, 1996). The protocol of transformation is based on the preparation of electro-competent cells and subsequent electroporation and on the optimization
of several parameters such as growth conditions, washing solutions, and electroporation voltage. The Bifidobacterium strains used are described in Table 1. Plasmid pNZ8048 is a broad-host shuttle vector, which possesses the nisin-inducible nisA promoter and a chloramphenicol resistance gene as the selection marker (de Ruyter et al., 1996). Escherichia coli strain DH10B, used as host strain for propagating the shuttle vector, was cultivated in LB medium (Savino et al., 2011) supplemented with chloramphenicol (Sigma) at a final concentration of 10 μg mL−1. The susceptibility to chloramphenicol of the bifidobacterial strains PRL2010 and PRL2011 was tested by means of a Minimal Inhibitor Concentration (MIC) assay, according to a previously find more described procedure (Serafini et al., 2011). Bifidobacteria were cultivated in de Man–Rogosa–Sharpe (MRS) medium supplemented with 0.05% cysteine-HCl (cMRS) in an anaerobic chamber (Concept 400, Ruskin; 2.99% H2, 17.01% CO2 and 80% N2) at 37 °C for EPZ 6438 24–72 h. In case of cultivation of bifidobacterial transformants, chloramphenicol was added to the growth medium cMRS agar at a final concentration of 3 μg mL−1. Plasmid DNA was isolated from E. coli as well as from bifidobacterial transformants using
a Qiagen Plasmid Mini Kit. For Bifidobacteria, an additional incubation step in 20 mg mL−1 lysozyme at 37 °C for 40 min was performed before beginning the Qiagen kit protocol (Guglielmetti et al., 2008). An overnight culture of Bifidobacterium (10%) was used to inoculate fresh MRS broth supplemented with 0.05% (final concentration) cysteine-HCl and 16% (v/w) fructo-oligosaccharides (FOS) (Actilight®; Beneo-Orafti), a commercial product comprising a mix of short-chain FOS (1-kestose, nystose,
and fructosylnystose; FOS) or 10% galacto-oligosaccharides (GOS) (Sigma), and cultivated overnight at 37 °C under anaerobic Metformin order conditions. This overnight culture was diluted 1 : 10 in fresh MRS broth supplemented with 16% FOS or 10% GOS and cultivated at 37 °C until an OD600 nm of 0.6–0.7 was reached. Then, bacteria were chilled on ice, harvested by centrifugation (4500 r.p.m. for 15 min), and washed twice with washing buffer composed of 1 mM citrate buffer supplemented with 16% FOS or 10% GOS (pH 6.0). Finally, cells were resuspended in about 1/250 of the original culture volume of ice-cold washing buffer, dispensed in Eppendorf tubes and incubated at 4 °C for 30 min to 3 h. Plasmid DNA (200 ng) was mixed with 80 μL bacterial suspension in a precooled Gene Pulser disposable cuvette with an interelectrode distance of 0.2 cm (Eppendorf). A high-voltage electric pulse was delivered employing a Gene Pulser apparatus (BioRad, UK) using 25 μF capacity and a parallel resistance of 200 Ω. Following electroporation, bacteria were diluted with 920 μL cMRS broth.