Estimating the antimicrobial effect of grape seed extract on L. monocytogenes ΔsigB on xanthan gum gels

The substitution of chemical preservatives with natural antimicrobials has emerged as an important topic of interest for both researchers and the food industry. The utilisation of grape seed extract (GSE) has the potential to serve as an effective natural antimicrobial agent, while also offering the advantage of being a sustainable antimicrobial strategy, since GSE is a by-product of the fruit industry. The aim of this study was to quantitatively investigate the antimicrobial efficacy of GSE (1 % w/v) that was added in xanthan gum (XG)-based viscoelastic models of various XG concentrations (3, 5, 7 % w/v XG) against the wild-type (WT) strain and isogenic ΔsigB mutant of the foodborne pathogen L. monocytogenes. The gene regulator SigB is responsible for the general stress response of L. monocytogenes and its adaptation to environmental stresses. The GSE treatment effectively inactivated both strains (microbial inactivation ≥3 log CFU/ml) in all viscoelastic models regardless of the model firmness. However, the mutant strain ΔsigB was more sensitive to GSE treatment evidenced by the reduced viable population count and the increased percentage of sublethal injury in comparison to the WT. Lastly, at 7 % w/v XG (GSE-free) concentration, which was the highest gelling agent concentration used in this study, the mutant formed smaller colonies on the model surface as compared to the WT, suggesting the impact of SigB on the microbial growth/colony formation, especially on stiffer surfaces. The results of our study shed light on the impact of matrix surface structure on the response of L. monocytogenes and its ΔsigB mutant to the waste product GSE. Therefore, this study contributes to the development of enhanced and sustainable antimicrobial control strategies.