 This is a Phase 2b, randomized, double-blind, placebo controlled, parallel-group, multi-center proof-of-concept study to assess the potential of SYN-004 in the prevention of CDI and the unwanted side effects of IV antibiotic treatment in at risk patients who are hospitalized for LRTI and receiving IV ceftriaxone alone or in combination with a macrolide. Subjects will be 50 years or older. The entire duration of the study may be up to 59 days. All patients will be evaluated for the occurrence of CDI and AAD by testing according to local diagnostic standards and monitoring for diarrhea 3 or more unformed stools per 24 hour period. 