METABOLIC-REGULATION OF LACTOCOCCUS-LACTI S GROWN IN PURE OR COCULTURE IN MILK

The cell responds to environmental changes by triggering or repressing the expression of its genes in order to adapt its metabolism to the n ew conditions. The measure of promoter activities could thus allow an indirect assessment of the external signals that the cell has integrat ed and the modification of its metabolic potential. We have developed a method using this idea to evaluate the bacterial metabolism independ ently of its externalized products. Promoter activity is measured by f ollowing the expression of luciferases as reporter genes. Two differen t luciferase genes were used, luxAB from the bacteria Vibrio harveyi a nd luc from the eucaryote Photinus pyralis. The activity of the procar yotic and the eucaryotic enzymes is detectable by on-line measurement with whole cells when the cells provide the cofactors FMNH and ATP, re spectively. This method is very sensitive, allowing the detection of w eak promoter activity, or moderate transcription at low cell density. To demonstrate that this method is efficient, we studied promoter acti vities modulated by the presence of available amino acids with bacteri al culture in milk. This allows us to see when the cells are starving, either in pure cultures or in mixed cultures with competing bacteria. As the two luciferases can be detected independently in the same cult ure, this method should allow the study of the interaction between str ains in coculture at the molecular level. (C) Inra/Elsevier, Paris.