Characterisation of the Antimicrobial Effects of Liquorice Extract: Selective Inhibition of a Broad Range of Gram-Positive Bacteria

Introduction: Liquorice extract is commonly used as a food flavouring and we have, previously, shown that a waste material from the production of liquorice extract has antimicrobial activity against the Gram-positive Listeria spp. Reduction in the bioluminescence levels of strains carrying the bacterial luxABCDE genes indicated that growth inhibition was due to a reduction in levels of metabolism. Bacillus spp., Brochthrix spp., and Lactic Acid Bacteria (all Gram-positive) commonly contribute to the spoilage of cooked meats and Listeria monocytogenes contamination is a particular problem associated with sliced deli meats.

Purpose:  The purpose of this study was to further investigate the mechanism of action of the liquorice extract and towards its application as a food preservative for sliced meats.

Methods: Bacteria were treated with an inhibitory (50 µg ml^-1) or sub-inhibitory (12.5 µg ml^-1) concentration of the extract. Growth was assessed by viable count or optical density. Metabolic levels were monitored by measuring bioluminescence levels of strains carrying the bacterial luxABCDE genes. Effects on membrane integrity were monitored using a LIVE/DEAD^®viability stain and fluorescence microscopy.

Results: LIVE/DEAD^® staining of L. monocytogenes, exposed to 50 µg ml^-1, resulted in an increasing proportion of red-stained (dead) cells over time. Challenge tests indicated that 50 µg ml^-1 of extract, also, inhibited the growth of a range of Gram-positive bacteria including Bacillus subtilis, Staphylococcus aureus and Enterococcus faecalis. LIVE/DEAD^® staining of B. subtilis confirmed that this was, also, due to damage to the membrane integrity. No effect was seen on the growth of the Gram-negative bacteria Escherichia coli, Salmonella Typhimurium or Pseudomonas fluorescens.

Significance: This novel antimicrobial extract could be of value to inhibiting the growth of L. monocytogenesand other Gram-positive spoilage organisms, when applied to the surface of sliced deli meats.