Growth is central to life, shaping physiology, ecology, and evolution. In this talk, I discuss our efforts to elucidate the causes and consequences of bacterial growth across scales. Starting from resource allocation models and the molecular and energetic demands of protein synthesis, I first introduce how bacterial cells adjust their composition with environmental conditions and metabolic state to promote efficient protein synthesis and growth. I then discuss two proposed consequences of these dynamics: First, the adjustments of cell shape with changes in cell composition such that macromolecular densities within the cytoplasm and on the cell membrane remain within narrow physiological ranges. Our density maintenance theory can rationalize observed changes in the cell width of Escherichia coli across growth conditions. Second, the excretion of fermentation products that gut bacteria release to enable protein synthesis and growth within the anaerobic environment of the human large intestine. Most of these fermentation products are taken up by the host, making bacterial protein synthesis the major driver of bacteria-host interactions.