Comparative study of the abundance of various bacterial morphotypes in an eutrophic freshwater environment determined by AODC and TEM

Transmission electron microscopy (TEM) and epifluorescence microscopy were used to obtain comparative measurements of total bacterial counts, and to e numerate abundances of various bacterial morphotypes in an eutrophic freshw ater habitat. Although particulate matter would have been expected to inter fere with counting by obscuring large areas of the electron microscope grid s, estimates of total bacterial abundance made by TEM were on average 1.2 t imes greater than those obtained using the acridine orange direct counting method (AODC). However, the precision of the AODC method was greater than t hat for TEM, with a coefficient of variation (C.V.) of 4.0% versus 8.8%, re spectively. The total bacterial abundance ranged from 1.1 to 3.2 x 10(6) ml (-1). As was the case for total bacterial density, the numbers of rod- and vibrio-shaped cells were lower when counted in the epifluorescence microsco pe, indicating the presence of potential starvation forms or ultramicrobact eria. Greatest variations in counts made by TEM and AODC were found for fil amentous and coccoid bacteria. Counts of filamentous bacteria made by AODC were only about half of those detected by TEM. In contrast, cocci were on a verage 1.5 times greater when counted by AODC compared to TEM estimates. Bo th counting differences were probably caused by the morphology and low dens ity of filamentous and coccoid bacteria (1.7 and 1.4 x 10(5) ml(-1), respec tively), which led to an uneven distribution on polycarbonate filters as we ll as on electron microscope grids. Besides, cocci might easily be mistaken for large viral particles when counted by AODC. Hence, the study supports the use of TEM over AODC for obtaining accurate estimates of total bacteria l abundance and especially bacterial morphotypes in natural waters. (C) 200 0 Published by Elsevier Science B.V. All rights reserved.