ACCURACY OF EPIFLUORESCENCE MICROSCOPY COUNTS FOR DIRECT ESTIMATES OFBACTERIAL NUMBERS

The aim of this study was to determine the extent and source of inaccu racies for total counts of microorganisms by filtration followed by ep ifluorescent microscopy. The extent of these inaccuracies were determi ned by comparing the microscopic method with viable plate counts while their possible sources were investigated using flow cytometry and flu orescent microbeads. One factor responsible for these inaccuracies was found to be the microscope conversion factor which did not take into account the total filter surface area on which the bacteria were impac ted. Approximately a two-fold underestimation was observed when the fi ltration area rather than the effective area over which the cells are distributed were compared. Thus it is recommended that the latter be a ccurately measured when calculating the conversion factor in order to reduce the inaccuracy of direct microscopic counts.