Salmonella and B. thermophilum in reactor effluents compared to theoretical washout curves during colonization tests. RBL67 was added once to TR2 and TR4 to reach 108 CN/mL, while TR4 was additionally supplied with 5.2 g of FOS/day. N-15 was added once to TR1 to reach 106 cfu/mL. Salmonella viable cell counts in reactor effluents was measured by plating on CHROMAgar . B. thermophilum numbers were estimated by qPCR. Measured concentrations were compared to a theoretical washout curve.