Quantification of plasmid loss in Escherichia coli cells by use of flow cytometry

A method was developed to study plasmid stability in Escherichia coli cells, which utilised the high speed analysis properties of flow cytometry. To discriminate between plasmid-harbouring cells and plasmid-free cells a plasmid-encoded Lac repressor protein was used to regulate the expression of a chromosomally inserted green fluorescent protein gene in the host cells. Flow cytometric analysis enabled detection and quantification of plasmid-free cells due to their green fluorescent phenotype. The reported system offers real-time analysis in combination with a very low detection level of plasmid loss in bacterial populations. This could be useful in future investigations of plasmid stability and population selection in bacterial communities. (C) 2004 Federation of European Microbiological Societies. Published by Elsevier B.V. All rights reserved.