Correlation Analysis between the Antimicrobial Resistance and Virulence of Pathogenic Streptococcus Isolates from Cows

Bovine Streptococcus are one of the main pathogens causing bacterial disease such as mastitis and endometritis in dairy farming. The virulence factors produced by Streptococcus are related to the occurrence of inflammation. To investigate the correlation between antimicrobial resistance and virulence traits of bovine Streptococcal isolates. Induced resistance was conducted for Streptococcus pneumonia ATCC49619 and erythromycin-sensitive strains by gradually increasing the antimicrobial concentration. Plasmid conjugation test was carried out by membrane filtration method. The correlation between antimicrobial resistance and virulence traits was analyzed by LD50 and related genes. Sensitive Streptococcus isolates to erythromycin and S. pneumoniae ATCC49619 were induced to resistance in vitro, MIC value was from <= 0.5 mu g/mL up to >= 64 mu g/mL, and ermB or mefA resistant gene were carried. Transfer rate of resistance was 100% by plasmid conjugant, conjugants had obtained the resistance phenotype and the related resistance genes from the donor bacteria. The LD50 of conjugants and induced resistance strains compared with parental strain, the virulence was lower than sensitive strains. The present study demonstrated that the virulence of resistant Streptococcus strains obtained by different drug resistance transfer methods was lower than that of their parents.