**2. Materials and Methods**

#### *2.1. Strains and Growth Conditions*

Bacterial strains used in this study and their origins are summarized in Table S1 (Supplementary Materials). The lactic acid bacteria (LAB) were routinely grown in either MRS broth (Man, Rogosa & Sharpe) (Hy-labs, Rehovot, Israel) or MRS broth solidified using 1.5% agar (Difco, New-Jersy, USA). In addition, the wild-type (WT) strain NCIB3610 of *B. subtilis* and its derivatives (Table S1, Supplementary Materials) were regularly cultured in Lysogeny broth (LB containing: 10 g of tryptone, 5 g of yeast extract, and 5 g of NaCl per liter) (Difco) or LB solidified with 1.5% agar. Prior to generating starter cultures, LAB and *B. subtilis* cells were grown on the agar-solidified plates for 48 h or overnight, respectively, both at 37 ◦C. A starter culture of each strain was prepared using a single bacterial colony; the cells of LAB were inoculated into 5 mL of MRS broth overnight without agitation, while the cells of *B. subtilis* were inoculated into LB medium overnight at 30 ◦C, 150 rpm, until the cultures reached an OD600 of approximately 1.5. For co-culture experiments, the modified MRS (MMRS) medium (pH = 7) was used due to its biofilm-promoting capability and its suitability for co-culture cultivation of *B. subtilis* and LAB [33]. Thus, *B. subtilis* cells were mixed with an equal amount of the LAB cells to a final concentration of 10<sup>8</sup> cells/mL of each strain within MMRS. The cells in mixed cultures were incubated aerobically at 37 ◦C at 50 rpm for 8 h [33]. Cells of *S. aureus* ATCC 25923 were regularly cultured in tryptic soy broth (TSB) (Difco) solidified with 1.5% agar overnight at 37 ◦C. A starter culture was prepared using a single bacterial colony inoculated into 10 mL of TSB medium overnight at 37 ◦C, 150 rpm.

#### *2.2. Visualizing Biofilm-Forming Cells Using Confocal Laser Scanning Microscopy (CLSM)*

Unlabeled cells of LAB and the CFP-tagged *B. subtilis* cells (YC189) were grown in co-culture into MMRS broth as described above. Cell suspensions of each bacterium grown as mono-species culture served as control samples. One milliliter of each culture was collected and centrifuged at 5000 rpm for 2 min. After removing supernatant, the cells were washed with 1 mL of DW (distilled water) and then the following centrifugation (at 5000 rpm for 2 min) re-suspended in 100 μL of DW. A suspension of 5 μL from each sample was placed on a microscopy glass slide and visualized in a transmitted light microscope using Nomarski differential interference contrast (DIC) and 458-nm laser for CFP excitation (Leica, Wetzler, Germany).
