The expression of most virulence factors during pathology is the result of the bacterial response to environmental changes. The understanding of the conditions affecting gene expression and bacterial physiology in the digestive tract is a clue toward control of pathology. We developed tools with the luciferase gene from Vibrio harveyii as reporter, which allows the measure of promoter activity in digestive tract. These tools can be used in most G+ bacteria such as Streptococci, Clostridii and aerobe Bacilli. In this work, Lactococcus lactis was chosen as model because it is genetically the best known bacteria of the streptococcal group. Transcriptional fusions were constructed between luciferase and different promoters active in various metabolic conditions. The luciferase activities were compared in culture broth and in mouse digestive tract. Three promoters were chosen for this study: P his , P ald , and P mleA the features of which are presented in Table 1. P his , the promoter of histidine biosynthesis operon, can be active during stringent response, is closed in media containing histidine and open under histidine starvation (Delorme under publication). Two forms of the P his promoter were used, the first one is an intact and fully regulated promoter, the second one drives a constitutive expression. P ald , the promoter of acetolactate decarboxylase is open in rich media and closed in stringent conditions (Goupil under publication). P mleA , the promoter of malate fermentation genes is closed in absence of malate and open when malate is added to the medium, it is not sensitive to stringent control (Renault under publication)