Biology Glossary - X

X adhesion
 An adhesin, distinct from both mannose-sensitive adhesins and P fimbriae, which occurs on a proportion of pyelonephritic strains of Escherichia coli ; some of these ‘Xspecific’ strains of E. coli bind specifically to neuraminyl a- (2 → 3)-galactosides.

Gram-negative endosymbiotic bacteria which occur in the cytoplasm of certain strains of Amoeba proteus; they do not confer a killer characteristic. Amoebae may be dependent on their endosymbionts. X-bacteria are sensitive to e.g. chloramphenicol, and are killed at temperatures above ca. 26°C.

X-band (in ESR)
see electron spin resonance.

X body (virol.)
Syn. viroplasm.

X factor
A growth factor required for aerobic growth by certain Haemophilus spp; the requirement is usually satisfiedmby haemin or by protoporphyrin IX (essential for the synthesis of e.g. cytochromes and catalase). Some strains (e.g. of H. aegyptius) lack ferrochelatase and thus cannot insert iron into protoporphyrin IX; these strains therefore require haemin. ‘X factor’ is available in media containing lysed RBCs (e.g. chocolate agar).


X-linked lymphoproliferative syndrome
see xlp syndrome.

An apparatus for cell disruption similar in principle to the hughes press.

X-ray spectrochemical analysis
 A method used e.g. to determine the in vivo location of certain types of atom within a specimen. When the electron beam irradiates a specimen, atoms of a given element give rise to a characteristic X-ray emission spectrum; the specimen, as a whole, also responds with a non-specific background continuum radiation of X-rays (Bremsstrahlung).The emitted X-rays are analysed (in terms of either energy or wavelength) and their source(s) within the specimen can be mapped with an accuracy which depends e.g. on the diameter and energy of the electron beam and on the thickness of the specimen.

 see ionizing radiation.

[Gk. xylon, wood]
The tube-shaped, nonliving portion of the vascular system in plants that carries water and minerals from the roots to the rest of the plant.

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