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Claire Hunt and Susan de Burgh, Bayer Australia Ltd, 875 Pacific Hwy, PYMBLE NSW 2073

Posted Flock & Herd December 2019


More than $150 million is lost to Australian dairy farmers each year through poor udder health and the major cause is mastitis. Antimicrobial therapy remains an essential tool in the management of mastitis, however it raises the issue of antimicrobial resistance (AMR). Demonstration of evidence-based usage and commitment to ethical product stewardship is necessary to maintain safe and effective veterinary use of antimicrobials.

In a recent large survey of dairy cattle in SE Australia, the most common bacterial pathogens cultured from clinical mastitis cases were: Streptococcus uberis (54%), Staphylococcus aureus (15%), Escherichia coli (12%) and Streptococcus dysgalactiae (9%) - i.e. predominantly gram-positive bacteria (Charman et.al. 2012).

The aim of these two studies was to determine the current susceptibility of the most common gram-positive mastitis pathogens to two common antimicrobials used in the dairy industry: penicillin and cloxacillin. In addition, the development of a farm-specific anti-microbial monitoring program is discussed.


Between 2016 - 2018 up to 30 bacterial isolates of Streptococcus uberis, Staphylococcus aureus, S. agalctiae and S. dysgalactiae were sampled from mastitis cases on commercial dairy farms in SE Australia. The Minimum Inhibitory Concentrations (MICs) of penicillin and cloxacillin were determined as the lowest concentration of the antibiotic that was found to inhibit bacterial growth.


Table 1. MICs for Cloxacillin
Bacterial pathogen MIC50 MIC90 MIC range Susceptibility*
Streptococcus uberis 0.5 4 0.125 - 4 100%
Staphylococcus aureus 0.25 0.25 <0.06 - 0.25 100%
Streptococcus agalactiae 2 2 0.125 - 2 100%
Streptococcus dysgalactiae 0.06 0.5 <0.06 - 2 100%
Table 2. MICs for Penicillin
Bacterial pathogen MIC50 MIC90 MIC range Susceptibility*
Streptococcus uberis 0.0312 0.125 0.0156 - 2 97%
Staphylococcus aureus 0.0312 4 0.0156 - 4 87%
Streptococcus agalactiae 0.0625 0.0625 0.0625 - 0.0625 100%
Streptococcus dysgalactiae 0.0078 0.0156 0.0078 - 0.125 100%

*Susceptibility was determined for each antimicrobial based on clinical breakpoints defined in the guidelines of the Clinical and Laboratory Standards Institute (CLSI).


Common gram-positive bacterial pathogens cultured from mastitis cases of Australian dairy cattle showed 100% susceptibility to cloxacillin and high susceptibility to penicillin. The lowest susceptibility for penicillin was found in isolates of S. aureus. Resistance of S. uberis to penicillin in mastitis cases has been shown to be increasing globally (77% intermediate susceptibility or resistant, Canada, Cameron et. al. 2016, and 35.6% intermediate susceptibility, Europe, de. Jong et al. 2018).

Clinical significance of the results

MIC testing of local mastitis pathogens gives a snap-shot of arising resistance. One mechanism for the accelerated development of antibiotic resistance is exposing bacteria to repeated, inappropriate antibiotic treatments causing them to become more resilient and harder to kill. Antibiotics remain a valuable tool in the dairy industry and, when used responsibly, are vital for maintaining animal health and welfare.

The use of narrow-spectrum, targeted antimicrobials and using antimicrobials wherever possible that are not on the 'restricted' list for human health are practices recommended by the recently published Veterinary Antimicrobial Prescribing Guidelines (prepared through collaboration between University of Melbourne's Asia Pacific Centre for Animal Health (APCAH) and the National Centre for Antimicrobial Stewardship).

Currently in Australia most mastitis prescriptions are made without local knowledge of the true quantitative antimicrobial susceptibility on that particular farm (MIC). The only commercial testing available previously for antimicrobial susceptibility was the agar disc diffusion assay, which had significant limitations and problems with interpretation.

Bayer is promoting the effective, sustainable, and responsible use of antimicrobials on Australian dairy farms by offering a new test, the DAIRY ANTIBIOGRAM. This test offers an easily accessible MIC assay of common mastitis pathogens tailored to a specific farm, based on a bulk milk tank sample.

The DAIRY ANTIBIOGRAM gives Australian dairy veterinarians a unique opportunity to build reliable susceptibility testing into their mastitis consultancy, and to use this data to guide responsible antibiotic use.


  1. Cameron M, Saab M, Heider L, McClure J, Rodriguez-Lecompte JC, Sanchez J. 2016. Antimicrobial Susceptibility Patterns of Environmental Streptococci Recovered from Bovine Milk Samples in the Maritime Provinces of Canada. Front. Vet. Sci. 203:79
  2. Charman, N, Dyson, R, Hodge, A, Robertson, N. and Chaplin, S. 2012 A Survey of Mastitis Pathogens in the South Eastern Australian Dairy Industry. Dairy Australia Countdown Mastitis Symposium, Melbourne
  3. de Jong A, El Garch F, Simjee S, Moyaert H, Rose M, Youala M, Siegwart E; VetPath Study Group. 2018. Monitoring of antimicrobial susceptibility of udder pathogens recovered from cases of clinical mastitis in dairy cows across Europe: VetPath results. Veterinary Microbiology 213: 73-81

Conflict of interest statement

Employment of the authors by Bayer as indicated in institution details above.


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