Determination of total protein content of bacterial cells by SYPRO staining and flow cytometry

An assay has been developed for measuring protein biomass of marine plankto nic bacteria by flow cytometry. The method was calibrated by using five spe cies of Bacteria (an Arcobacter sp., a Cytophaga sp., an Oceanospirillum sp ., a Pseudoalteromonas sp., and a Vibrio sp.) recently isolated from seawat er samples and grown in culture at different temperatures. The intensity of SYPRO-protein fluorescence of these bacteria strongly correlated with thei r total protein content, measured by the bicinchoninic acid method to be in the range of 60 to 330 fg of protein cell(-1) (r(2) = 0.93, n = 34). Accor ding to the calibration, the mean biomass of planktonic bacteria from the N orth Sea in August 1998 was 24 fg of protein cell(-1).