Effect of licorice extract on cell viability, biofilm formation and exotoxin production by Staphylococcus aureus

Staphylococcus aureus is one of the most significant
clinical pathogen, as it causes infections to humans and
animals. Even though several antibiotics and other treatments
have been used to control S. aureus infections and intoxication,
bacterium is able to adapt, survive and produces
exotoxins. Licorice (Glycyrrhiza glabra L.) has been used
traditionally in various medicinal (antimicrobial) preparations,
and Glycyrrhizic acid (GA) is the major active constituents
present in it. In the present investigation the effect of licorice
extract on methicillin susceptible S. aureus (FRI 722) and
methicillin resistant S. aureus (ATCC 43300) growth and toxin
production was studied. The MIC of licorice extract was
found to be 0.25 and 2.5 mg GA ml−1 against S. aureus FRI
722 and S. aureus ATCC 43300, respectively. Inhibition of
biofilm formation was observed even at very low concentration
(25 μg GA ml−1). Gradual decrease in expression and
production of exotoxins such as α and β hemolysins and
enterotoxin B was observed with the increasing concentrations
of licorice extract, however, suboptimal concentration
induced the expression of some of the virulence genes. This
study indicated efficacy of licorice extract in controlling
growth and pathogenicity of both methicillin susceptible and
methicillin resistant S. aureus, however, the mechanisms of
survival and toxin production at suboptimal concentration
needs further study.