Methods and compositions are provided for the selective lysis of eukaryotic cells and the separation of microbial cells. Blood cells and/or other eukaryotic cells in a sample, may be selectively lysed by adding, to the sample, a blood lysis reagent including saponin and an alkaline buffer, and optionally sodium polyanethole sulfonate and a non-ionic surfactant, thereby forming a mixture, and agitating the mixture. Microbial cells in the mixture may then be separated, for example, using a separation method such as centrifugation or filtration, and optionally detected or cultured in growth media. Blood lysis reagent compositions are provided that are suitable for preserving the intactness of microbial cells upon mixing with the sample. In example embodiments in which the sample is a blood sample, the blood lysis reagent composition may be selected to avoid or reduce the presence of visible blood debris upon centrifugation or filtration.