We found also that bacteria which are not host-associated tend to have low abundances and low diversity. This was also observed by other studies on similarly low abundance, soil-associated bacteria  (Fig. 6B). It is of course possible that they have been misclassified; however, they could also be actively transmitted from the environment to the human host. These observations relate mainly to the environmental factors that are likely to affect the bacterial community, but also the impact of host-associated factors has been observed. For example the population of Vibrio cholerae is influenced by the health of the aquatic environment and host-associated factors, such as age, sex, seasonality, and contraceptive usage . These factors could, in an equilibrium state, affect the stability and resilience of the population.]
A possible explanation for the observed fact that the most diverse community tends to be that of open environments is that some parts of the human body are more niche-like and more stable than the rest of the body. The skin for example, which has been thought of as a stable surface and the part of the body with the highest bacterial load, could be such a stable niche. Another observation is that the airway, which is generally considered not to be a stable environment, has a diversity comparable to the gut. Here other factors than the home equilibrium of bacteria may be important, such as infectious agents. The fact that the airway stays relatively free of bacterial contamination following mechanical preparation is probably the reason for the low diversity of the bacteria in the cultivation-based analysis. However, it has also been shown that the commensal community of the airway is not stable but changes over time from a stable and relatively diverse to a highly variable and very stable community .
Host-associated bacteria have been associated with a number of human diseases, such as periodontal disease, diarrhea, gastrointestinal disorders, and respiratory infections. The saliva of healthy people has been found to contain a diverse array of oral bacteria, and many of them have been found to play a key role in the symbiotic relationship between the human host and the oral microbial community . d2c66b5586