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Biosynthetic reactions form a network of pathways that lead from 12 precursor metabo-lites (provided by the fueling reactions) to the many amino acids, nucleotides, sugars, amino sugars, fatty acids, and other building blocks needed for macromolecules (see Fig 3 – 1). In addition to the carbon precursors, large quantities of reduced nicotinamide adenine dinucleotide phosphate (NADPH), ATP, amino nitrogen, and some source of sul-fur are needed for biosynthesis of these building blocks. These pathways are similar in all species of living things, but bacterial species differ greatly as to which pathways they possess. Because all cells require the same building blocks, those that cannot be produced by a given cell must be obtained preformed from the environment. Nutritional require-ments of bacteria, therefore, differ from species to species and serve as an important prac-tical basis for laboratory identification.
Relatively few unique reactions in the domain of biosynthesis are present to form the basis for specific therapeutic attack on the microorganism rather than the host.
The effec-tiveness of sulfonamides and trimethoprim is one of these exceptional situations; many bacteria must synthesize folic acid rather than use it preformed from their environment, as human cells do, which renders these bacteria susceptible to agents that interfere with the biosynthesis of folic acid.
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