Its dimensions are those of a cylinder 1.0-2.0 micrometers long, with radius about 0.5 micrometers. Another gram-negative rod, less metabolically independent than E. coli , is Hemophilus influenzae , which has half the length and diameter. Bacteria such as mycoplasma, which have a more modest metabolic capability, are even smaller. If the environment of the culture of microorganisms is rich, the growth rate will be fast relative to the growth rate in minimal medium. The cytoplasm, which contains the soluble proteins and the protein synthesis apparatus, is the dominant space-filling aspect of the microbial cell.
Ribosomes dominate the cytoplasm and are major space-occupying cellular elements.
Compared to the cytoplasmic proteins and the ribosomes, cellular ingredients that do not occupy much volume are the DNA, the cell wall, and the membranes with their component transport systems. Nucleoids are composed of compacted DNA neutralized with either positive ions or basic proteins. The numbers and kinds of genes expressed at any one time are determined by the composition of the environment. Enzymes are repressed unless needed, induced when needed. If one is thinking about organisms living in a biochemically rich soup or organisms like parasitic bacteria that live inside cells and draw on the processes and contents of inhabited cells, the organism could be smaller than if it were a free-living cell that makes all its own substance from scratch.