Streptococcus pneumoniae Folate Biosynthesis Responds to Environmental CO 2 Levels

Abstract

ABSTRACT Although carbon dioxide (CO 2 ) is known to be essential for Streptococcus pneumoniae growth, it is poorly understood how this respiratory tract pathogen adapts to the large changes in environmental CO 2 levels it encounters during transmission, host colonization, and disease. To identify the molecular mechanisms that facilitate pneumococcal growth under CO 2 -poor conditions, we generated a random S. pneumoniae R6 mariner transposon mutant library representing mutations in 1,538 different genes and exposed it to CO 2 -poor ambient air. With Tn-seq, we found mutations in two genes that were involved in S. pneumoniae adaptation to changes in CO 2 availability. The gene pca , encoding pneumococcal carbonic anhydrase (PCA), was absolutely essential for S. pneumoniae growth under CO 2 -poor conditions. PCA catalyzes the reversible hydration of endogenous CO 2 to bicarbonate (HCO 3 − ) and was previously demonstrated to facilitate HCO 3 − -dependent fatty acid biosynthesis. The gene folC that encodes the dihydrofolate/folylpolyglutamate synthase was required at the initial phase of bacterial growth under CO 2 -poor culture conditions. FolC compensated for the growth-phase-dependent decrease in S. pneumoniae intracellular long-chain ( n > 3) polyglutamyl folate levels, which was most pronounced under CO 2 -poor growth conditions. In conclusion, S. pneumoniae adaptation to changes in CO 2 availability involves the retention of endogenous CO 2 and the preservation of intracellular long-chain polyglutamyl folate pools.