A bioreactor is provided that includes a main reactor having a configuration selected from the group consisting of stirred and unstirred tank reactor, trickle bed reactor (TBR), cocurrent contactor (CCC), moving bed bioreactor (MBBR), and a bubble column reactor. The bioreactor also includes a growth reactor continuous with the main reactor and having a configuration selected from the group consisting of stirred and unstirred tank reactor, trickle bed reactor (TBR), cocurrent contactor (CCC), moving bed bioreactor (MBBR), and a bubble column reactor. A method is also provided where acetogenic bacteria are contacted with syngas in a growth fermentor section of a reactor vessel that is continuous with a main fermentor section of a reactor vessel.