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Two-component systems (TCSs) have the potential to be an effective target of the antimicrobials, and thus received much attention in recent years. VicK/VicR is one of TCSs in  Streptococcus pneumoniae  ( S. pneumoniae ), which is essential for pneumococcal survival. We have previously obtained several Traditional Chinese Medicine monomers using a computer-based screening. In this study, either alone or in combination with penicillin, their antimicrobial activities were evaluated based on  in vivo  and  in vitro  assays. The results showed that the MICs of 5′-(Methylthio)-5′-deoxyadenosine, octanal 2, 4-dinitrophenylhydrazone, deoxyshikonin, kavahin, and dodecyl gallate against  S. pneumoniae  were 37.1, 38.5, 17, 68.5, and 21 μg/mL, respectively. Time-killing assays showed that these compounds elicited bactericidal effects against  S. pneumoniae  D39 strain, which led to a 6-log reduction in CFU after exposure to compounds at four times of the MIC for 24 h. The five compounds inhibited the growth of  Streptococcus pyogenes, Streptococcus mitis, Streptococcus mutans  or  Streptococcus pseudopneumoniae , meanwhile, deoxyshikonin and dodecyl gallate displayed strong inhibitory activities against  Staphylococcus aureus . These compounds showed no obvious cytotoxicity effects on Vero cells. Survival time of the mice infected by  S. pneumoniae  strains was prolonged by the treatment with the compounds. Importantly, all of the five compounds exerted antimicrobial effects against multidrug-resistant clinical strains of  S. pneumoniae . Moreover, even at sub-MIC concentration, they inhibited cell division and biofilm formation. The five compounds all have enhancement effect on penicillin. Deoxyshikonin and dodecyl gallate showed significantly synergic antimicrobial activity with penicillin  in vivo  and  in vitro , and effectively reduced nasopharyngeal and lung colonization caused by different penicillin-resistant pneumococcal serotypes. In addition, the two compounds also showed synergic antimicrobial activity with erythromycin and tetracycline. Taken together, our results suggest that these novel VicK inhibitors may be promising compounds against the pneumococcus, including penicillin-resistant strains.

Antibacterial effects of Traditional Chinese Medicine monomers against Streptococcus pneumoniae via inhibiting pneumococcal histidine kinase (VicK)