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.
X-bacteria
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-gal
See XGAL.
X-linked lymphoproliferative
syndrome
see xlp syndrome.
X-Press
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.
X-rays
see ionizing radiation.Xylem
[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.
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.
