Comparison of invasiveness among surface-adherent variants of Listeria monocytogenes in Caco-2 cell culture assays

The persistence of Listeria monocytogenes in meat processing plants is believed to be partly linked with their adherence properties to abiotic surfaces that may allow contamination of ready-to-eat (RTE) foods. Isolates of raw meats, RTE meats and environmental samples from RTE meat processing facilities that demonstrated differences in adherence to abiotic surfaces (glass, plastic, stainless steel, and rubber) were tested for cellular adherence and invasion in the Caco-2 human cell line. Strains of L. monocytogenes were classified into strong (CW50, 99-38, CW77, SM5 and CW62) and weakly adherent (CW34, CW35, CW72, SM3, J7 and J126) based on a microplate adherence assay using the fluorescing substrate, 5,6-CFDA. These strains were tested for adherence and invasion in cell culture assays with Caco-2 cells. At long incubation time (2 h) and high multiplicity of infection (MOI, 100:1) we observed equivalent cellular adherence and invasiveness. After optimizing conditions for time of infection (15-, 30-, 60-, 90-, and 120-min) and MOI (100:1, 10:1, 1:1, and 0.1:1), we found that under the conditions of 15-min infection time at an MOI of 10:1 (Listeria:Caco-2 cells), we again observed equivalent cellular adherence by the two distinct surface-adherent groups, but we only recovered invading Listeria from the group that was strongly adherent to environmental surfaces. The data indicate that a surface factor that provides for strong environmental surface adherence may also be involved with internal cellular virulence and survival and implicates strongly adherent strains as possibly being of greater invasiveness than weakly adherent strains.