Abstract
Although carbon dioxide (CO) 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 levels it encounters during transmission, host colonization, and disease. To identify the molecular mechanisms that facilitate pneumococcal growth under CO-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-poor ambient air. With Tn-seq, we found mutations in two genes that were involved in S. pneumoniae adaptation to changes in CO availability. The gene pca, encoding pneumococcal carbonic anhydrase (PCA), was absolutely essential for S. pneumoniae growth under CO-poor conditions. PCA catalyzes the reversible hydration of endogenous CO to bicarbonate (HCO) and was previously demonstrated to facilitate HCO-dependent fatty acid biosynthesis. The gene folC that encodes the dihydrofolate/folylpolyglutamate synthase was required at the initial phase of bacterial growth under CO-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-poor growth conditions. In conclusion, S. pneumoniae adaptation to changes in CO availability involves the retention of endogenous CO and the preservation of intracellular long-chain polyglutamyl folate pools.
