Quercetin reduces adhesion and inhibits biofilm development by Listeria monocytogenes by reducing the amount of extracellular proteins

The aim of the present study was to investigate the inhibitory mechanisms of quercetin against L. monocytogenes biofilm formation, by affecting secretion of polymeric substances (EPS) and spatio-temporal organization. The minimum concentration of quercetin inhibiting the establishment of bacterial biofilm was determined to be 0.8 mM; therefore, a lower concentration of the flavonoid (0.2 mM) was used in order to investigate its effect on cell attachment and EPS production on stainless steel and the spatio-temporal organization on glass, while avoiding bias of loss of cellular viability. In applying this concentration, reductions of 1.32 and 1.94 Log10 CFU/cm2 of initial adhesion and maximum cell density on stainless steel were observed on the test surfaces at 2 h (initial adhesion) and 24 h (maximum cell density), respectively. EPS on mature biofilms (24 h) was mainly composed by proteins (97.83%), followed by polysaccharides (2.03%) and DNA (0.14%). In addition, quercetin reduced total extracellular protein content by 41% compared with control; while, DNA and polysaccharide contents in biofilms were not affected. Also, relative to control, quercetin impaired surface colonization, early stages of biofilm formation and the development of a more complex architecture during its formation. This proved that quercetin inhibited the formation of L. monocytogenes biofilms by reducing initial attachment, the amount of extracellular proteins found in the EPS matrix and the overall spatio-temporal organization of L. monocytogenes.