DETECTION OF INDUCED BETA-GALACTOSIDASE ACTIVITY IN INDIVIDUAL NON-CULTURABLE CELLS OF PATHOGENIC BACTERIA BY QUANTITATIVE CYTOLOGICAL ASSAY

One Escherichia coli and two F'lac(+) Salmonella strains were carbon a nd nitrogen stressed at 37 degrees C over 35 days in the presence or a bsence of chloramphenicol; the number, activity and culturability of c ells in the resultant populations were studied. Active cells were enum erated by fluorescence microscopy after treatment with the lac inducer IPTG and cytological assay for beta-galactosidase. In all experiments , active and total cell counts remained within a three-fold range of e ach other and their initial values, while culturability fell by >10(8) -fold and 10(3)-fold in chloramphenicol-treated and untreated preparat ions, respectively, Quantitative image analysis revealed different dis tributions of cell-specific fluorescence and indicated a progressive d ecline in the levels of induced enzyme activity in both E. coli and Sa lmonella enteritidis. It was concluded that the non-culturable cells s tudied retained inducible enzyme activity and that this activity did n ot result from a starvation-induced programme of gene expression. Whet her or not such active but nonculturable cells are viable, they are cl early responsive and have the potential to influence their environment . The assay described can be applied to heterogeneous populations and environments and shows considerable potential for the study of gene ex pression at the single cell level.