Transients are acquired from the environment and

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sites, which they occupy indefinitely. Transients are acquired from the environment and establish themselves briefly but tend to be excluded by competition from residents or by the host's innate or immune defense mechanisms. The term carrier state is used when potentially pathogenic organisms are involved, although its implication of risk is not always justified. For example, Streptococcus pneumoniae, a cause of pneumonia, and Neisseria meningitidis, a cause of meningitis, may be isolated from the throat of 5% to 40% of healthy people. Whether these bacteria represent transient flora, resident flora, or carrier state is largely semantic. The possibility that their presence could be the prelude to disease is impossible to determine in advance. It is important for students of medical microbiology and infectious disease to understand the role of the normal flora because of its significance both as a defense mechanism against infection and as a source of potentially pathogenic organisms. It is also important for physicians to know the sites and composition of flora to avoid interpretive confusion between normal flora species and pathogens when interpreting laboratory culture results. The following excerpt indicates that the English poet W.H. Auden understood the need for balance between the microbial flora and its host. He was influenced by an article in Scientific American about the flora of the skin. On this day tradition allots to taking stock of our lives, my greetings to all of you, Yeasts, Bacteria, Viruses, Aerobics and Anaerobics: A Very Happy New Year to all for whom my ectoderm is as middle earth to me.
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For creatures your size I offer a free choice of habitat, so settle yourselves in the zone that suits you best, in the pools of my pores or the tropical forests of arm-pit and crotch, in the deserts of my fore-arms, or the cool woods of my scalp. Build colonies: I will supply adequate warmth and moisture, the sebum and lipids you need, on condition you never do me annoy with your presence, but behave as good guests should, not rioting into acne or athlete's-foot or a boil. —W.H. Auden, Epistle to a Godson Origin and Nature The healthy fetus is sterile until the birth membranes rupture. During and after birth, the infant is exposed to the flora of the mother's genital tract and to other organisms in the environment. During the infant's first few days of life, the flora reflects chance exposure to organisms that can colonize particular sites in the absence of competitors. Subsequently, as the infant is exposed to a broader range of organisms, those best adapted to colonize particular sites become predominant. Thereafter, the flora generally resembles that of other individuals in the same age group and cultural milieu. Local physiologic and ecologic conditions determine the nature of the flora. These conditions are sometimes highly complex, differing from site to site, and sometimes vary with age. Conditions include the amounts and types of nutrients available, pH, oxidation–reduction potentials, and resistance to local antibacterial substances such as bile and lysozyme. Many bacteria have adhesin-mediated affinity for
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