Complex sentence: The second step occurs only when the required bacteria exist, then the ring cleavage of benzene and the further desulphonation of the mono-and dicarboxyilic sulphophenyl acids happen.
Split into simpler sentences: The second step occurs only when the required bacteria exist. Then, the ring cleavage of benzene and the further desulphonation of the mono-and dicarboxyilic sulphophenyl acids happen.