This invention relates to compounds of the formula wherein a, R1, R2, R3, R4, R5, R6 and X are each as defined above, and to pharmaceutically acceptable salts thereof, useful as potent antibacterial and antiprotozoal agents that may be used to treat various bacterial and protozoal infections and disorders related to such infections. The invention also relates to pharmaceutical compositions containing the compounds of formula I and to methods of treating bacterial and protozoal infections by administering the compounds of formula I.