Objective: Streptococcus dysgalactiae is a major pathogen in bovine mastitis. The purpose of this study was to survey the prevalence, antimicrobial resistance, as well as the spread of resistance and virulence-associated gene of S. dysgalactiae. Methods: A total of 60 S. dysgalactiae strains were obtained from 830 milk samples from Holstein cows with clinical mastitis. Antimicrobial resistance was examined by the disk diffusion method. Antimicrobial resistance and virulence genes were investigated by PCR, agarose gel electrophoresis and 16S rRNA gene sequencing. Results: All isolates were resistant to tetracycline and showed a high level of resistance to aminoglycoside antibiotics, where 81.67% of the strains were multi-resistant to these ten sorts of antibiotics. In addition, the most prevalent resistance gene in S. dysgalactiae was aphA-1 (98.33%), followed by blaTEM (96.67%), ermB (83.3%), aadA1/aadA2 (78.33%) and tetL (73.33%). Totally, seven virulence genes with 25 combination patterns were detected in these isolates, and each isolates harbored at least one virulence gene. 21.67% of the isolates carried three or more virulence genes, while one strain with seven virulencerelated genes and belonged to cfb+lmb+eno+napr+bca+scpB+cyl. Conclusion:These findings indicate that S. dysgalactiae isolated from clinical bovine mastitis cases in Northwest China show a variety of molecular ecology and are highly resistant to antibiotics commonly used in dairy farms. This research will help investigators better understand the pathophysiology S. dysgalactiae in bovine mastitis and choose the appropriate antibiotics to treat mastitis.

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Nocardia farcinica, one of the most common etiological pathogens of nocardiosis in pulmonary infection, usually causes severe disseminated diseases in immunocompromised individuals. To date, there is a lack of rapid and reliable diagnostic tools for the detection of N. farcinica in clinical laboratories. In the present study, we reported the development of rapidity, simplicity, and high specificity real-time fluorescence recombinase-aided amplification (RT-RAA) assay for the detection of N. farcinica. RT-RAA technique was developed to amplify the specific gene of pbr1 and detected double labeled amplicons within 20 min by designing a specific labeled primer set and the corresponding probe. We identified 86 strains of N. farcinica and 46 closely related strains, analyzed the specificity of the developed assay, and estimated the clinical diagnostic efficacy of the method with simulated sputum samples. The results showed no cross-reaction was observed and specificity was 100%. The limit of detection of RT-RAA was 10 copies per reaction of Nocardia genomic DNA, which was up to 10 times higher sensitivity than conventional real-time quantitative PCR. In conclusion, the recombinase-mediated isothermal amplification technique is a fast, simple, and sensitive method for the detection of N. farcinica, which may have potential application for the detection of N. farcinica in clinical laboratories, especially in resource-poor areas.