2.5. Folate determination
After growth in MRS or LAPTg broth, strains were washed 3
times with saline solution (0.85% wt/v NaCl), resuspended in this
solution at the original culture volume, and used to inoculate at 4%
(v/v) folic acid casei medium (FACM, Difco, USA) that were then
incubated without agitation at 30 C for 18 h. After growth, this
washingeresuspension procedure was repeated, and the resulting
LAB suspension was used to inoculate at 2% (v/v) the respective
fresh vitamin-free media. This last step was repeated 7 times with
the cultures showing good growth (observed by increased
turbidity); strains that did not grow in vitamin-free media were not
used in further studies. After the last incubation, 2 samples were
taken to determine the concentration of extracellular and intra-
cellular folates. A sample (500 mL) of LAB-containing vitamin-free
media was taken, and 500 mL of folate protecting buffer (0.1 mol/L
phosphate buffer, pH 6.8, containing 1.5% (w/v) ascorbic acid) to
prevent vitamin oxidation and degradation was added, mixed and
centrifugated for 5 min at 5000 g. The supernatant was collected
(extracellular sample) and the pellet was resuspended in 500 mL of
protecting buffer (intracellular sample). Both samples were then
boiled (100 C) for 5 min, centrifuged for 6 min at 10 000 g, and
stored at 70 C until used for vitamin determinations.