Bactericidal Effect of Clove Oil against Multidrug-Resistant Streptococcus suis Isolated from Human Patients and Slaughtered Pigs

Abstract

<i>Streptococcus suis</i> is a zoonotic pathogen that is currently considered an emerging multidrug-resistant (MDR). Increasing antibiotic resistance can lead to the unsuccessful treatment of <i>S. suis</i> infection. Recently, many investigations of medicinal plants were conducted for the treatment of infection as a result of the increase of antibiotic-resistant bacteria. The aims of this study were to determine the chemical composition of essential oil from <i>Syzygium aromaticum</i> (L.) Merr. & L.M. Perry and the antibacterial activities of clove oil on MDR <i>S. suis</i>. Using gas chromatography coupled to a mass spectrometer, eugenol (97.76%) was found to be the major active ingredient of clove oil. <i>In vitro</i> antibacterial activities of clove oil against MDR <i>S. suis</i> were evaluated. Using the agar disc diffusion test, the clove oil showed a maximum zone of inhibition at 15% (v/v) oil concentration. In a broth microdilution method, the minimum bactericidal concentration of clove oil against all MDR <i>S. suis</i> isolates was 0.1% (v/v). A time-kill analysis was performed, and the killing kinetics of clove oil showed that MDR <i>S. suis</i> was completely reduced after 15 min of exposure to clove oil. In addition, clove oil exhibited a strong antibacterial activity at all pH values applied following incubation of MDR <i>S. suis</i> in pH-adjusted media with clove oil. Moreover, scanning electron microscopy revealed the nonviable <i>S. suis</i> isolates clearly showed atypical form and cell membrane lysis after incubation with clove oil. This study confirms the efficacy of clove oil as a natural antimicrobial against MDR <i>S. suis</i> and suggests the possibility of employing it as a promising alternative product for control of infectious diseases caused by <i>S. suis</i> in animal and human patients.