Comparison of metabolic networks of an uninfected and a MS2 phage-infected Escherichia coli cell. Thick flux arrows represent high reaction fluxes, whereas the thinner flux arrows represent low reaction fluxes. Biosynthesis of amino acids is upregulated in infected cells. NADPH is a pre-cursor that is required in the biosynthesis of most of the amino acids. The pentose phosphate pathway provides NADPH and is therefore upregulated in infected cells. This up-regulation is due to the diversion of a major portion of glucose-6-phosphate (g6p) to the pentose phosphate pathway instead of the glycolytic pathway. The glycolytic pathway is not affected because the loss of g6p is made up by the gain in the amounts of the by-products of the pentose phosphate pathway, fructose-6-phosphate and glyceraldehyde 3-phosphate, which are fed back into the glycolytic pathway.