Abstract
Food contamination by Listeria monocytogenes remains a major concern for some food processing chains, particularly for ready-to-eat foods, including processed foods. Bacterial adhesion on both biotic and abiotic surfaces is a source of contamination by pathogens that have become more tolerant or even persistent in food processing environments, including in the presence of adverse conditions such as cold and dehydration. The most distinct challenge that bacteria confront upon entry into food processing environments is the sudden downshift in temperature, and the resulting phenotypic effects are of interest. Crystal violet staining and the BioFilm Ring Test® were applied to assess the adhesion and biofilm formation of 22 listerial strains from different serogroups and origins under cold-stressed and cold-adapted conditions. The physicochemical properties of the bacterial surface were studied using the microbial adhesion to solvent technique. Scanning electron microscopy was performed to visualize cell morphology and biofilm structure. The results showed that adhesion to stainless-steel and polystyrene was increased by cold stress, whereas cold-adapted cells remained primarily in planktonic form. Bacterial cell surfaces exhibited electron-donating properties regardless of incubation temperature and became more hydrophilic as temperature decreased from 37 to 4°C. Moreover, the adhesion of cells grown at 4°C correlated with affinity for ethyl acetate, indicating the role of cell surface properties in adhesion.
Highlights
In recent decades, the foodborne pathogen Listeria monocytogenes has become a notable threat to food manufacturers, those making ready-to-eat (RTE) foods (Jofré et al, 2016; Vongkamjan et al, 2016)
Certain bacteria adapt to inhabit environments by assuming different forms that are favorable to survival, such as planktonic cells, sessile biofilm communities or spore formation
L. monocytogenes adapts to the harsh environments employed by food processing facilities, such as antibacterial agents or refrigeration, and reports have demonstrated that exposing bacteria to sublethal stress leads to cross-protection or cross-adaptation to various stresses and lethal factors (Lou and Yousef, 1997; Lundén et al, 2003)
Summary
The foodborne pathogen Listeria monocytogenes has become a notable threat to food manufacturers, those making ready-to-eat (RTE) foods (Jofré et al, 2016; Vongkamjan et al, 2016). Total biomass production by L. monocytogenes strains is augmented with increased incubation temperature, regardless of the adhesion surface, including hydrophilic stainless-steel coupons and hydrophobic polystyrene culture plates. One study showed that storage of L. monocytogenes strains at –20◦C for 6 and 24 months increased adhesion and biofilm formation on various surfaces, including polystyrene microtiter plates and stainless-steel (Slama et al, 2012). Stationary cells were harvested by centrifugation at 5,000 × g for 10 min at 4◦C and suspended by vortexing in fresh pre-cooled BHI broth for use as cold-adapted samples (Figure 1, box B). Some cold-adapted cells were streaked onto BHI agar with a sterile inoculating loop and incubated at 37◦C overnight This culture was exposed to sudden cold stress as described above and designated cold-stressed cells (Figure 1, box C).
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