Abstract
Natural antimicrobials, such as grape seed extract (GSE) and garlic extract (GE), are often used as (a part of) novel food preservation technologies, especially due to their antilisterial effect. However, little is known on the extent of sublethal injury (SI) these extracts cause to Listeria monocytogenes, possibly leading to overestimated efficacies for such novel technologies. The influence of both extracts on the sublethal injury of L. monocytogenes strain LMG23775 was investigated, also using propidium iodide staining to investigate the nature of the injury. Minimum inhibitory concentrations were 500 mg gallic acid equivalents (GAE)/L and 7.5 μg allicin/mL for GSE and GE, respectively. These concentrations slowed down the growth of L. monocytogenes, while only causing a significant amount of SI for GSE. Pure extracts caused inactivation of the cells, with GSE being the most effective. Sublethal injury from pure GSE was mainly caused by membrane damage. In pure GE, a significant amount of SI, not caused by membrane damage, was also present, albeit less pronounced than in pure GSE. In conclusion, both extracts cause a significant amount of sublethal injury to L. monocytogenes, which is not taken into account in relevant studies investigating novel food preservation applications involving natural plant extracts.
Highlights
Food preservation strategies to which foodborne pathogens are subjected throughout the food chain may result in physiological alterations of varying degrees, i.e., sublethal injury (SI) [1,2]
For grape seed extract (GSE) and garlic extract (GE), the minimum inhibitory concentration (MIC) was expressed as mg gallic acid equivalents (GAE)/L and μg allicin/mL, respectively, since gallic acid and allicin have been identified as the main antibacterial compounds of the respective extracts [10,30]
Even though the current study only focused on SI inflicted to one specific strain of L. monocytogenes (i.e., LMG23775) and results should be validated for other strains, the large amount of SI observed after treatments with GSE-MIC, pure GSE, and pure GE confirms the risk of over-estimating the antibacterial effect of natural plant extracts in novel food preservation applications
Summary
Food preservation strategies to which foodborne pathogens are subjected throughout the food chain may result in physiological alterations of varying degrees, i.e., sublethal injury (SI) [1,2]. Since the selective media which are commonly used to detect pathogenic microorganisms are unable to support the growth of sublethally injured target microorganisms, SI causes serious limitations in food diagnostics due to the possible underestimation of contamination. Sci. 2019, 9, 2731 and false negative results [2]. Cells can become sublethally injured due to the specific food environment, processing or preservation technologies, and the injury can be either structural (i.e., altered membrane permeability), metabolic (i.e., damage to functional cell components), or a combination of both [1]
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