The present invention relates to a method of detecting a microorganism, in particular Staphylococcus aureus, in a sample, comprising the steps of a) incubating the sample with a slow off-rate modified aptamer (SOMAmer) comprising a fluorescent label, b) optionally washing the sample, c) analyzing the sample by a fluorescence-based detection method.The invention further relates to a slow off-rate modified aptamer comprising a fluorescent label, wherein the SOMAmer comprises a nucleotide sequence specific for Staphylococcus aureus, its use for detecting Staphylococcus aureus cells in a sample, and to a microarray or biosensor and a kit comprising at least one of such SOMAmers comprising a fluorescent label.