Objective: The aim of this study was to screen a group of medicinal plants’ extracts used in the treatment of ailments related to infections in the Brazilian popular medicine. And to carry out in vivo toxicity and antibacterial studies on Conyza bonariensis (Asteraceae) leaves and roots methanolic extracts selected based on the screening. Methods: Eleven methanolic extracts obtained from nine plants, reportedly used in the treatments of infections from the state of Mato Grosso, Brazil, were initially screened for their in vitro antibacterial and antifungal activities employing disc diffusion and broth micro dilution assays. Preliminary phytochemical analysis was carried out. The most promising extract based on our results and previous literature reports was then evaluated in the in vivo antibacterial activities using mouse model of bacterial infection induced by Staphylococcus aureus and Escherichia coli. In addition, in vivo acute toxicity was conducted to evaluate the safety profile of the extracts. Results: All of the extracts tested were active against at least one of the bacterial and fungal strain tested with activities ranging from moderate to weak. Phytochemical analyses of MECbl and MECbr demonstrated the presence of free steroids and coumarins in MECbl and flavonoids, tanins, free steroids, reduced anthraquinones and coumarins in MECBr. Oral administration of MECbl and MECbr up to 5000 mg/kg did not provoked any toxicological events in the mice, thus suggesting that the LD50 is higher than 5000 mg/kg. In vivo antibacterial assay demonstrated superior prophylactic activity of MECbl compared to MECbr. Conclusion: MECbl and MECbr are safe when administered acute orally at doses up to 5000 mg/kg. Methanolic extracts of Conyza bonariensis possessed in vitro antibacterial and antifungal activities. Considerable in vivo antibacterial activities were observed in bacterial infection model for both MECbl and MECbr, effects comparable to that of meropenem, in some cases. Both extracts present in common free steroids and coumarins. The current in vivo antibacterial activity study further lend supports to the use of Conyza bonariensis in the treatment of infections in many traditional medicines.