The concept of microbial content of the lung is still controversial. What make this more complicated are controversial results obtaining from different methodologies about lung microbiome and the definition of "lung sterility". Lungs may have very low bacteria but are not completely germ-free. Bacteria are constantly entering from the upper respiratory tract, but are then quickly being cleared. We can find bacterial DNA in the lungs, but it is much harder to ask about living bacteria. Here, we propose that if there is any trafficking of the microorganisms in the lung, it should be a "Transient But Not Resident (TBNR)" model. So, we speculate a "Yin Yang model" for the lung immune system and TBNR. Despite beneficial roles of microbiome on the development of lung immune system, any disruption and alteration in the microbiota composition of upper and lower airways may trigger or lead to several diseases such as asthma, chronic obstructive pulmonary disease and mustard lung disease.
Keywords: Asthma; COPD; TBNR microbiome; lung immune system; lung microbiome; lung sterility; mustard.