Simple Summary
We conducted a study to investigate the role and characteristics of Bacillus cereus in bovine mastitis within dairy herds in Inner Mongolia, China. The objectives were to determine its prevalence, antimicrobial resistance profile, virulence gene carriage, and pathogenic potential. A total of 340 bacterial strains were isolated from clinical mastitis milk samples. Bacillus cereus was identified as the predominant pathogen. Antimicrobial susceptibility testing revealed significant resistance to tetracycline, but consistent susceptibility to gentamicin, amikacin, and roxithromycin. PCR assay confirmed that all B. cereus isolates carried key virulence genes. Mouse model challenges demonstrated that strains harboring a higher number of virulence genes induced more severe histopathological damage in liver and kidney tissues and a stronger inflammatory response. Our findings establish Bacillus cereus as a major etiological agent of mastitis in this region with a distinct resistance and virulence pattern. This study provides crucial epidemiological data to inform targeted mastitis control strategies and support animal health and food safety.
Abstract
This study aimed to investigate the epidemiological characteristics and antimicrobial resistance patterns of Bacillus cereus (B. cereus) in bovine mastitis within Inner Mongolian dairy herds, with a focus on virulence gene distribution and their clinical implications. A cross-sectional epidemiological investigation was conducted across three large-scale dairy farms. A total of 340 bacterial strains were isolated from milk samples collected from 108 cows with clinical mastitis, all of which underwent comprehensive PCR testing. Antimicrobial susceptibility testing was performed using the Kirby–Bauer disk diffusion method against eight antibiotics. Virulence gene profiling was conducted for all B. cereus isolates, and murine challenge experiments were performed to assess virulence-factor-dependent pathogenicity. Bacteriological analysis identified B. cereus as the predominant pathogen (104 strains, 30.58%), followed by Staphylococcus spp. (74 strains, 21.76%). Antimicrobial susceptibility testing revealed high resistance to tetracycline (38.46%), cotrimoxazole (15.38%), and ciprofloxacin (7.69%), while complete sensitivity (100%) was observed for gentamicin, amikacin, and roxithromycin. Virulence gene profiling demonstrated universal presence of nheA, nheB, and entFM genes in all isolates, with bh1D detected in only 21.15% (22/104) of strains. Murine challenge experiments confirmed virulence-factor-dependent pathogenicity, with strains harboring nine virulence factors inducing significant upregulation of hepatic inflammatory markers (p < 0.05) and histopathological alterations in hepatic and renal tissues compared to strains with three virulence factors. Our findings highlight B. cereus as an emerging virulent pathogen in Inner Mongolian dairy herds, necessitating enhanced surveillance of virulence factors and antimicrobial stewardship in mastitis management. This study provides critical epidemiological data to inform clinical veterinary practices and targeted intervention strategies.