Antimicrobial activity of Chromolaena odorata crude extracts against Streptococcus suis

Streptococcus suis can cause severe diseases in both pigs and humans, develop resistance to antibiotics, and persistent infection by forming biofilms. Chromolaena odorata has been reported to have several beneficial medicinal properties due to its high content of phenolic compounds, especially in the leaves. This study evaluated the antimicrobial activity of C. odorata leaves crude extract against 28 clinically S. suis isolates. Antimicrobial efficacy was assessed by determining the minimum inhibitory concentration (MIC) and minimal bactericidal concentration (MBC). The bactericidal kinetics and antibiofilm efficacy of extract were examined using a time-kill and the crystal violet assay, respectively. C. odorata extract inhibited 100 % of all tested S. suis strains with the MIC and MBC values in the range 3.9-62.5 mg/mL. The time-kill tests showed a complete elimination of all S. suis strains after 24 h exposure to MIC of the extract, indicating the rapid bactericidal activity of C. odorata crude extract against the bacteria. Crystal violet staining showed that this extract had a significant inhibitory effect on the biofilm formation ability of S. suis serotype 2 (P1/7) at sub-MIC concentrations. Scanning electron microscopy revealed that extract destroyed bacterial cell integrity and destructed cellular structure, including significantly wrinkled surface, intracellular content leakage, and cell lysis. Based on these finding, this extract has antibacterial activity against S. suis strains. Thus, C. odorata crude extract may serve as a potential alternative choice for therapeutic or decontamination purposes in both medical and veterinary applications.