Disclosed is trans-7-oxo-6-(sulphoxy)-1,6-diazabicyclo[3,2,1]octane-2-carbonitrile and its salts of formula (I), wherein M is a cation selected from hydrogen, sodium, or potassium. Also disclosed is the use of compounds of formula (I) for treating or preventing bacterial infections, particularly bacterial infections caused by bacterial producing beta-lactamase enzymes.