Nanobacteria, Ultramicrobacteria and Starvation Forms: A Search for the Smallest Metabolizing Bacterium

Very small forms of bacteria have been reported from marine and freshwater systems as well as from soils, subsurface terrestrial environments' and more recently from samples of kidney stones. Also, such small cells could be obtained in the laboratory during starvation experiments, indicating that bacteria which survive periods of nutrient deprivation manifest a decrease of cell size. Even though the various reports brought about poorly defined designations for small bacteria, they have stimulated the discussion on how small a living bacterium could be. The information derived from the literature concerning starvation forms, ultramicrobacteria and nanobacteria is analysed in the light of own field and laboratory observations. It could be shown that despite conceptual shortcomings and problems with definitions of what is meant by "small", starvation forms and ultramicrobacteria are clearly distinguishable according to physiological characteristics, which could not be achieved for nanobacteria or nanobes. Furthermore, it is documented that ultramicrobacteria are not the smallest procaryotes and that the size of bacterial starvation forms are much closer to the calculated minimal sizes required to ensure independent viable life. A bacterial cell which is growing and dividing needs to be large enough to accommodate DNA and RNA, enzymes for replication transcription and translation, solvent for ubstances as well as a minimum set of proteins and plasamtic space to run the operations. Many authors assume that this requires a cell with a diameter not smaller than 200 nm and with a volume between 0.014 and 0.06 μm3. Most cells with diameters equal or below 0.2 μm are rods while cocci are numerically of minor importance in natural aquatic systems. It seems that the rod morphotype has more potential to produce viable cells with minimal volumes than spherical morhotypes. This supports the assumption that not only size but also cell shape is important to achieve functional minimum cell volumes. Many cell parameters can be estimated with relatively high precision, but one should remember that absolute calibration is still not possible. Even if we assume that there are errors involved in most biometric measurements, there is still a trend which indicates that rod shaped cells can function with diameters below 0.2 μm and cell volumes well below 0.02 μm3. Such dimensions could be an indication that the lower size limit of a viable bacterium may be close to the size of the s