Staphylococcus aureus belongs to the group of major contagious mastitis pathogens, whereas the coagulase-negative staphylococci (CNS) are also capable of causing opportunistic bovine mastitis. Many of these strains are resistant to penicillin or ampicillin because of the long-term use of beta-lactam antibiotics in agricultural and healthcare settings. Based on the simple and highly specific coagulase genotyping by PCR-RFLP used for discriminating among Staph. aureus strains, the relationship between phenotypic antibiogram and the polymorphism of coagulase gene was determined in this study. The staphylococci strains (835 Staph. aureus and 763 CNS) were isolated from 3,047 bovine mastitic milk samples from 153 dairy farms in 8 provinces from 1997 to 2004 in the Republic of Korea. Twenty-one (2.5%) Staph. aureus and 19 (2.4%) CNS strains were resistant to methicillin [oxacillin minimum inhibitory concentration (MIC) > or = 4 microg/mL]. The mecA gene was also found in 13 methicillin-resistant Staph. aureus (MRSA) and 12 methicillin-resistant CNS (MRCNS) isolates with a significantly higher detection rate of the mecA gene in MRSA with high MIC (> or = 16 microg/mL) compared with those with MIC < or = 8 microg/mL. Methicillin-resistant Staph. aureus and MRCNS were also more resistant to other antibiotics (ampicillin, cephalothin, kanamycin, and gentamicin) than methicillin-susceptible staphylococci. Among 10 different coa PCR-RFLP patterns (A to J) in 706 Staph. aureus strains, the main types were A (26.9%), B (17.0%), G (10.5%), and H (15.4%), with the frequent observation of the A and H types (6 and 10 isolates) in MRSA. This study indicates that major epidemic Staph. aureus clones may be spread between different dairy farms, and the profile of coa genotype can be applied for epidemiological investigations and control of bovine mastitis, particularly one caused by MRSA with specific prevalent coa types.

The objective of this study was to assess trends in the prevalence and distribution of gram-negative bacteria isolated from bovine mastitis and their antimicrobial susceptibilities during a 6-yr period between 2003 and 2008 in Korea. Escherichia coli, Pseudomonas fluorescens, Klebsiella pneumoniae, Enterobacter cloacae, Acinetobacter lwoffi/junii, Pseudomonas aeruginosa, and Serratia marcescens were the most commonly observed pathogens during this period. Generally, gram-negative bacteria showed low susceptibilities to most of the antimicrobials tested in this study, except amikacin and gentamicin. Although these 2 aminoglycosides were broadly active against gram-negative bacteria, less than half of those bacteria showed susceptibilities to streptomycin. The beta-lactams, except piperacillin, had the lowest activity among antimicrobials tested in this study. Susceptibilities to chloramphenicol and trimethoprim were fairy high in all genera of gram-negative bacteria, except Acinetobacter spp. and Pseudomonas spp., whereas relatively high resistance to tetracycline was observed uniformly among gram-negative bacteria. There was no significant change in the prevalence of bacterial and the proportion of antimicrobial resistance among gram-negative bacteria isolates during a 6-yr period.

Based on our previous study evaluating the in vivo cure efficacy of chitosan on bovine mastitis, a more water-soluble chitosan-oligosaccharide (OCHT) with a high degree of deacetylation and low molecular weight was prepared to obtain high antibiotic efficacy. The growth of Staphylococcus aureus isolated from bovine mastitis was inhibited within 10 min of treatment with OCHT in concentrations ranging from 0.0001 to 0.5%. Additionally, electron microscopic observation indicated that the surface of the OCHT-treated bacteria was expanded, distorted, and lysed compared to that of the control bacteria. In mice, the proportion of monocytes was elevated, and the levels of interleukin-6 and interferon-gamma sharply increased l h after the peritoneal inoculation of the OCHT (0.5 to 1 mg per mouse). Mice challenged intraperitoneally with S. aureus (2.5 x 10(8) colony forming units) after oral treatment with OCHT (0.5 to 2 mg per day) for 7 days showed a higher survival rate (70-100%) than that of the control (10%). We suggest that the OCHT prepared in this study is a potential agent for the prevention and treatment of bovine mastitis based on its strong antibacterial activity against S. aureus as well as the immunostimulative effect it exhibits on murine infection by S. aureus.

Methicillin (oxacillin)-resistant staphylococci (MRS) have emerged as major clinical and epidemiological pathogens, and there have been frequent reports of MRS infections in the veterinary field. The MRSA-Screen latex agglutination test (Denka Seiken Co., Ltd., Tokyo, Japan) was compared with an oxacillin agar screen test, MIC determination, and mecA PCR assay, the "gold standard." In an analysis of 15 mecA-positive and 48 mecA-negative S. aureus animal isolates, as well as 9 mecA-positive and 147 mecA-negative, coagulase-negative staphylococcal animal isolates, the latex agglutination test surpassed the widely used oxacillin agar screen method and MIC determination, with a sensitivity and a specificity of 100%. The MRSA-Screen test is a reliable and rapid method of detecting MRS in the veterinary field.

The aim of this study was to determine the antimicrobial resistance of streptococci isolated from mastitic bovine milk samples. A total of 178 isolates belonging to 6 different Streptococcus species were examined: S. uberis (n = 99), S. bovis (n = 30), S. oralis (n = 24), S. salivarius (n = 13), S. intermedius (n = 7), and S. agalactiae (n = 5). Only 8.9% (16/178) of the isolates were susceptible to all antimicrobial agents tested in this study, and S. agalactiae and S. intermedius isolates were all resistant to at least 1 antimicrobial agent tested. Overall, the most frequently observed resistance was to tetracycline (61.2%), followed by lincomycin (43.2%), gentamycin (35.3%), oxacillin (34.3%), and erythromycin (28.6%). Cephalothin and penicillin were the only antimicrobial agents to which most of the streptococci (>or=92%) were susceptible. Wide differences in the prevalence of resistance are apparent among the individual species: S. salivarius displayed exceptionally high resistance to cephalothin (23.0%) and oxacillin (76.9%) and S. agalactiae (20%) and S. intermedius (14.2%) to penicillin. Streptococcus salivarius and S. agalactiae were all susceptible to erythromycin, but others showed various rates of resistance ranging from 12.5% to 42.8%. Resistance to 3 or more of 7 antimicrobial agents was observed in all species (37.6%, 67/178).