Antibacterial and synergistic effects of Smallanthus sonchifolius leaf extracts against methicillin-resistant Staphylococcus aureus under light intensity

Smallanthus sonchifolius (yacon) is a perennial plant mostly cultivated in South America, primarily for use of the tubers as a food crop and the leaves as fodder for livestock. The antibacterial activities of the methanol extract of yacon leaves (S. sonchifolius) and its n-hexane, ethyl acetate, n-butanol and water fractions were evaluated against 6 strains of methicillin-resistant Staphylococcus aureus (MRSA) and 1 standard methicillin-susceptible S. aureus (MSSA) strain by using the disc diffusion method and minimal inhibitory concentrations (MICs) assay in the presence and absence of light. No activity was detected when the two methods were performed without light; however, under illumination at 4000 lux, the n-hexane fraction of yacon (HFY) had a MIC of 15.6 μg/ml. HFYL, prepared by exposure of HFY to 4000 lux for 18 h, was more effective than HFY in terms of antimicrobial activity against the 6 MRSA strains and 1 standard MSSA strain. HFYL mixed with ampicillin or oxacillin showed a synergistic effect with all fractional inhibitory concentrations values being below 0.5. The present study demonstrates the enhancement and antimicrobial activity of yacon leaves against MRSA in the presence of light.