Compounds of formula (I):
wherein R¹ is optionally substituted phenyl and R is a cycloalkyl group having an alkyl substituent in the 1-position; or R¹ is phenyl substituted by at least one fluorine atom and/or at least one trifluoromethyl group and R is hydrogen or an organic radical, and their salts and esters, are useful in the treatment of bacterial infections in humans and animals.