Rapid and simple method for double staining of bacteria with 4',6-diamidino-2-phenylindole and fluorescein isothiocyanate-labeled antibodies.

Bacteria were either heat fixed on microscope slides or filtered with 0.2 micron-pore-size Nuclepore filters. The samples were stained with 4',6-diamidino-2-phenylindole (DAPI) for total staining and with polyvalent rabbit antibodies and fluorescein isothiocyanate-conjugated swine anti-rabbit antibodies for specific staining. By switching between two different optical filter packages in the microscope, only one sample was needed for determining both total and specific counts of bacteria. False-positive counts and other artifacts that occur with antibody staining were easily distinguished when individual fluorescent particles were checked for DAPI fluorescence. The method for applying the general stain to membrane filters was performed quickly and simply by using a DAPI-soaked polypropylene filter that lay beneath the Nuclepore filter which collected the sample.