lysis buffer (20 mM Tris–Cl, pH 8.0, and 300 mM NaCl), and lysed by sonication (Branson Ultrasonics).

After centrifugation at 15 000 g for 15 min, the supernatant was added to Ni-NTA Superflow resin (Qiagen, Germany) and gently mixed in a column for 1 h at 4 °C. The resin was washed with lysis buffer four times and eluted with elution buffer (20 mM Tris–Cl, pH 8.0, 300 mM NaCl, and 170 mM imidazole). The buffer was changed to storage buffer [20 mM Tris–Cl, pH 8.0, 300 mM NaCl, and 30% (v/v) glycerol] by dialysis, and the purified protein was stored NU7441 at −80 °C until use. The lytic activity of the endolysin was determined by measuring decreases in the optical density of the cell suspension after the addition of endolysin. Bacterial cells were grown to the exponential

phase, harvested, washed twice, and resuspended in 50 mM glycine (pH 9.5) to adjust the OD600 nm = 0.8–1.0, as described previously (Loessner et al., 1997). To test the lysis of Gram-negative bacteria, harvested cells were incubated with 0.1 M EDTA for 5 min prior to the washing and resuspension steps. The endolysin solution (100 μL) was added to 900 μL of cell suspension. In control samples, one hundred microliter of resuspension buffer BCKDHA was added instead of the endolysin solution. Unless indicated otherwise, 5 μg of LysBPS13 was added per 1 mL reaction. The OD600 nm was measured after incubation at room temperature for 5 min, and the lytic activity was calculated using the following equation: 100 × (OD600 nm of control without enzyme − OD600 nm of reaction mixture)/OD600 nm of control without enzyme. When determining the optimal pH for endolysin activity, the following buffers were used for cell suspension instead of the glycine buffer: 0.1% (w/v) Trifluoroacetic acid (TFA) for pH 2.0; 50 mM sodium acetate for pH 4.0 and 5.0; 50 mM MES for pH 6.0; 50 mM potassium phosphate for pH 7.0; 50 mM Tris–Cl for pH 7.5, 8.0, and 8.5; 50 mM glycine for pH 9.0 and 9.5; and 50 mM CAPS for pH 10.0 and 10.5. Different temperatures (4–55 °C) were applied to test the effect of temperature on the enzymatic activity of 0.1 μg LysBPS13. When necessary, EDTA (300 mM), NaCl (0–300 mM), or detergents (0.1%) were added.