_Hydrogenomonas_ bacteria can be used for at least part of the astronauts’ diet. The washed bacteria have a mild taste and are being studied for their total energy content, protein and lipid digestibility, and vitamin content. Carbon and nitrogen balances, and respiratory quotient are to be determined in animals fed the bacteria as their sole food source. No toxic constituents have been discovered. Sonicated and cooked bacteria, when fed to white rats as 12 percent of the solids of a nutritionally balanced diet, were eaten readily and produced no ill effects. Net utilization of the protein appears to be somewhat lower than casein and about the same as legume proteins. The composition of _Hydrogenomonas eutropha_ is shown in table XI. The composition of the bacteria varies with the age and growth phase of the cells and with the medium and gas available. It is possible to modify the growth conditions to grow the type of bacteria desired for nutritive purposes. _Hydrogenomonas_ cells contain about 75 percent water. Of the dry weight, about 74 percent is protein, calculated as 6.25 times the nitrogen content. Table XI shows the amino acid composition to be comparable with other bacterial proteins, except for higher tryptophan and methionine values. Table XI—_Analysis of_ Hydrogenomonas eutropha _Cells Grown in Continuous Culture_ [From [ref.194]] ----------------------------------------------------------------- Constituent Percent by weight ----------------------------------------------------------------- Moisture 74.55 ----------------------------------------------------------------- Fat .44 ----------------------------------------------------------------- Ash 1.73 ----------------------------------------------------------------- Nitrogen 3.02 (wet) ------------------- 11.87 (dry) ----------------------------------------------------------------- Protein (N × 6.25) 18.90 (wet) ------------------- 74.26 (dry) ----------------------------------------------------------------- Amino acids (dry weight)⁸ ----------------------------------------------------------------- Alanine 4.47 ----------------------------------------------------------------- Arginine 3.41 ----------------------------------------------------------------- Aspartic acid 4.32 -----------------------------------------------------------------