## 4 Discussion  
Biofilm formation by _Listeria_ spp. in food processing environments is a serious threat for the food industry since these structures contribute to the persistence of microorganisms, increasing the risk of contamination of foodstuffs [30]. The ability of _Listeria_ to produce biofilms at the low temperatures used during food processing and storage (for example, 12 \({}^{\circ}\)C) increases the likelihood of cross-contamination. In line with previous findings [19; 31; 32; 33; 34; 35], all the strains of _Listeria_ tested showed a capacity to form biofilms under the conditions studied. Nevertheless, a strong strain-to-strain variation in biofilm-forming ability was observed (Figure 1). An intra-specific variability in biofilm architecture has also been reported by other authors. Among the different structures seen are one-cell-thick layers, flat layers of cells consisting of several layers without a given structure, honeycomb structures, and networks of interwoven clumps [35; 36; 37; 38; 39; 40; 41; 42]. The great variability observed in the structure of the biofilms formed by _Listeria_ spp. shows that there is a clear need to assay various strains when drawing conclusions about the ability of these bacteria to produce biofilm. The three-dimensional images obtained show marked variations in the structure of the biofilms depending on the contact material. While on resin all the strains formed robust biofilms covering the entire surface, on glass, polystyrene and graphene the biofilms produced were more irregular, consisting of just small clumps of cells in some cases. The smallest production of biofilm was found on glass and especially graphene. It is noteworthy that the greater the amount of biofilm, the greater the likelihood of cross-contamination. Other studies have also recorded major differences between contact surfaces when investigating the formation of biofilms by _L. monocytogenes_. In previous research [19] undertaken using a strain of _L. monocytogenes_ of food origin (LM6), it was observed that the biofilms formed at 37 \({}^{\circ}\)C showed greater biovolume on glass surfaces than on polystyrene. It must be pointed out that this strain had low hydrophobicity (14.8% affinity for xylene), which may explain its limited attachment to the hydrophobic surfaces. [PERSON] et al. [43] observed biofilm formation at 4 \({}^{\circ}\)C on different surfaces, with greater amounts of biofilm recorded on stainless steel and glass than on polystyrene. Using the crystal violet assay technique, [PERSON] et al. [44] also observed that _L. monocytogenes_ was able to form biofilms at 4 \({}^{\circ}\)C and 12 \({}^{\circ}\)C, producing greater amounts on glass than on the more hydrophobic stainless steel and polystyrene. The aforementioned results do not coincide with the findings of the present study, in which the greatest production of biofilm was observed on the hydrophobic materials, resin and polystyrene. Intraspecific variations in the behaviour of _L. monocytogenes_ could be responsible for the differences found between reports. Some researchers have demonstrated that the hydrophobicity of the microbial cell surface plays a critical role in the adherence of bacteria to hydrophobic surfaces like polystyrene [45; 46]. Because bacterial adhesion to the surface is the first step in biofilm formation, it is to be expected that an increase in cell surface hydrophobicity should be accompanied by a greater bacterial ability to form biofilms on abiotic surfaces [28; 47; 48]. However, controversial results for the effects of cell surface hydrophobicity on biofilm formation on cell contact surfaces have been reported because biofilm formation depends on several factors [49], such as temperature. To this effect, it has been observed that biofilm formation is greater at temperatures above 20 \({}^{\circ}\)C than at refrigeration temperatures, for example 12 \({}^{\circ}\)C, the temperature used in this study [15; 44; 50; 51].