This invention relates to novel antimicrobial compounds of formula; ##STR00001## wherein X, R.sub.1, R.sub.3, R.sub.5, R.sub.6, and R.sub.8 are defined in the claims, and to their optical isomers, diastereomers or enantiomers, as well as pharmaceutically-acceptable salts, hydrates, and biohydrolyzable esters, amides and imides thereof, and to compositions and uses of such compounds. The invention also relates to compounds derived from these compounds having antimicrobial uses.