<P> Numerous genera of bacteria and fungi live on and in the human body as part of its natural flora . The fungal genus Aspergillus is capable of living under considerable environmental stress, and thus is capable of colonising the upper gastrointestinal tract where relatively few examples of the body's gut flora can survive due to highly acidic or alkaline conditions produced by gastric acid and digestive juices . While Aspergillus normally produces no symptoms, in individuals who are immunocompromised or suffering from existing conditions such as Tuberculosis, a condition called Aspergillosis can occur, in which populations of Aspergillus grow out of control . </P> <P> Staphylococcus aureus, a common bacterial species, is known best for its numerous pathogenic strains that can cause numerous illnesses and conditions . However, many strains of S. aureus are metabiotic commensals, and are present on roughly 20 to 30% of the human population as part of the skin flora . S. aureus also benefits from the variable ambient conditions created by the body's mucous membranes, and as such can be found in the oral and nasal cavities, as well as inside the ear canal . Other Staphylococcus species including S. warneri, S. lugdunensis and S. epidermidis, will also engage in commensalism for similar purposes . </P> <P> Whether the relationship between humans and some types of gut flora is commensal or mutualistic is still unanswered . </P> <P> Some biologists argue that any close interaction between two organisms is unlikely to be completely neutral for either party, and that relationships identified as commensal are likely mutualistic or parasitic in a subtle way that has not been detected . For example, epiphytes are "nutritional pirates" that may intercept substantial amounts of nutrients that would otherwise go to the host plant . Large numbers of epiphytes can also cause tree limbs to break or shade the host plant and reduce its rate of photosynthesis . Similarly, phoretic mites may hinder their host by making flight more difficult, which may affect its aerial hunting ability or cause it to expend extra energy while carrying these passengers . </P>

Which of the following relationship is an example of commensalism