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. 2022 Apr 6;11(4):440.
doi: 10.3390/pathogens11040440.

Antimicrobial Synergy between Aminoglycosides and Licorice Extract in Listeria monocytogenes

Myungseo Park  1 , Liz Horn  2 , Victoria Lappi  2 , Dave Boxrud  2 , Craig Hedberg  1 , Byeonghwa Jeon  1
Affiliations

Antimicrobial Synergy between Aminoglycosides and Licorice Extract in Listeria monocytogenes

Myungseo Park et al. Pathogens. .

Abstract

Listeria monocytogenes is a foodborne pathogen that can develop serious invasive infections. Among foodborne pathogens, L. monocytogenes exhibits the highest case fatality despite antibiotic treatment, suggesting the current therapy should be improved. Although ampicillin and gentamicin are used as a combination therapy to treat listeriosis, our results showed there is no synergy between the two antibiotics. We discovered that aqueous extract of licorice generated significant antimicrobial synergy when combined with aminoglycosides, such as gentamicin, in L. monocytogenes. In the presence of 1 mg/mL licorice extract, for instance, the minimum inhibitory concentration (MIC) of gentamicin was reduced by 32-fold. Moreover, antimicrobial synergy with licorice extract made gentamicin-resistant clinical isolates of L. monocytogenes susceptible to gentamicin. Given the common use of licorice as a food sweetener in Western countries and a herb in Oriental medicine, our findings suggest that licorice extract can be potentially used as an antibiotic adjuvant to improve the efficacy of antimicrobial treatment of listeriosis.

Keywords: Listeria; aminoglycosides; antimicrobial synergy; drug potentiation; plant extract.

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Conflict of interest statement

The authors declare no conflict of interest.

Figures

Figure 1
Figure 1
Absence of antimicrobial synergy between ampicillin and gentamicin. The heat map of a checkerboard titration assay shows the growth (OD600) of L. monocytogenes ATCC 19115 in the presence of gentamicin and ampicillin combinations.
Figure 2
Figure 2
Evaluation of the antimicrobial synergy of licorice extracts in L. monocytogenes ATCC 19115 in combination with (A) gentamicin, (B) kanamycin, (C) tetracycline, and (D) ciprofloxacin. Heat maps of checkerboard titration assays show the optical density at 600 nm (OD600) of L. monocytogenes. The results are representative of at least three independent experiments.
Figure 3
Figure 3
Synergistic growth inhibition and bacterial killing activities of gentamicin and licorice extract combinations against L. monocytogenes. (A) Synergistic growth inhibition of L. monocytogenes by gentamicin and licorice extract combinations. The results show the means and standard deviations of triplicate samples. The experiment was repeated three times and generated similar results. GEN: gentamicin. (B) Synergistic bactericidal activity of gentamicin and licorice extract combinations.
Figure 4
Figure 4
Antimicrobial synergy between gentamicin and aqueous extract of licorice against gentamicin-resistant L. monocytogenes strains isolated from clinical cases: (A) PNUSAL008195 and (B) PNUSAL002708. The heat maps show the optical density at 600 nm (OD600) of L. monocytogenes in the presence of gentamicin and licorice combinations. The results are representative of at least three independent experiments.
See this image and copyright information in PMC

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