title: Susceptibility to antimicrobials of mastitis-causing Staphylococcus aureus, Streptococcus uberis and Str. dysgalactiae from New Zealand and the USA as assessed by the disk diffusion test.
abstract: OBJECTIVE
  To compare the antimicrobial susceptibility patterns of three common mastitis pathogens (Staphylococcus aureus, Streptococcus uberis and Str. dysgalactiae) isolated from milk samples from New Zealand and the USA.
  
  
  METHODS
  A total of 182 S. aureus, 126 Str. uberis and 89 Str. dysgalactiae isolates from New Zealand (107, 106 and 41, respectively) and the USA (75, 20 and 48, respectively) were assessed using the disk diffusion test.
  
  
  RESULTS
  Susceptibility varied among the bacterial species. All isolates were susceptible to the amoxicillin-clavulanic acid combination. Resistance to lincomycin was most frequent (susceptibility of 8.6%) across all species. Non-susceptible (i.e. resistant or intermediate) isolates of S. aureus were identified for the three non-isoxazolyl penicillins (amoxicillin, ampicillin and penicillin: 20.6% and 36.0%) and lincomycin (99.9% and 94.6%) for NZ and the USA, respectively. Resistance to erythromycin (5.3%) and tetracyclines (6.7%) was detected only in isolates from the USA. There were differences in susceptibility between Str. uberis and Str. dysgalactiae; all streptococcal isolates demonstrated resistance to aminoglycosides (neomycin 52.4% and streptomycin 27.9%) and enrofloxacin (28%). Resistance of Str. dysgalactiae to tetracycline was almost 100.0% and to oxytetracycline 89.9%.
  
  
  CONCLUSIONS
  Most of the isolates tested were susceptible to most of the antimicrobials commonly used for treatment of bovine mastitis, with the exception of the lincosamides. Susceptibility to a selected class-representative antimicrobial and at the genus level should be interpreted with caution. Differences between NZ and the USA confirm the value of national surveys to determine the susceptibility patterns of mastitis pathogens.
authors.affiliation: Institute of Veterinary, Animal and Biomedical Sciences, Massey University, Palmerston North, New Zealand.
authors.forename: K R
authors.initials: KR
authors.lastname: Petrovski
authors.affiliation: Institute of Veterinary, Animal and Biomedical Sciences, Massey University, Palmerston North, New Zealand.
authors.forename: A
authors.initials: A
authors.lastname: Grinberg
authors.affiliation: Institute of Veterinary, Animal and Biomedical Sciences, Massey University, Palmerston North, New Zealand.
authors.forename: N B
authors.initials: NB
authors.lastname: Williamson
authors.affiliation: Institute of Veterinary, Animal and Biomedical Sciences, Massey University, Palmerston North, New Zealand.
authors.forename: M E
authors.initials: ME
authors.lastname: Abdalla
authors.affiliation: Institute of Veterinary, Animal and Biomedical Sciences, Massey University, Palmerston North, New Zealand.
authors.forename: N
authors.initials: N
authors.lastname: Lopez-Villalobos
authors.affiliation: Institute of Veterinary, Animal and Biomedical Sciences, Massey University, Palmerston North, New Zealand.
authors.forename: T J
authors.initials: TJ
authors.lastname: Parkinson
authors.affiliation: School of Pharmacy, University of Otago, Dunedin, New Zealand.
authors.forename: I G
authors.initials: IG
authors.lastname: Tucker
authors.affiliation: Veterinary Diagnostic Laboratory, University of Minnesota, MN, USA.
authors.forename: P
authors.initials: P
authors.lastname: Rapnicki
mesh_terms: D000818:Animals; D000900:Anti-Bacterial Agents / Q000494:pharmacology*; D002417:Cattle; D052978:Disk Diffusion Antimicrobial Tests; D024881:Drug Resistance, Bacterial; D005260:Female; D005516:Food Microbiology; D008414:Mastitis, Bovine / Q000188:drug therapy / Q000382:microbiology; D008892:Milk / Q000382:microbiology*; D009520:New Zealand; D013203:Staphylococcal Infections; D013211:Staphylococcus aureus / Q000187:drug effects* / Q000302:isolation & purification; D013290:Streptococcal Infections; D013291:Streptococcus / Q000187:drug effects* / Q000302:isolation & purification; D014481:United States
chemical_list: D000900:Anti-Bacterial Agents
keywords: Staphylococcus aureus; Streptococcus dysgalactiae; Streptococcus uberis; antimicrobials; dairy cows; mastitis; susceptibility patterns