We applied transmission electron microscopy and densitometric image analysis to measure the cell volume (V) and dry weight (DW) of single bacterial cells. The system was applied to measure the DW ofEscherichia coli DSM 613 at different growth phases and of natural bacterial assemblages of two lakes, Piburger See and Gossenköllesee. We found a functional allometric relationship between DW (in femtograms) and V (in cubic micrometers) of bacteria (DW = 435 · V 0.86); i.e., smaller bacteria had a higher ratio of DW to V than larger cells. The measured DW of E. coli cells ranged from 83 to 1,172 fg, and V ranged from 0.1 to 3.5 μm3(n = 678). Bacterial cells from Piburger See and Gossenköllesee (n = 465) had DWs from 3 fg (V = 0.003 μm3) to 1,177 fg (V = 3.5 μm3). Between 40 and 50% of the cells had a DW of less than 20 fg. By assuming that carbon comprises 50% of the DW, the ratio of carbon content to Vof individual cells varied from 466 fg of C μm−3 forVs of 0.001 to 0.01 μm3 to 397 fg of C μm−3 (0.01 to 0.1 μm3) and 288 fg of C μm−3 (0.1 to 1 μm3). Exponentially growing and stationary cells of E. coli DSM 613 showed conversion factors of 254 fg of C μm−3 (0.1 to 1 μm3) and 211 fg of C μm−3 (1 to 4 μm3), respectively. Our data suggest that bacterial biomass in aquatic environments is higher and more variable than previously assumed from volume-based measurements.

Thymic nurse cells (TNC), a paradigmatic cell type of cortical epithelium, are large lymphoid-epithelial cell complexes of thymocytes enclosed within vacuoles lined by the epithelial cell membrane. TNC express major histocompatibility complex (MHC) class I and class II molecules on their surface and vacuole-lining membranes at high density and it was suggested that TNC provide an optimal microenvironment for positive selection of T cells. In this report we present electron microscopical data demonstrating that chicken TNC display morphological structures of exocytosis previously shown for hormone-secreting cells. In TNC, however, exocytosis is restricted to the capillary cleft between the epithelial cell and engulfed thymocytes. Thus, besides physical contact between the epithelial cell and enclosed thymocytes, TNC may additionally influence the development of thymocytes through release of soluble factors in a restricted microenvironment. By employing the 3-(2,4-dinitroanilino)-3'-amino-N-methyl-propylamine technique which at the ultrastructural level detects acidic organelles involved in processing of antigens presented by MHC class II molecules, we also show that TNC contain acidic compartments similar to classical antigen-presenting cells, i.e. early and late endosomes and lysosomes, albeit in a lower amount than in thymic dendritic cells. This fact provides evidence that TNC not only are capable of antigen presentation but also possess the intracellular machinery for antigen processing.

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Eighteen fungal strains were isolated from water and soil samples and tested for their ability to enrich chromium. The microorganism with the highest enrichment capacity, a zygomycete (Mucor hiemalis MP/92/3/4), was chosen for detailed investigations. Some basic tests such as the pH-dependence, the kinetics of the enrichment and the metal selectivity were carried out with the two most frequent oxidation states of chromium, the trivalent cation (Cr 3+) and the hexavalent anion (CRO42-). With Cr 3+ the enrichment showed a saturation kinetic reaching 70% of the maximum capacity after about 30 min, whereas with CrO42-a linear time course with a much lower metal enrichment was observed. The highest level of enrichment for Cr 3+ was observed at pH 5.5 (21.4 mg/g dry wt), and for CrO42-at pH 1 (4.3 mg/g dry wt). Investigations concerning the metal enrichment selectivity resulted in the following series of decreasing ion uptake: Cr 3+ > Cu 2+ > Pb 2+ > Ag + > AI 3+ > Co 2+ > Zn 2+ > Ni 2+ > Fe 2+ > Mo s+ > Cd 2+ > Cs + > Cr2072-> CrO42-> VO 3-, calculated on a molar basis. Trivalent chromium caused a staining of the outer cell wall region in transmission electron microscopy. The localization of chromium in the stained outer layers of the cell wall could be verified by electron energy loss spectroscopy. The enrichment of Cr a+ by M. hiemalis seemed to be mainly a passive biosorption to the cell wall, whereas for the uptake of CrO4 2-intracellular accumulation as well as biosorption is possible.