Farrell's medium

Farrell's medium is a selective bacteriological medium for Brucella species which is prepared by the addition of six antibiotics to a basal bacteriological medium such as serum dextrose agar. In order to prepare 1 liter of the Farrell's medium, the following quantities are added to 1 liter of serum dextrose agar: polymyxin B sulfate (5000 units =5 mg), bacitracin (25,000 units = 25 mg), natamycin (50 mg), nalidixic acid (5 mg), nystatin (100,000 units), and vancomycin (20 mg). Vancomycin inhibits the growth of Gram positive bacteria on this medium, while nystatin inhibits the growth of fungi. Other antibiotics inhibit the growth of Gram negative bacteria other than Brucella species, thus favoring the exclusive growth of the latter in this medium.