## 3 Results  
### Biofilm Determination  
The three-dimensional images of the biofilms formed by nine _Listeria_ strains on various contact surfaces (glass, polystyrene, graphene and resin) are shown in Figure 1. On resin, all the strains produced compact structures that covered the entire available surface area. However, a strong strain-to-strain variation in the three-dimensional biofilm structures was observed on glass, polystyrene and graphene. On these materials some strains (LM28, LM29, LM32, LM33 and LI) formed biofilms containing several small aggregates, while others (LM26, LM30, LM31 and LM34) produced rough biofilms with a patchy coverage and confluent growth areas where the bacterium formed clumps. The numerical data confirmed the visual observations. Figure 2 shows the box and whisker diagrams of the distribution of the biovolume values (total biovolume, biovolume for live cells, and biovolume for dead cells) found for each strain and surface material. Major differences were observed between strains (\(P<0.001\)) with respect to their capacity to form biofilm. Considering the data for the four materials simultaneously, the total biovolume varied from \(2.61\pm 1.93\)\(\upmu\)m\({}^{3}\)/\(\upmu\)m\({}^{2}\) for LM33 to \(8.37\pm 3.11\)\(\upmu\)m\({}^{3}\)/\(\upmu\)m\({}^{2}\) for LM34. The biovolume of live cells ranged from \(2.09\pm 2.06\)\(\upmu\)m\({}^{3}\)/\(\upmu\)m  
Figure 1: Three-dimensional projections of the biofilm structures of nine _Listeria_ spp. isolates on four materials (glass, polystyrene, graphene monolayer coating and epoxy resin coating). The biofilms were studied after 120 h of incubation at 12 \({}^{\circ}\)C. The images (126.8 \(\upmu\)m \(\times\) 126.8 \(\upmu\)m) were reconstructed from confocal z-stacks using IMARIS software, with shadow projections to the right. for LM33 to 6.32 \(\pm\) 2.40 \(\upmu\)m\({}^{3}\)/\(\upmu\)m\({}^{2}\) for LM34, while the biovolume of inactivated cells ranged from 0.52 \(\pm\) 0.41 \(\upmu\)m\({}^{3}\)/\(\upmu\)m\({}^{2}\) for LM33 to 2.51 \(\pm\) 1.98 \(\upmu\)m\({}^{3}\)/\(\upmu\)m\({}^{2}\) for LM34. Figure 2: Box and whisker diagrams showing the distribution of the biovolume values observed for eight _L. monocytogenes_ (LM) strains and one _L. Ivanovii_ (LI) strain. The boxes run from the 25 th to the 75 th percentile and are intersected by the median line. Whiskers extend below the box range, from the lowest to the highest values, respectively. Average values are indicated with (\(\times\)). Outliers are indicated as individual data points (\(\circ\)). Average values in the same graph for the same physiological state (total, live or dead cells) with no letter in common are significantly different (\(P\) < 0.05). In total, nine determinations were performed for each strain, contact material and physiological state. Similar values (\(P>0.05\)) for total biovolume were observed in the biofilms formed on glass (average 3.39 \(\pm\) 1.69 \(\upmu\)m\({}^{3}\)/\(\upmu\)m\({}^{2}\)) and graphene monolayer coating (2.93 \(\pm\) 1.14 \(\upmu\)m\({}^{3}\)/\(\upmu\)m\({}^{2}\)). These figures were lower (\(P<0.05\)) than those recorded for polystyrene (4.39 \(\pm\) 4.14 \(\upmu\)m\({}^{3}\)/\(\upmu\)m\({}^{2}\)). The greatest biovolume (\(P<0.01\)) was found on the epoxy resin coating (7.35 \(\pm\) 1.45 \(\upmu\)m\({}^{3}\)/\(\upmu\)m\({}^{2}\)). There were also differences among groups of samples (\(P<0.01\)) when only the live cells were considered, with the greatest biovolume found for biofilms formed on resin (6.97 \(\pm\) 1.33 \(\upmu\)m\({}^{3}\)/\(\upmu\)m\({}^{2}\)), followed by those formed on polystyrene (3.30 \(\pm\) 2.92 \(\upmu\)m\({}^{3}\)/\(\upmu\)m\({}^{2}\)). The biofilms produced on the glass and graphene surfaces showed similar values (\(P>0.05\)) for biovolume (2.33 \(\pm\) 1.25 \(\upmu\)m\({}^{3}\)/\(\upmu\)m\({}^{2}\) and 1.69 \(\pm\) 0.83 \(\upmu\)m\({}^{3}\)/\(\upmu\)m\({}^{2}\), respectively).