Fluorescent detection of nisin by genetically modified Lactococcus lactis strains in milk and a colonic model: Application of whole-cell nisin biosensors

Detection of bioactive peptides in complex ecosystems like intestinal environment is a difficult task. In this study, we developed two new bioreporters for nisin based on Lactococcus lactis NZ9000 transformed with the vector pNZ:Nis-aFP or pNZ:Nis-mCherry, that encoded for the anaerobic fluorescent protein evoglow-Pp1 (aFP) or the fluorescent protein mCherry, respectively. The biosensors were used to study nisin A production by L. lactis INIA 650 in milk and in a colonic model. The use of L. lactis NZ9000 pNZ:Nis-aFP as a biosensor allowed the detection of nisin produced by L. lactis INIA 650 in milk, but not in the in vitro colonic model. In milk, this reporter was induced by direct addition of 10 ng/ml nisin while, in the colonic model, nisin concentrations of 50 ng/ml were necessary. However, the reporter system based on pNZ:Nis-mCherry showed a higher sensibility, detecting nisin concentrations of 1 ng/ml produced by L. lactis INIA 650 in colonic media using agar diffusion or cross streak bioassays.