attenuation
1. (in medicine) A process of reducing the disease-producing ability of a microorganism. It can be achieved by chemical treatment, heating, drying, irradiation, by growing the organism under adverse conditions, or by serial passage through another organism. Attenuated bacteria or viruses are used for some vaccines.

2. (in mycology) The conversion by yeasts of carbohydrates to alcohol, as in brewing and wine and spirit production.

3. (in genetics) A mechanism for regulating gene expression in prokaryotes, observed especially in functional gene clusters (operons), such as the trp genes that encode enzymes responsible for synthesizing tryptophan in E. coli bacteria. Attenuation comes into play when the product of the enzymes (in this case tryptophan) is present to excess in the medium; transcription of the operon is drastically reduced, perhaps by as much as 90% of the maximum rate. This attenuation is thought to be caused by interaction of tryptophan in the medium with the initial part of the RNA transcript, encoded by an attenuator region upstream of the structural genes.

attenuation The reduction in amplitude or energy of a signal. Attenuation of seismic waves (seismic attenuation) occurs as a result of spherical divergence, absorption, energy losses at interfaces through reflection and refraction, and by internal scatterers. For electromagnetic waves see SKIN DEPTH.

attenuation The reduction in amplitude of a signal when it passes through a medium that dissipates its energy. It is usually measured in decibels (attenuation then being negative while gain is positive).

attenuation (ă-ten-yoo-ay-shŏn) n. reduction of the disease-producing ability (virulence) of a bacterium or virus so that it may be used for immunization.