Influence of cell surface properties on the adhesion potential of environmental Listeria isolates to dairy floors

The prevalence of Listeria in dairy manufacturing environment poses a cross-contamination risk. The current study compares 3 Listeria isolates from dairy plants; L. monocytogenes (Lm), L innocua (Li), and L. welshimeri (Lw), for their cell surface properties and adhesion on different floor types; clay brick, poured concrete, and grout. For evaluating cell attachment, different floor chips (1x1cm2 ) were immersed in sterile distilled water, containing 6 logs/mL of the individual Listeria strain, and held for 1h at 22.4°C in a shaker incubator. The attached cells were retrieved from floor chips using 3M sponge sticks and plated on brain heart infusion agar. The counts obtained were reported as cfu/cm2 . The cell surface hydrophobicity was determined by the hexadecane method. The absorbance of aqueous layers was measured, and % hydrophobicity was calculated. The zeta potential was determined by the Zeta sizer Nano series instrument (Malvern Panalytical, UK). In addition, the contact angle of HPLC water on the dairy floor types was measured using the Sessile Drop device. For each of the analyses, 3 trials were conducted, with samples drawn in triplicates, and the means were compared by ANOVA. The results from the attachment study indicated the highest adhesion (log cfu/ cm2 ) of all 3 isolates on poured concrete (Lm 3.54 ± 0.04, Li 3.27 ± 0.16 and Lw 3.70 ± 0.24), as compared with grout (Lm 3.28 ± 0.51, Li 2.67 ± 0.41 and Lw 2.71 ± 0.53), and clay brick (Lm 2.91 ± 0.16, Li 2.73 ± 0.04 and Lw 2.89 ± 0.40). The water contact angles on all 3 surfaces were less than 90° (clay brick 43.3 ± 0.78, poured concrete 47.9 ± 3.23 and grout 64.5 ± 2.85) indicating them to be hydrophilic and thus supporting the attachment. Of the 3 isolates, Lm and Lw resulted in a greater attachment than Li, which was also supported by their respective hydrophobicity values (20.13%,21.15%, and 13.88%). The zeta potential values, however, were similar (−15.8 mV, −16.2 mV and −15.5 mv, respectively) and did not appear to influence attachment. The study provides critical information for selecting the type of floors that may reduce the colonization and biofilm formation by the environmental Listeria in dairy plants