Complex sentence: Bacteria are unable to take up folic acid from the environment (i.e. the infection host) thus are dependent on their own "de novo" synthesis-inhibition of the enzyme starves the bacteria of two bases necessary for DNA replication and transcription.
Split into simpler sentences: Bacteria are unable to take up folic acid from the environment (i.e. the infection host) and are thus dependent on their own "de novo" synthesis. Inhibition of the enzyme starves the bacteria of two bases necessary for DNA replication and transcription.