The present invention relates to a cell which is genetically modified with respect to its wild type and which comprises a gene sequence coding for a fluorescent protein, wherein the expression of the fluorescent protein depends on the amount of protein that is secreted across the cytoplasmic membrane into the extracytosolic space. The present invention also relates to a method for the identification of a cell having an increased secretion of protein across the cytoplasmic membrane into the extracytosolic space, a method for the identification of a culture medium composition that is optimized for the recombinant production of protein, a method for the identification of culture conditions that are optimized for the recombinant production of protein, a method for the identification of a compound that is characterized by an antibiotic activity due to its property to damage the membrane of a bacterial cell or to analyse the effect of such a compound on a population of genetically different bacterial cells or genetically identical cells in different physiological states or different growths phases, a method for the production of a cell which is genetically modified with respect to its wild type with optimized secretion of protein across the cytoplasmic membrane into the extracytosolic space, a cell obtained by this method, a method for the production of proteins and a method for the preparation of a mixture.