Bovine mastitis is a serious disease associated with both high incidence and economic loss, posing a major challenge to the dairy industry worldwide. Staphylococcus aureus is one of the most common pathogens to cause bovine mastitis, and antimicrobial therapy plays an important role in mastitis control caused by S. aureus but has become less effective due to widespread drug resistance. The purpose of this study was to detect antibiotic resistance, drug resistance gene, and virulence gene of S. aureus strains, which would be helpful to mastitis control.

This study aimed to investigate the antimicrobial resistance and virulence genes of Enterococcus faecalis isolated from subclinical bovine mastitis cases in China. Enterococcus faecalis isolates were identified by 16S rRNA amplification and sequencing. Antimicrobial susceptibility was determined by the disc diffusion method. Antimicrobial resistance and virulence genes were tested by PCR. Overall, E. faecalis was recovered from 81 of 1,787 (4.5%) mastitic milk samples. The isolates showed high resistance against tetracycline (87.7%) and erythromycin (79.0%). The most prevalent resistance genes found in the E. faecalis were tetK (96.3%), tetL (79.0%), and tetM (87.7%) for tetracycline and ermC (97.5%) for erythromycin. Moreover, gelE (70.4%), esp (85.2%), efaA (91.4%) were the most common virulence genes. This is the first report to characterize E. faecalis recovered from subclinical bovine mastitis cases in China.

Escherichia coli is the leading causative agent of bovine mastitis worldwide. Quinolone-resistant E. coli is becoming a potential threat to veterinary and public health. The aim of this study was to investigate the characteristics of quinolone-resistant E. coli isolated from bovine mastitis cases in China. Antimicrobial susceptibility of the isolates against 15 antimicrobial agents was determined by disc diffusion method. Phylogenetic grouping was detected by PCR. Extended-spectrum β-lactamase-producing isolates were determined by double-disc synergy test. In addition, the plasmid-mediated quinolone resistance (PMQR) and β-lactamase-encoding genes, as well as mutations of quinolone resistance-determining regions in GyrA, GyrB, ParC, and ParE, were measured by PCR and DNA sequencing. Overall, 75 (22.9%) out of 328 E. coli isolates were confirmed as ciprofloxacin-resistant from 2,954 mastitic milk samples. Phylogenetic group analysis showed that the majority of these strains belonged to phylogenetic group A (57.3%) and group B1 (24.0%). All the resistant isolates were identified as multidrug resistant, showing high resistance to cephalosporins and non-β-lactams. Forty-nine (65.3%) of the quinolone-resistant isolates were positive for PMQR genes; aac-(6')-Ib-cr was the most common PMQR determinant detected in 33 (44.0%) isolates. Eighteen (24.0%), 4 (5.3%), 3 (4.0%), and 1 (1.3%) of the quinolone-resistant isolates were harboring oqxA/B, qepA4, qnrS, and qnrB2, respectively. Additionally, 55 (73.3%) of the quinolone-resistant E. coli isolates were found to be extended-spectrum β-lactamase producers. The preponderant β-lactamase-encoding gene, blaTEM, was detected in 44 (58.7%) isolates; blaCTX-M, blaCMY, and blaSHV were found in 35 (46.7%), 22 (29.3%), and 2 (2.7%) isolates, respectively. Moreover, the most frequently identified substitutions were S83L/D87N or S83L in GyrA, detected in all of the quinolone-resistant isolates. Meanwhile, 74 (98.7%), 33 (44.0%), and 6 (8.0%) of the isolates were carrying substitutions S80I in ParC, S458A in ParE, and S492N in GyrB, respectively. All 58 (77.3%) isolates with a high level of ciprofloxacin resistance (>32 µg/mL) carried single or double mutations in GyrA combined with single mutation in ParC. To the best of our knowledge, this is the first report on the high occurrence of PMQR determinants and quinolone-determining resistant regions mutations in quinolone-resistant E. coli isolated from bovine mastitis in China.

The aim of this study was to evaluate the influences of season, parity, lactation, udder area, milk yield, and clinical symptoms on bacterial intramammary infection (IMI) in dairy cows. A total of 2,106 mastitis pathogens in 12 species were isolated from 125 dairy farms distributed in 30 different cities in China, and the information about these factors was recorded at the same time. Mastitis pathogens were isolated from 63.43% of the milk samples, whereas Streptococcus agalactiae accounted for 38.61% of all pathogens, followed by Str. dysgalactiae (28.16%), Staphylococcus aureus (19.10%), Escherichia coli (6.90%), and other pathogens (7.23%). According to our investigation, IMI was more common in spring with the isolation rate of pathogens at 81.04%, and lowest in winter (52.34%). Cows were more likely to be infected by environmental pathogens (E. coli or Str. uberis) in summer, in rear quarters and in cows with higher daily milk yield or lower somatic cell count. In addition, Str. dysgalactiae exhibited a higher prevalence with increased parity. Different clinical symptoms of quarters with bacterial IMI were seen in this study, and mastitis pathogens were isolated from healthy quarters.

The aim of this study was to investigate the effects of N-acetylcysteine (NAC) on antibiotic susceptibility of bovine mastitis pathogens including Staphylococcus aureus, Streptococcus dysgalactiae, Escherichia coli, and Streptococcus agalactiae. Minimum inhibitory concentrations (MIC) were tested by the agar-based E-test method. The presence of 10mM NAC reduced the MIC of penicillin and ampicillin but enhanced the MIC of erythromycin and ciprofloxacin for all of the strains. In addition, NAC-mediated modulation of MIC of kanamycin, tetracycline, and vancomycin was diverse, depending on the target bacterial pathogen and antibiotic being used. The results suggest that NAC is an important modulator of antibiotic activity against the major bovine mastitis pathogens.