Isolation and Characterization of Vancomycin-Resistant Enterococcus faecium from Cattle: Antimicrobial Resistance, Virulence Genes, and Pathogenicity

This study aimed to investigate the antimicrobial resistance, virulence gene profile, and pathogenicity of <i>Enterococcus faecium</i> isolated from a large-scale cattle farm in Xinjiang, China, to provide a scientific basis for the prevention and control of <i>E. faecium</i> infections in the region. Nineteen rectal swabs were aseptically collected from diarrheic calves for bacterial isolation. Isolates were identified through morphological observation, biochemical characterization, and PCR amplification. Homology analysis was conducted using 16S rRNA gene sequencing. Antimicrobial susceptibility was evaluated by the disk diffusion method, and key virulence genes were detected using PCR. Pathogenicity was assessed through intraperitoneal inoculation of mice, followed by histopathological examination. Three isolates were identified as <i>E. faecium</i>, consistent with morphological and molecular results. Biochemical tests indicated that the strains could metabolize sucrose, maltose, lactose, melibiose, and raffinose, but not sorbitol. Phylogenetic analysis revealed that isolates SCQ3 and SCQ4 shared 99.3% homology with <i>E. faecium</i> strain MF678878.1, while SCQ11 showed 91% similarity to strain JP2. Antimicrobial susceptibility testing indicated that SCQ11 was resistant to vancomycin. PCR analysis identified the presence of multiple virulence genes, including <i>psaA</i>, <i>hyp</i>, <i>asal</i>, <i>sprE</i>, <i>nuc</i>, <i>cbh</i>, <i>srtA</i>, <i>hyl</i>, <i>scm</i>, and <i>agg</i>. In vivo pathogenicity testing demonstrated that the vancomycin-resistant strain exhibited strong virulence in mice, with gross lesions observed in the liver, spleen, and intestines. Histopathological examination confirmed varying degrees of tissue damage, particularly in the liver and spleen. All three <i>E. faecium</i> isolates exhibited multidrug resistance, with one strain showing vancomycin resistance and harboring a high number of virulent genes. This strain demonstrated significant pathogenicity in vivo. These findings highlight the potential public health threat posed by multidrug-resistant <i>E. faecium</i> in livestock and provide essential data for regional prevention and control strategies.