## 4 Conclusions  
The study of foodborne pathogens that have biofilm formation capacity, such as _L. monocytogenes_, is important for food safety and consumer health. Failure in following sanitation procedures may result in biofilm formation on food preparation equipment and surfaces, in the presence of sub-inhibitory concentrations of disinfectants, such as BAC, thus leading to the contamination and/or recontamination of food products. Resistance to biocides caused by the application of disinfectants may be an important factor for the persistence of _L. monocytogenes_ in food production environments [13; 18].  
By using the RSZ method in polystyrene microplates, the _L. monocytogenes_ BAC-R mutant (S2\({}^{\text{BAC}}\)) was shown to produce significantly more biofilm in the presence of sub-inhibitory BAC concentrations compared to that in the culture medium without BAC. This effect was not observed in its isogenic BAC-S wild-type strain (S2-1). When the enumeration SSC method was used, the presence of sub-inhibitory BAC did not have a significant effect on the number of viable cells in the biofilms for both strains. However, the presence of sub-inhibitory BAC resulted in the higher resistance of the biofilms of S2\({}^{\text{BAC}}\). Although this strain did not show a significant difference in the number of viable cells present in the biofilm produced with and without BAC (SSC method), it showed more metabolic activity under sub-inhibitory BAC concentrations (RSZ method).  
In summary, sub-inhibitory BAC can affect certain resistant strains by stimulating biofilm formation on polystyrene, while on stainless steel, a higher resistance of the biofilm can be observed. Thus, growth conditions on different surfaces may regulate resistance of biofilms of strains with different susceptibilities. More research on biofilm's susceptibility under conditions closer to the food industry environment are welcomed, including but not limited to assays with biofilms formed in different types of surfaces.  
Contact of microorganisms with sublethal concentrations of disinfectants present in the environment can increase their ability to survive and persist in the niches or reservoirs of food-processing plants. Therefore, our study supports the association between selective pressure exerted by biocides in food-production environments and the persistence of _L. monocytogenes_.  
L.B. ([PERSON]): Executed the experiments, and interpreted the results; S.O.: Executed the experiments, and interpreted the results; V.M.: Executed the experiments, and interpreted the results; L.B. ([PERSON]): Designed the experiments, interpreted the results, and wrote the manuscript; J.V.M.-S.-Designed the experiments, interpreted the results, and wrote the manuscript. All authors have read and agreed to the published version of the manuscript.  
This research was funded by Research Project grant RTI2018-098267-R-C31 from the Spanish Ministry of Science, Innovation and Universities and by project FORBIOTA-Portuguese E-Infrastructure for Information and Research on Biodiversity (POCI-01-0145-FEDER-022127), funded by Operational Thematic Program for Competitiveness and Internationalization (POCI), under the PORTUGAL 2020 Partnership Agreement, through the European Regional Development Fund (FEDER).  
We thank [PERSON] (INIA, Madrid, Spain) and [PERSON] (Laboratory of Microbiology, ISA-ULisboa) for technical assistance.  
The authors declare no conflict of interest.