DETERMINATION OF BACTERIAL-CELL NUMBER AND CELL-VOLUME BY MEANS OF FLOW-CYTOMETRY, TRANSMISSION ELECTRON-MICROSCOPY, AND EPIFLUORESCENCE MICROSCOPY

Comparative measurements of bacterial total counts and volumes by flow cytometry (FCM), transmission electron microscopy (TEM), and epifluor escence microscopy (EFM), were undertaken during a four week mesocosm experiment. Total counts of bacteria measured by TEM, EFM, and FCM wer e in the range of 1.10(6)-6.10(6) cells ml(-1), 1.10(6)-3.10(6) cells ml(-l), and 5.10(5) cells ml(-1) respectively. The mean volume of the bacterial community, measured by means of EFM and TEM, increased from 0.12-0.15 mu m(3) at the start of the experiment to 0.39-0.53 mu m(3) at the end. Generally, there was good agreement between the two method s and regression analyses gave r = 0.87 (p < < 0.01) for cell volume a nd r = 0.97 (p < < 0.01) for cell number. DAPI stained bacteria with v olumes less than 0.2 mu m(3) were not detected by flow cytometry and t hese counts were generally an order of magnitude lower than counts mad e by TEM and EFM. For samples where the mean bacterial cell volume was larger than 0.3 mu m(3), all three methods were in agreement both wit h respect to counts and volume estimates.