Molecular epidemiology, characterization of virulence factors and antibiotic resistance profile of Streptococcus agalactiaeisolated from dairy farms in China and Pakistan

Streptococcus agalactiaeis one of the most common pathogens that cause bovine mastitis worldwide. Identifying pathogen prevalence and virulence factors is critical for developing prevention and control approaches. Herein, 1 161 milk samples from various dairy farms in China (n=558) and Pakistan (n=603) were collected between 2019–2021 and were subjected to S. agalactiaeisolation. Prevalence, serotyping, virulence genes, and antibiotic-resistant genes of S. agalactiaewere evaluated by PCR assay. All isolates were characterized for haemolysis, biofilm production, cytotoxicity, adhesion, and invasion on bovine mammary epithelial cells. The prevalence of S. agalactiae-induced mastitis in cattle was found to be considerably higher in Pakistan than in China. Jiangsu and Sindh provinces had the highest area-wise prevalence in China and Pakistan, respectively. Serotypes Ia and II were prevalent in both countries, whereas serotype III was found only in Pakistan. Moreover, all isolates tested positive for PI-2b gene but negative for PI-1 and PI-2a genes. All isolates harboured cfb, cylE, hylB, and fbsBvirulent genes, whereas many of them lacked bibA, riband bca. However, the absence of bacand scpgenes in Chinese isolates and cspAin Pakistani isolates was noted, while spb1and lmbwere not detected in isolates of both countries. Pakistani isolates, particularly serotype Ia-positive, had a considerably higher ability to produce biofilm, haemolysis, cytotoxicity, adhesion, and invasion than Chinese isolates. Most of the isolates were phenotypically resistant to tetracycline, erythromycin, and clindamycin and genotypic resistance was confirmed by the presence of ermA, ermB, tetMand tetOgenes. Our study highlights the antimicrobial resistance profile and virulence-related factors contributing to the epidemiological spread of mastitis-causing S. agalactiaein China and Pakistan. The findings may facilitate future studies designed to develop improved treatment and control strategies against this pathogen.