Reading some articles, now I can give some comments on my own question.

The apparent differences in the distribution of strains representing the L. monocytogenes lineages has lead to a number of studies aimed at identifying phenotypic differences among the different lineages. Interestingly, lineage II e,g 1/2a isolates seem to carry more plasmids than lineage I e.g 4b isolates and these plasmids often confer resistance to toxic metals and possibly other compounds that may be found in the environment as well as food ecosystems. Moreover, lineage II isolates seem to be more resistant to bacteriocins than lineage I isolates, which probably confers an advantage in environments where bacteriocin-producing organisms are abundant. Therefore one of the possible reason that L.mono 4b is not present in food environment such as cold smoked salmon could be dute to natural flora of this product which is dominated by bacterion producing LAB.

It could be something as simple as ability to adhere to surfaces in food processing plants allowing a less prevalent, but strongly adherent strain to be a persistant in-plant contaminant, therefore having a greater likelihood to contaminate foods and make people ill (or some other similar phenomenon). We have isolated strains of Listeria monocytogenes from meat processing plants and examined their ability to adhere to surfaces using a microplate adherence assay. The more strongly adherent strains were also more invasive in tissue culture assay and live mouse assays. We are currently studying surface proteomics to help identify proteins that may contribute to the molecular basis of adherence. Meat processing plants have a constant flow of microbial flora on the incoming raw ingredients. They also have daily sanitation regimens. If some organisms escape the sanitation due to strong adherence, biofilm formation to escape lethality of sanitizers, they can become enriched, widely distributed and persistent in the facility, and may contaminate the food produced.