Reported prices in the literature on the number of cells in the body differ by orders of magnitude and are very seldom supported by any measurements or calculations. human body? Beyond order of magnitude statements that give no main research or uncertainty estimates, very few detailed estimates have been performed (the one exception [1] is TET2 usually discussed below). Similarly, the ubiquitous statements regarding 1014C1015 bacteria residing in our body track back to an aged back-of-the-envelope calculation [2C4]. The aim of this study is usually to critically revisit 67469-75-4 IC50 former estimates for the number of human and bacterial cells in the human body. We give up-to-date detailed estimates where the calculation logic and sources are fully documented and uncertainty ranges are produced. By updating the cell counts in the body, we also revisit the 10:1 value that has been so thoroughly repeated as to accomplish the status of an established common knowledge fact [4]. This ratio was criticized recently in a letter to the journal [5], but an alternate detailed estimate that will give concrete values and estimate the uncertainty range is usually needed. Here, we provide an account of the methodologies employed hitherto for cell count and revise past estimates. Doing so, we repeat and reflect on the assumptions in previous back-of-the-envelope calculations, also known as Fermi problems. We find such estimates as effective sanity inspections and a way to improve our quantitative understanding in biology. A major part of the available books used in the derivation of human cell figures was based on cohorts of exclusively or mostly men, and as we use these sources, our analysis starts with adult men. As discussed below, relatively moderate quantitative differences apply for women due to changes in characteristic body mass, blood volume, and the genital microbiota. For our analysis, we used the definition of the standard research man as given in the books [6] as: “Reference Man is usually defined as being between 20C30 years of age, weighing 70 kg, is usually 170 cm in height. Our analysis revisits the estimates for the number of microbial cells, human cells, and their ratio in the body of such a 67469-75-4 IC50 standard man. We begin our analysis by revisiting the number of bacteria through surveying earlier sources, comparing counts in different body organs and finally focusing on the content of the colon. We then estimate the total number of human cells in the body, comparing calculations using a “representative” cell size to aggregation by cell type. We then contrast the cell number distribution by tissue type to the mass distribution. In closing, we revisit the ratio of bacterial to human cells and evaluate the effect of gender, age, and obesity. Results Origin of Prevalent Claims in the Literature on the Number of Bacterial Cells in Humans Microbes are found throughout the human body, mainly on the external and internal surfaces, including the gastrointestinal tract, skin, saliva, oral mucosa, and conjunctiva. Bacteria overwhelmingly outnumber eukaryotes and archaea in the human microbiome by 2C3 orders of magnitude [7,8]. We therefore sometimes operationally refer to the microbial cells in the human body as bacteria. The diversity in locations where microbes reside in the body makes estimating their overall number daunting. Yet, once their quantitative distribution shows the dominance of the colon as discussed below, the problem becomes much simpler. 67469-75-4 IC50 The vast majority of the bacteria reside in the colon, with previous estimates of about 1014 bacteria [2], followed by the skin, which is estimated to harbor ~1012 bacteria [9]. As we showed recently [4], all papers regarding the number of bacteria in 67469-75-4 IC50 the human gastrointestinal tract that gave reference to the value stated could be traced to a single back-of-the-envelope.