Medicina Molecular e Genética

Cold Stress and Presence of Pseudomonas fluorescens Affect Listeria monocytogenes Biofilm Structure and Response to Chitosan

Abstract

Puga CH, Orgaz B, Muñoz S and SanJose C

Life in biofilms (BF) provides microorganisms with protection against different adverse conditions and agents. In food industry, as they can host and transfer to food both pathogenic and spoilage microorganism, they have to be constantly kept under control. Many hygienic practices and disinfectants aim at preventing and/or destroying BF, and chitosan has a promising future in this respect. Listeria monocytogenes (Lm) is a dangerous foodborne pathogen that can live in BF and survive many restrictive conditions used to preserve foods, such as refrigeration. In this work, nine Lm strains, persistently or sporadically isolated from a meat processing plant, were cultured at 20°C and 4°C to obtain mature BF either in isolation or with Pseudomonas fluorescens (Pf), both species being simultaneously inoculated at similar low population levels. Pf was more compatible with the persistent Lm strains than with the rest, enhancing or maintaining their viable counts in the corresponding dual species BF. All dual species BF formed at 4°C were much thinner than those formed at 20°C, but contained more cells per cm3 of BF biomass. Chitosan damage was observed both as reduction of Lm viable cells and by confocal laser scanning microscopy (CLSM) with Live/Dead stains. In Lm monospecies BF, 1 h chitosan exposure reduced viable counts between 3 and 6 Log when cultured at 20°C and 2-4 Log when at 4°C. Both temperature of BF formation and Lm strain affected their susceptibility to chitosan in dual species BF. CLSM showed focalized chitosan injuries in binary BF, particularly in those with persistent Lm strains.