The invention relates to a crystalline form of the (R)-4-(5- (cyclopropylethynyl)isoxazol-3-yl)-N-hydroxy-2-methyl-2- (methylsulfonyl)butanamide (i.e. the compound of formula (A)) with low hydroscopicity. The compound and compositions thereof are useful to treat bacterial infections and in particular Gram negative bacterial infections, including mult-drug resistant strains. The crystalline form is prepared by dissolving an amorphous form of the compound in a halogenated organic solvent (e.g. dichloromethane) and precipitating the crystalline form with a hydrocarbon solvent (e.g. heptane).(A)
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