## 2 Materials and Methods
### Adhesion, Exclusion and Competence Assays on Caco-2 Cells  
The _E. coli_ O157:H7 bacterial strain was used as pathogenic model for competence, displacement and exclusion assays. _E. coli_ was obtained from DMSZ (ref. 19206, Heidelberg, Germany) and cultured in TSB + YE broth (Scharlab,S.L., Barcelona, Spain) at 37 \({}^{\circ}\)C in aerobic atmosphere with agitation. _L. plantarum_ CNTA 628 was cultured in MRS broth (Merck KGaA, Darmstadt, Germany) at 37 \({}^{\circ}\)C in anaerobic conditions.  
The determination of the adhesion of _E. coli_ was performed on Caco-2 cells, a human colon adenocarcinoma cell line capable of forming monolayer cultures. Caco-2 cells (ACC 169) were purchased from DSMZ (Heidelberg, Germany). The cell line was maintained in standard culture conditions (37 \({}^{\circ}\)C in 5% CO\({}_{2}\) atmosphere) with DMEM (10569, Gibco, Thermo Fisher Scientific, Carlsbad, CA, USA) supplemented with 20% fetal bovine serum (A310500064, Gibco, Thermo Fisher Scientific, Carlsbad, CA, USA) and 1% penicillin-streptomycin(P0781, Sigma-Aldrich, Saint Louis, MI, USA). Passaging was performed by standard trypsinization.  
First, overall _E. coli_ adhesion to Caco-2 cells was quantified. Fresh _E. coli_ 18 h and 19 h _L. plantarum_ CNTA628 cultures were centrifuged and washed once with distilled water containing 0.1% peptone (Merck KGaA, Darmstadt, Germany). Caco-2 cells were seeded on 24-well plates (Cat. 3524, Corning, NY, USA) at a concentration of 3 \(\times\) 10\({}^{4}\) cells/well. After they reached confluence (ca. 1 week), cells were further cultured for 1 week to become fully differentiated. At least 96 h before the tests, cultured medium without antibiotics was used. Then, bacterial dilutions were prepared in DMEM. Caco-2 cells were washed twice in DPBS (without calcium or magnesium, A314190094, Gibco), and 0.5 mL of the corresponding bacterial suspension was added to each well, resulting in a final concentration of 1 \(\times\) 10\({}^{7}\) UFC/mL. Cells were incubated for 60 min at 37 \({}^{\circ}\)C. The final ratio resulting after incubation was 1:10 cell/bacteria. Afterwards, Caco-2 cells were washed 3 times with DPBS to remove loose bacteria.  
For competition assays, the previous procedure was followed, adding _L. plantarum_ simultaneously with _E. coli_[34], and incubated for 1 h at 37 \({}^{\circ}\)C. In exclusion assays, _L. plantarum_ was added to Caco-2 cells, incubated for 1 h, then washed twice with DPBS and further incubated with _E. coli_ for 1 h. Displacement assays were performed similarly but _E. coli_ was added in the first step. Finally, the number of _E.coli_ or _L. plantarum_ attached to Caco-2 cells was quantified by harvesting with 0.05% Trypsin-EDTA (Sigma Chemical Co., St. Louis, MO, USA) for 5 min, then seeding serial dilutions on VRBD agar plates or MRS agar plates, respectively. VRBD plates were incubated 24 h in aerobiosis and MRS plates for 2 days in anaerobiosis, both at 37 \({}^{\circ}\)C.  
Approximately, 1:10 cell/bacteria concentration of _L. plantarum_ was tested. The bacteria concentrations tested were 5.8 \(\times\) 10\({}^{6}\)\(\pm\) 2.2 \(\times\) 10\({}^{6}\) CFU/well for _L. plantarum_ and 2.4 \(\times\) 10\({}^{7}\)\(\pm\) 1.3 \(\times\) 10\({}^{7}\) CFU/well for _E. coli_. The % of bacterial adhesion in competition, exclusion and displacement was calculated respective to the total adhesion of each bacterial strain individually onto Caco-2 cells.