A compound of formula (I) or a pharmaceutically or veterinarily acceptable salt thereof for the inhibition of activity of bacterial PDF enzyme: ##STR00001## wherein A represents a group of formula (IA) or (IB) ##STR00002## and wherein R.sub.1 is hydrogen, R.sub.2 is n-butyl, benzyl or cyclopentylmethyl, R.sub.3 is hydrogen, R.sub.4 is tert-butyl, iso-butyl, benzyl or methyl, R.sub.5 is hydrogen or methyl and R.sub.6 is methyl.