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Lipoteichoic acid (LTA) is a cell surface polymer of Gram-positive bacteria. LTA participates in host-microbe interactions including modulation of host immune reactions. It was previously reported that the major human pathogenStreptococcus pneumoniaeand the closely related oral commensalsS. mitisandS. oralisproduce Type IV LTAs. Herein, using liquid chromatography/mass spectrometry (LC/MS)-based lipidomic analysis, we found that in addition to Type IV LTA biosynthetic precursors,S. mitis,S. oralis, andS. pneumoniaealso produce glycerophosphate (Gro-P)-linked dihexosyl-diacylglycerol (DAG), which is a biosynthetic precursor of Type I LTA. Mutants incdsAandpgsAproduce dihexosyl-DAG but lack (Gro-P)-dihexosyl-DAG, indicating that the Gro-P moiety is derived from phosphatidylglycerol (PG), whose biosynthesis requires these genes.S. mitis, but neitherS. pneumoniaenorS. oralis, encodes an ortholog of the PG-dependent Type I LTA synthase,ltaS. By heterologous expression analyses, we confirmed thatS. mitis ltaSconfers poly-(Gro-P) synthesis in bothEscherichia coliandStaphylococcus aureus, and thatS. mitis ltaScan rescue the severe growth defect of aS. aureus ltaSmutant. However, despite these observations, we do not detect a poly-(Gro-P) polymer inS. mitisusing an anti-Type I LTA antibody. Moreover, (Gro-P)-linked dihexosyl-DAG is still synthesized by aS. mitis ltaSmutant, demonstrating thatS. mitisLtaS does not catalyze the transfer of Gro-P from PG to dihexosyl-DAG. Finally, aS. mitis ltaSmutant has increased sensitivity to human serum, demonstrating thatltaSconfers a beneficial but currently undefined function inS. mitis. Overall, our results demonstrate thatS. mitis,S. pneumoniae, andS. oralisproduce a (Gro-P)-linked glycolipid via a PG-dependent,ltaS-independent mechanism.ImportanceLTA is an important cell wall component synthesized by Gram-positive bacteria. Disruption of LTA production can confer severe physiological defects and attenuation of virulence. We report here the detection of a biosynthetic precursor of Type I LTA, in addition to the previously characterized Type IV LTA, in the total lipid extracts ofS. pneumoniae,S. oralis, andS. mitis. Our results indicate that a novel mechanism is responsible for producing the Type I LTA intermediate. Our results are significant because they identify a novel feature ofS. pneumoniae,S. oralis, andS. mitisglycolipid biology. |
