All microbial nutrients are compounds constructed from the chemical elements. However, just a handful of elements dominate living systems and are essential: hydrogen (H), oxygen (O), carbon (C), nitrogen (N), phosphorus (P), and sulfur (S).
Besides water, which makes up 70-80% of the wet weight of a microbial cell, cells consist primarily of macromolecules, proteins, nucleic acids, lipids, and polysaccharides; the building blocks (monomers, 单体) of these macromolecules are the amino acids, nucleotides, fatty acids, and sugars, respectively. Proteins dominate the macromolecular composition of a cell, making up 55% of total cell dry weight. Interestingly, as important as DNA is to a cell, it contributes a very small percentage of a cell‘s dry weight; RNA is far more abundant (大量的).