Image Analysis of Bacterial Cell Size and Diversity

Abstract

Bacteria as they occur in nature are very small, compared with bacteria cultured under enriched conditions of the laboratory, i.e., in vitro. In general, planktonic bacterial cells in the natural environment are less than 1 µm in length and, therefore, biovolumes of planktonic bacteria are usually less than 0.2 – 0.3 µm³. Planktonic bacteria are very small organisms, even among planktonic constituents of aquatic ecosystems. They constitute less than 1/100 of the biovolume of small phytoplankton and less than 1/10 of that of microflagellates (Fig. 1). Compared with large phytoplankton, cell size differences reach more than thousands-fold. However, as planktonic bacteria are plentiful in water, in the order of 10⁵ to 10⁷ per ml (xcVanEs and Meyer-Reil, 1982), their overall biovolume in aquatic ecosystems can be significant. In marine systems, small bacteria, with a diameter of ca. 0.2 – 0.6 µm usually predominate, but, even among these bacteria, significant variation in size exists. Therefore, the bacterial biomass of a given population cannot be estimated accurately from a roughly estimated mean value, even if the diameters range from 0.2 to 0.6 µm, since such differences can lead to a ten-fold increase in biovolume. When bacteria are observed under an epifluorescence microscope, very large cells are often found, even in samples of pelagic marine waters. Thus, it is necessary to determine the cell size of bacteria precisely. The development of a personal computer-assisted image analyzer, connected to an epifluorescent microscope via a charge-coupled device (CCD) camera, has enabled the size of small bacterial cells to be measured automatically (xcSieracki et al., 1985; xcBjørnsen, 1986).