Sv. Avery, CESIUM ACCUMULATION BY MICROORGANISMS - UPTAKE MECHANISMS, CATION COMPETITION, COMPARTMENTALIZATION AND TOXICITY, Journal of industrial microbiology, 14(2), 1995, pp. 76-84
The continued release of caesium radioisotopes into the environment ha
s led to a resurgence of interest in microbe-Cs interactions. Caesium
exists almost exclusively as the monovalent cation Cs+ in the natural
environment Although Cs+ is a weak Lewis acid thai exhibits a low tend
ency to form complexes with Ligands, its chemical similarity to the bi
ologically essential alkali cation Kt facilitates high levels of metab
olism-dependent intracellular accumulation. Microbial Cs+ (K+) uptake
is generally mediated by monovalent cation transport systems located o
n the plasma membrane. These differ widely in specificity for alkali c
ations and consequently microorganisms display large differences in th
eir ability to accumulate Cs+; Cs+ appears to have an equal or greater
affinity than K+ for transport in certain microorganisms. Microbial C
s+ accumulation is markedly influenced by the presence of external cat
ions, e.g. K+, Na+, NH4+ and H+, and is generally accompanied by an ap
proximate stoichiometric exchange for intracellular K+. However, stimu
lation of growth of Ki-starved microbial cultures by Cs+ is limited an
d it has been proposed that it is not the presence of Cs+ in cells tha
t is growth inhibitory but rather the resulting loss of K+. Increased
microbial tolerance to Cs+ may result from sequestration of Cs+ in vac
uoles or changes in the activity and/or specificity of transport syste
ms mediating Cs+ uptake. The precise intracellular target(s) for Cs+ i
nduced toxicity has yet to be clearly defined, although certain intern
al structures, e.g. ribosomes, become unstable in the presence of Csand Cs+ is known to substitute poorly for K+ in the activation of many
K+-requiring enzymes.