Food microstructure and fat content affect growth morphology, growth kinetics, and the preferred phase for cell growth of Listeria monocytogenes in fish-based model systems

Food microstructure significantly affects microbial growth dynamics, but knowledge concerning the exact influencing mechanisms at a microscopic scale is limited. The food microstructural influence on Listeria monocytogenes (green fluorescent protein strain) growth at 10°C in fish-based food model systems was investigated by Confocal Laser Scanning Microscopy. The model systems had different microstructures, i.e., liquid, xanthan (high-viscosity liquid), aqueous gel, and emulsion and gelled emulsion systems varying in fat content. Bacteria grew as single cells, small aggregates, and micro-colonies of different sizes (based on colony radii (μm), i.e., I: 1.5-5.0, II: 5.0-10.0, III: 10.0-15.0; and IV: ≥15). In the liquid, small aggregates and Size I micro-colonies were predominantly present, while Size II and III micro-colonies were predominant in the xanthan and aqueous gel. Cells in the emulsions and gelled emulsions grew in the aqueous phase and on the fat-water interface. Microbial Adhesion to Solvents Assay demonstrated limited bacterial nonpolar solvent affinities, implying that this behaviour was probably not caused by cell surface hydrophobicity. In systems containing 1 and 5% fat, the largest cell volume was mainly represented by Size I and II micro-colonies, while at 10 and 20% fat, a few Size IV micro-colonies comprised nearly the total cell volume. Microscopic results (concerning, e.g., growth morphology, micro-colony size, inter-colony distances, preferred phase for growth) were related to previously obtained macroscopic growth dynamics in the model systems for a L. monocytogenes strain cocktail, leading to more substantiated explanations for the influence of food microstructural aspects on lag phase duration and growth rate. Importance Listeria monocytogenes is one of the most hazardous foodborne pathogens due to the high fatality rate of the disease (i.e., listeriosis). In this study, the growth behaviour of L. monocytogenes was investigated at a microscopic scale in food model systems that mimic processed fish products (e.g., fish pate, fish soup), and results were related to macroscopic growth parameters. Many studies have previously focused on the food microstructural influence on microbial growth. The novelty of this work lies in (i) the microscopic investigation of products with a complex composition and/or structure using Confocal Laser Scanning Microscopy, and (ii) the direct link to the macroscopic level. Growth behaviour (i.e., concerning bacterial growth morphology and preferred phase for growth) was more complex than assumed in common macroscopic studies. Consequently, the effectiveness of industrial antimicrobial food preservation technologies (e.g., thermal processing) might be overestimated for certain products, which may have critical food safety implications.