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A radio-resistant bacterium, designated as strain WMA-LM9, was isolated from desert soil. 16S rRNA gene sequencing indicated that the bacterium belongs to genus Deinococcus with maximum similarity to Deinococcus radiopugnans. Deinococcus sp. strain WMA-LM9 was found to be resistant to a ultraviolet (UV) dose of 5 x 10(3) J/m(2), hydrogen peroxide (50 mM) and mitomycin C (10 mu g/ml). A carotenoid pigment was extracted using chloroform/methanol/acetone (7:5:3) and purified by high-performance liquid chromatography on a C-18 analytical column. The compound was characterised as mono-esterified astaxanthin by H-1, C-13 nuclear magnetic resonance and mass spectrometry. It was tested for antioxidant activity, total flavonoids and phenolic content, radioprotective potential in correlation to the prevention of protein oxidation and DNA strand breaks in vitro. The carotenoid pigment showed a very potent antioxidant activity and significantly stronger scavenging ability against superoxides, with an IC50 (concentration causing 50% inhibition of the desired activity) of 41.6 mu g/ml. The total phenolic and flavonoid contents were 12.1 and 7.4 mu g in terms of gallic acid and quercetin equivalents per milligram of dried mass, respectively. astaxanthin also showed a higher inhibitory action against oxidative damage to collagen, elastin and bovine serum albumin than did beta-carotene. The carotenoid also inhibited breaks to DNA strands, as indicated by the results of the DNA damage prevention assay. We conclude that astaxanthin from Deinococcus sp. strain WMA-LM9 has protective effects against radiation-mediated cell damage, and it also protects cellular protein and DNA against oxidative stress and other anti-oxidant activities.