<P> Humans are colonized by many microorganisms; the traditional estimate is that the average human body is inhabited by ten times as many non-human cells as human cells, but more recent estimates have lowered that ratio to 3: 1 or even to approximately the same number . Some microbiota that colonize humans are commensal, meaning they co-exist without harming humans; others have a mutualistic relationship with their human hosts . Conversely, some non-pathogenic microbiota can harm human hosts via the metabolites they produce, like trimethylamine . Certain microbiota perform tasks that are known to be useful to the human host; the role of most resident microorganisms is not well understood . Those that are expected to be present, and that under normal circumstances do not cause disease, are sometimes deemed normal flora or normal microbiota . </P> <P> The Human Microbiome Project took on the project of sequencing the genome of the human microbiota, focusing particularly on the microbiota that normally inhabit the skin, mouth, nose, digestive tract, and vagina . It reached a milestone in 2012 when it published its initial results . </P> <P> Though widely known as flora or microflora, this is a misnomer in technical terms, since the word root flora pertains to plants, and biota refers to the total collection of organisms in a particular ecosystem . Recently, the more appropriate term microbiota is applied, though its use has not eclipsed the entrenched use and recognition of flora with regard to bacteria and other microorganisms . Both terms are being used in different literature . </P> <P> As of 2014, it was often reported in popular media and in the scientific literature that there are about 10 times as many microbial cells in the human body as there are human cells; this figure was based on estimates that the human microbiome includes around 100 trillion bacterial cells and that an adult human typically has around 10 trillion human cells . In 2014, the American Academy of Microbiology published a FAQ that emphasized that the number of microbial cells and the number of human cells are both estimates, and noted that recent research had arrived at a new estimate of the number of human cells--approximately 37.2 trillion, meaning that the ratio of microbial - to - human cells, if the original estimate of 100 trillion bacterial cells is correct, is closer to 3: 1 . In 2016, another group published a new estimate of the ratio being roughly 1: 1 (1.3: 1, with "an uncertainty of 25% and a variation of 53% over the population of standard 70 - kg males"). </P>

Where is bacteria located in the human body