*2.1. Bacterial Strains*

The bacterial strains used in this study were provided by the Szeged Microbiological Collection (SZMC). The Gram-positive *L. monocytogenes* SZMC 21307 and *S. aureus* SZMC 110007 and the Gram-negative *E. coli* SZMC 0582 and *P. putida* SZMC 291T were used to form mono- and dual-species biofilms.

For pre-culturing and biofilm formation, tryptic soy broth (TSB) containing (in %) peptone from casein 1.7 (Merck; Budapest, Hungary), peptone from soy meal 0.3 (Oxoid; Hampshire, UK), D(+)-glucose 0.25 (VWR; Debrecen, Hungary), NaCl 0.5 (VWR), and K2HPO4 0.25 was used. Pre-culturing was performed for 18–20 h at the optimum temperature for the bacteria to achieve high viable cell counts.

Dual-species biofilms were formed in TSB broth with *L. monocytogenes* paired with *E. coli* or *S. aureus* at 37 ◦C, or with *P. putida* at 30 ◦C. Quantification of bacteria in the supernatants before and after treatments was done by spreading on selective media (Palcam for *L. monocytogenes*, Chromocult for *E. coli*, Pseudomonas Selective Agar for *P. putida*, and Baird Parker Agar for *S. aureus*). Lab M Limited (Heywood, UK) provided the first three media and Biolab (Budapest, Hungary) provided Baird Parker Agar.
