Cells can display different variations in behavior toward different external cues. The multidisciplinary field of metabolomics provides a unique insight into the molecular feedbacks and cross-talk between metabolic cascades that allow organisms to adapt toward different sources of biotic and abiotic stress.Unfortunately, there is still a substantial technological limitation (in terms of sensitivity and throughput), when trying to understand the complex and highly interrelated metabolic processes occurring within a clonal cell population under stress, and how these metabolic processes could lead to the generation of resistant cells toward a specific stress cue (i.e. stochastically-induced phenotypic heterogeneity).The objective of this proposal is to overcome the substantial technological limitation in single cell metabolomic analysis by developing an automated microfluidic device couple to a mass spectrometer for obtaining high quality “metabolic profiles”; in order to build models of signal cascades and of metabolic activity occurring in single cell organisms under different nutrient stress conditions. The proposed single cell metabolomic instrumentation belong to a group of analytical platforms, we called Direct Single Cell - Mass Spectrometry (DiSC-MS) platforms, currently the forerunners of single cell metabolomic studies are in Japan and in the USA, while only a handful of groups in Europe are also active in this field.Although the proposal focuses on monitoring the metabolic profiles of E. coli and S. cerevisiae to identify resistant phenotypes within a large clonal population toward a nutritional shift, the developed analytical method may also be used to better understand, how some populations of bacteria can exploit their regulatory metabolic feedback to create multiple types of coexisting phenotypes that are drug resistant.