Anti-biofilm activities of quercetin and tannic acid against <i>Staphylococcus aureus</i>

<div><p> <i>Staphylococcus aureus</i> is a leading cause of nosocomial infections because of its resistance to diverse antibiotics. The formation of a biofilm is one of the mechanisms of drug resistance in <i>S. aureus</i>. The anti-biofilm abilities of 498 plant extracts against <i>S. aureus</i> were examined. Seventy-two plant extracts belonging to 59 genera and 38 families were found to significantly inhibit the formation of biofilms of <i>S. aureus</i> without affecting the growth of planktonic cells. The most active extract, from <i>Alnus japonica</i>, inhibited the formation of biofilms by three <i>S. aureus</i> strains by >70% at 20 μg ml⁻¹. Transcriptional analyses showed that extract of <i>A. japonica</i> repressed the intercellular adhesion genes <i>icaA</i> and <i>icaD</i> most markedly. Quercetin and tannic acid are major anti-biofilm compounds in the extract of <i>A. japonica</i>. Additionally, the extract of <i>A. japonica</i> and its component compound quercetin, reduced hemolysis by <i>S. aureus</i>. This phenomenon was not observed in the treatment with tannic acid. This study suggests that various plant extracts, such as quercetin and tannic acid, could be used to inhibit the formation of recalcitrant biofilms of <i>S. aureus</i>.</p>