Antibiotic penem compounds are represented by the following formula: ##STR1## wherein R represents a group of the general formula: ##STR2## in which R.sub.1 is a linear or branched C.sub.1 -C.sub.6 alkyl group, R.sub.2 is a particular substituted or unsubstituted alkyl, aryl or aralkyl group, n is an integer of 1-6, Y is a 5- or 6-membered heterocyclic aliphatic group having 1 or 2 oxygen atoms in the ring thereof, and Z is a specific 5-substituted 2-oxo-1,3-dioxolen-4-yl group. Antibiotic compositions for oral administration are also described.