carotenoids are associated with several important biological functions as antenna pigments in photosynthesis or protectives against oxidative stress. Occasionally they were also discussed as part of the cold adaptation mechanism of bacteria. For two Staphylococcus xylosus strains we demonstrated an increased content of staphyloxanthin and other carotenoids after growth at 10 °C but no detectable carotenoids after grow at 30 °C. By in vivo measurements of generalized polarization and anisotropy with two different probes Laurdan and TMA-DPH we detected a strong increase in membrane order with a simultaneous increase in membrane fluidity at low temperatures accompanied by a broadening of the phase transition. Increased carotenoid concentration was also correlated with an increased resistance of the cells against freeze-thaw stress. In addition, the fatty acid profile showed a moderate adaptation to low temperature by increasing the portion of anteiso-branched fatty acids. The suppression of carotenoid synthesis abolished the effects observed and thus confirmed the causative function of the carotenoids in the modulation of membrane parameters. A differential transcriptome analysis demonstrated the upregulation of genes involved in carotenoid syntheses under low temperature growth conditions. The presented data suggests that upregulated synthesis of carotenoids is a constitutive component in the cold adaptation strategy of Staphylococcus xylosus and combined with modifications of the fatty acid profile constitute the adaptation to grow under low temperature conditions. Carotenoids represent a large group and comprise at least 800 described compounds 1,2 which are synthesized from isoprene units by phototrophic and chemoorganotrophic prokaryotes, plants, fungi, and algae 3. Most carotenoids consist of eight isoprene units resulting in a C40 backbone and usually display β-cyclisation 1,4. Only a few chemoorganotrophic bacteria are capable of producing C30, C45 or C50 carotenoids 4,5. Such a rare C30 carotenoid is produced by staphylococci 6,7. As lipophilic compounds, carotenoids are located in the cell membrane, but their orientation inside the membrane may vary depending on their chemical structure and on the thickness of the membrane 8,9. They perform important biological functions such as light harvesting as antenna pigments 10,11 , provide protection against oxidative stress 5,12 , provide protection from ultraviolet radiation 13,14 , and stabilization of pigment proteins 15. In addition to the functions mentioned above, the involvement of carotenoids in cold adaptation was suspected 16-18. We assumed that carotenoids might have a similar function in regulating membrane fluidity as sterols such as cholesterol or ergosterol in eukaryotic cells. They increase membrane order with concurrent maintenance of lateral lipid motility, which results in a liquid-ordered membrane state 19. Similar mechanisms were previously described for the bacterial sterol-like hopanoids 20,21 and isoprenoid quinones 22. In vitro studies wit...