Method

Supporting Information S1 Appendix p.4 top paragraph: "2. Mean bacterium mass in the colon-What fraction of colonic content is occupied by bacterial mass? What is the mean mass of a bacterium in the colon? The measurements of bacterial concentrations in the colon (table 2 BNID 112983), can be used to infer answers to those questions, provided two additional values: (1) the fraction of dried fecal mass that is dry bacteria and (2) the total water content in a bacterium. Overall dry mass fraction contributed by bacteria was directly measured to be 55% of fecal dry mass [primary source]. The dry mass percentage of cell mass varies for different types of bacteria [S1 Appendix refs 5,6] but can be assumed to be roughly equal to that of stool (29%), and thus the fraction of bacterial dry mass in dry feces is a good approximation to the fraction of bacterial mass in stool. Using the measured value of 4·10^11 bacteria per gram dry stool [primary source], [investigators] evaluate the average mass of bacteria in the Stephen and Cummings samples to be 4.6·10^-12 g (SEM 35%, CV 47%). Interestingly, this value for the average bacterial cell mass is several times higher than is usually taken for a model bacterium such as E. coli [BNID 101789 S1 Appendix refs 7,8]."