Antimicrobial activities of Eugenia caryophyllata extract and its major chemical constituent eugenol against Streptococcus pneumoniae

In this study, we investigate the antimicrobial activities of both Eugenia caryophyllata (Ec) extract and its major component eugenol (4-allyl-2-methoxyphenol) against Streptococcus pneumoniae. The minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) were determined by microdilution method. Pneumococcal biofilms were detected by crystal-violet microtiter plate assay, followed by colony-forming unit counts and visualized by scanning electron microscope (SEM). The synergistic effect of eugenol and penicillin was determined by checker-board method. Both the eugenol and the Ec extract inhibited pneumococcal growth in a concentration-dependent manner. The MIC and MBC of eugenol were 0.06% and 0.12%, respectively. Eugenol at a concentration of 0.12% completely killed S. pneumoniae within 60 min of exposure. The kill rate of planktonic cells was most rapid during the first 15 min of contact with eugenol. The addition of eugenol or Ec extract inhibited in vitro biofilm formation. In already established biofilms, the inhibitory effect of eugenol or Ec extract was more significant in terms of cell viability than in terms of disruption of the biofilm matrix. SEM analysis revealed non-viable and disruptive action of eugenol on the cell membrane of bacteria of biofilms. It was found that eugenol and penicillin produced a synergistic effect against S. pneumoniae. In conclusion, eugenol and Ec extract efficiently inhibited S. pneumoniae in planktonic growth and within biofilms.