Bovine mastitis is the most important disease of dairy farms and is typically caused by bacterial infection.1 This disease constitutes a serious problem in dairy herds incurring considerable economic losses due to reduced milk production and discarded milk.2 In the United States, economic losses due to mastitis are estimated at US$2 billion per year, whereas in the United Kingdom and Northern Ireland, the annual losses are £300 million and £14 million, respectively. Mammary gland infections implicate an additional cost of €693 per cow per year in the Republic of Ireland, whereas in the Netherlands, these additional costs vary between €164 and €235.3 Staphylococcus aureus and Escherichia coli are the most common etiological agents of subclinical and clinical bovine mastitis.4

Penicillin and its derivatives are recommended for the treatment of bovine mastitis caused by Gram-positive pathogens. However, a worldwide increase in S. aureus that are resistant to these antibiotics has been described. In the United States, >70% of isolates obtained from mastitis are penicillin resistant, whereas in Brazil and Ireland, the level is about 85%.5 Multiple drug resistance to streptomycin, sulphametoxazole, ampicillin or tetracycline for coliform mastitis strains has been described elsewhere.6 Therefore, new antibacterial compounds are being investigated as an alternative to treat infections of human and animal origin.

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