The article discusses the physiology of *Escherichia coli* in Luria-Bertani (LB) broth. LB broth supports *E. coli* growth up to an optical density at 600 nm (OD600) of 7. However, steady-state growth stops at an OD600 of 0.3, when the growth rate slows and cell mass decreases. This is due to the depletion of a utilizable carbon source, which is catabolizable amino acids, not sugars. The LB broth is a rich medium that supports fast growth and good yields for many species. It is composed of tryptone, yeast extract, NaCl, and distilled water. The study used the wild-type *E. coli* K-12 strain MG1655 to investigate the physiology of *E. coli* in LB broth. The doubling time during steady-state growth in LB broth was 20 minutes. Growth becomes unbalanced at an OD600 of around 0.3, indicating the end of steady-state growth. The study found that the average cell mass decreases at this point, indicating a decreasing growth rate. The physiological state of *E. coli* cells in LB broth varies significantly in the OD660 range of 0.3 to 1.0. The study also found that LB broth contains a low concentration of fermentable sugars, which are depleted at an OD600 of 0.3, forcing cell metabolism to switch to using amino acids as a carbon source. The study also found that LB broth contains a high concentration of catabolizable amino acids, which are the principal carbon sources in LB broth. The study concluded that steady-state growth in LB broth ends surprisingly early, at an OD600 of around 0.3, and is followed by an extended period during which the growth rate and average cell mass decrease gradually. When *E. coli* stops growing, at an OD600 of around 7, utilizable carbon sources are exhausted, although inorganic nutrients remain plentiful. LB broth lacks recoverable sugars but has high concentrations of catabolizable amino acids. These are probably depleted sequentially during the postexponential phase of growth. During this period, the physiological state of the cells varies constantly. Furthermore, even using the same strain at the same OD600, if the latter is above 0.3 the cells' physiological state will not necessarily be reproducible in different experiments since the composition of LB broth can vary.The article discusses the physiology of *Escherichia coli* in Luria-Bertani (LB) broth. LB broth supports *E. coli* growth up to an optical density at 600 nm (OD600) of 7. However, steady-state growth stops at an OD600 of 0.3, when the growth rate slows and cell mass decreases. This is due to the depletion of a utilizable carbon source, which is catabolizable amino acids, not sugars. The LB broth is a rich medium that supports fast growth and good yields for many species. It is composed of tryptone, yeast extract, NaCl, and distilled water. The study used the wild-type *E. coli* K-12 strain MG1655 to investigate the physiology of *E. coli* in LB broth. The doubling time during steady-state growth in LB broth was 20 minutes. Growth becomes unbalanced at an OD600 of around 0.3, indicating the end of steady-state growth. The study found that the average cell mass decreases at this point, indicating a decreasing growth rate. The physiological state of *E. coli* cells in LB broth varies significantly in the OD660 range of 0.3 to 1.0. The study also found that LB broth contains a low concentration of fermentable sugars, which are depleted at an OD600 of 0.3, forcing cell metabolism to switch to using amino acids as a carbon source. The study also found that LB broth contains a high concentration of catabolizable amino acids, which are the principal carbon sources in LB broth. The study concluded that steady-state growth in LB broth ends surprisingly early, at an OD600 of around 0.3, and is followed by an extended period during which the growth rate and average cell mass decrease gradually. When *E. coli* stops growing, at an OD600 of around 7, utilizable carbon sources are exhausted, although inorganic nutrients remain plentiful. LB broth lacks recoverable sugars but has high concentrations of catabolizable amino acids. These are probably depleted sequentially during the postexponential phase of growth. During this period, the physiological state of the cells varies constantly. Furthermore, even using the same strain at the same OD600, if the latter is above 0.3 the cells' physiological state will not necessarily be reproducible in different experiments since the composition of LB broth can vary.