## 2 Materials and Methods
### In Vitro Safety Testing: Hemolysis, Enzymes, Antibiotic Profile, Biofilm-Forming Ability  
The method according to [PERSON] et al. [23] was used for hemolysis checking. The strain was inoculated onto Brain heart agar supplemented with 5% defibrinated sheep blood (Difco, Sparks, MD, USA). After cultivation at 37 \({}^{\circ}\)C for 18 h, the presence/absence of cleared zones around grown colonies was evaluated. It was assessed as \(\alpha\), \(\beta\)-hemolysis, and negative (no hemolysis), \(\gamma\)-hemolysis.  
To eliminate and/or confirm safety under in vitro conditions, the API-ZYM panel enzyme system (BioMerieux, [PERSON], France) was used. This test includes these enzymes: alkaline phosphatase, esterase (C4), esterase lipase (C8), lipase (C14), leucine arylamidase, valine arylamidase, cystine arylamidase, trypsin, \(\alpha\)-chymotrypsin, acid phosphatase, Naphtol-AS-BI-phosphohydrolase, \(\alpha\)-galactosidasease, \(\beta\)-galactosidasease, \(\beta\)-glucuronidase, \(\alpha\)-glucosidase, \(\beta\)-glucosidase, N-acetyl-\(\beta\)-glucosaminidase, \(\alpha\)-mannosidase, and \(\alpha\)-fucosidase. The method was previously described by [PERSON] et al. [7]. The enzyme activity was evaluated by color intensity values assessed from 0 to 5. The values checked corresponded with relevant values in nanomoles (nmol, 5-40 nmol).  
Antibiotic tests include the standard disc diffusion method and E-strip method with the minimum inhibitory concentration (MIC), as well as EUCAST [24]. In the case of the agar diffusion test, 9 antibiotics were screened, such as novobiocin (5 \(\upmu\)g), ampicillin (10 \(\upmu\)g), erythromycincin, azithromycin (15 \(\upmu\)g), chloramphenicol, tetracycline, erythromycinycin, rifampicin (each in a concentration of 30 \(\upmu\)g), and gentamicin (120 \(\upmu\)g). BH-agar supplemented with sheep blood (Difco, Sparks, MD, USA) was inoculated with 100 \(\upmu\)L of the overnight strain broth culture. Disks were applied onto the agar plate surface. They were supplied by Oxoid Limited (Basingstoke, Hampshire, UK). After incubation at 37 \({}^{\circ}\)C for 18 h, susceptibility (inhibitory-zone diameter) and/or resistance was evaluated according to EUCAST [24].  
The following 7 antibiotics were used in the E-strip method: chloramphenicol (0.016-256 \(\upmu\)g/mL), gentamicin (0.064-1024 \(\upmu\)g/mL), erythromycincin (0.015-256 \(\upmu\)g/mL), vancomycin (0.016-256 \(\upmu\)g/mL), rifampicin (0.032-32 \(\upmu\)g/mL), tetracycline(0.016-256 \(\upmu\)g/mL), and ampicillin (0.016-256 \(\upmu\)g/mL). The same protocol was applied, but instead of disks, the appropriate strips were used. After overnight incubation at 37 \({}^{\circ}\)C, MIC (minimum inhibitory concentration) was evaluated. The strain _Lactococcus lactis_ CCM 1998 was the positive control strain.  
To test biofilm formation by the strain MK1/3, the quantitative plate assay was applied [25], as previously described by [PERSON] et al. [18]. Absorbance (A\({}_{570}\)) at 570 nm of the prepared samples was assessed using a Synergy TM4 Multi Mode Microplate reader (Biotek, USA). The strain was tested in two independent analyses, with 12 replicates. In each test, a sterile Brain heart infusion (broth) served as a negative control. As a positive control, the strain _Streptococcus equi_ subsp. _zoepidemicus_ CCM 7316 (provided by Dr. [PERSON], University of Veterinary Medicine and Pharmacy, Kosice, Slovakia) was used. Biofilm formation was classified as follows: highly-positive (A\({}_{570}\geq 1\)), low-grade positive (0.1 \(\leq\) A\({}_{570}<\) 1) and negative (A\({}_{570}<\) 1).