Role of osmotic stress in modulating antibiotic resistance and adhesion properties of Listeria monocytogenes in a dairy factory environment

Listeria monocytogenes is a significant foodborne pathogen capable of surviving various stress conditions encountered during food processing. The aim of this study was to evaluate the influence of osmotic pressure induced by NaCl (4% and 6%) on the phenotypic antibiotic resistance, biofilm-forming capacity, and slime production of L. monocytogenes isolates from food processing environments. A total of 10 isolates of L. monocytogenes were subjected to osmotic stress. Antimicrobial resistance was assessed using the disk diffusion method and MIC, and biofilm and slime production capacities were determined through biofilm production indices (BPI), and culture on stainless-steel 304 and polyurethane coupons and Congo red agar, respectively. Isolates were also assigned to serotypes using PCR. Genotypic variability of isolates was determined by enterobacterial repetitive intergenic consensus PCR (ERIC-PCR). All isolates tested belonged to serotype 3a. It was shown that MIC values varied depending on the antibiotics tested and NaCl concentrations. The results demonstrated that the isolates exhibited strain-specific adaptations to osmotic stress, with variable survival rates and alterations in their capacity to form biofilms. Biofilm formation remained robust under stress conditions, with certain isolates exhibiting increased capacity at 6% NaCl, particularly on polyurethane surfaces. This indicates potential adaptive responses and highlights the importance of the type of materials used in the food industry. Conversely, slime production was minimally induced by osmotic stress, with only 1 isolate exhibiting positive results. These findings highlight the resilience of L. monocytogenes to osmotic stress and its capacity to adapt to challenging conditions in food processing environments, which may complicate contamination control and sanitation efforts.