Chapter 1 Infection

Organisms of the normal flora may have a symbiotic

Info iconThis preview shows pages 4–6. Sign up to view the full content.

View Full Document Right Arrow Icon
Organisms of the normal flora may have a symbiotic relationship that benefits the host or may simply live as commensals with a neutral relationship to the host. A parasitic relationship that injures the host would not be considered "normal," but in most instances not enough is known about the organism–host interactions to make such distinctions. Like houseguests, the members of the normal flora may stay for highly variable periods. Residents are strains that have an established niche at one of the many body 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.
Background image of page 4

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full Document Right Arrow Icon
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
Background image of page 5
Image of page 6
This is the end of the preview. Sign up to access the rest of the document.

{[ snackBarMessage ]}

Page4 / 12

Organisms of the normal flora may have a symbiotic...

This preview shows document pages 4 - 6. Sign up to view the full document.

View Full Document Right Arrow Icon
Ask a homework question - tutors are online