Choline starvation induces the gene licD2 in Streptococcus pneumoniae

Polysaccharide Capsule Composition of Pneumococcal Serotype 19A Subtypes Is Unaltered among Subtypes and Independent of the Nutritional Environment

Serotype 19A strains have emerged as a cause of invasive pneumococcal disease after the introduction of the 7-valent pneumococcal conjugate vaccine (PCV7), and serotype 19A has now been included in the recent 13-valent vaccine (PCV13). Genetic analysis has revealed at least three different capsular serotype 19A subtypes, and nutritional environment-dependent variation of the 19A capsule structure has been reported. Pneumococcal vaccine effectiveness and serotyping accuracy might be impaired by structural differences in serotype 19A capsules. We therefore analyzed the distribution of 19A subtypes collected within a Swiss national surveillance program and determined capsule composition under different nutritional conditions with high-performance liquid chromatography (HPLC), gas chromatography-mass spectrometry (GC-MS), and nuclear magnetic resonance (NMR) spectroscopy. After the introduction of PCV7, a significant relative increase of subtype 19A-II and decrease of 19A-I occurred. Chemical analyses showed no difference in the composition as well as the linkage of 19A subtype capsular saccharides grown in defined and undefined growth media, which is consistent with a trisaccharide repeat unit composed of rhamnose, N-acetyl-mannosamine, and glucose. In summary, our study suggests that no structural variance dependent of the nutritional environment or the subtype exists. The serotype 19A subtype shift observed after the introduction of the PCV7 can therefore not be explained by selection of a capsule structure variant. However, capsule composition analysis of emerging 19A clones is recommended in cases where there is no other explanation for a selective advantage, such as antibiotic resistance or loss or acquisition of other virulence factors.

Fine-tuning of choline metabolism is important for pneumococcal colonization

The human pathogen Streptococcus pneumoniae (the pneumococcus) is rare in having a strict requirement for the amino alcohol choline, which decorates pneumococcal teichoic acids. This process relies on the lic locus, containing the lic1 and lic2 operons. These operons produce eight proteins that import and metabolize choline, generate teichoic acid precursors and decorate these with choline. Three promoters control expression of lic operons, with Plic1P1 and Plic1P2 controlling lic1 and Plic2 controlling lic2. To investigate the importance of lic regulation for pneumococci, we assayed the activity of transcriptional fusions of the three lic promoters to the luciferase reporter gene. Plic1P1 , whose activity depends on the response regulator CiaR, responded to fluctuations in extracellular choline, with activity increasing greatly upon choline depletion. We uncovered a complex regulatory mechanism controlling Plic1P1 , involving activity driven by CiaR, repression by putative repressor LicR in the presence of choline, and derepression upon choline depletion mediated by LicC, a choline metabolism enzyme. Finally, the ability to regulate Plic1P1 in response to choline was important for pneumococcal colonization. We suggest that derepression of Plic1P1 upon choline depletion maximizing choline internalization constitutes an adaptive response mechanism allowing pneumococci to optimize growth and survival in environments where choline is scarce.

Biosynthesis of teichoic acids in Streptococcus pneumoniae and closely related species: lessons from genomes

The cell wall of Streptococcus pneumoniae contains an unusually complex wall teichoic acid (WTA), which has identical repeating units as the membrane-anchored lipoteichoic acid (LTA). Both polymers share a common cytoplasmic pathway of precursor synthesis, but several TA enzymes have remained elusive. Bioinformatic analysis of the genome of various pneumococcal strains, including choline-independent mutant strains, has allowed us to identify the missing TA genes. We present here the deduced complete pathways of WTA and LTA synthesis in S. pneumoniae and point to the variations occurring in different pneumococcal strains and in closely related species such as Streptococcus oralis and Streptococcus mitis.

Versatility of choline metabolism and choline-binding proteins in Streptococcus pneumoniae and commensal streptococci

The pneumococcal choline-containing teichoic acids are targeted by cholinebinding proteins (CBPs), major surface components implicated in the interaction with host cells and bacterial cell physiology. CBPs also occur in closely related commensal species, Streptococcus oralis and Streptococcus mitis, and many strains of these species contain choline in their cell wall. Physiologically relevant CBPs including cell wall lytic enzymes are highly conserved between Streptococcus pneumoniae and S. mitis. In contrast, the virulence-associated CBPs, CbpA, PspA and PcpA, are S. pneumoniae specific and are thus relevant for the characteristic properties of this species.

The CiaRH system of Streptococcus pneumoniae prevents lysis during stress induced by treatment with cell wall inhibitors and by mutations in pbp2x involved in beta-lactam resistance

The two-component signal-transducing system CiaRH of Streptococcus pneumoniae plays an important role during the development of beta-lactam resistance in laboratory mutants. We show here that a functional CiaRH system is required for survival under many different lysis-inducing conditions. Mutants with an activated CiaRH system were highly resistant to lysis induced by a wide variety of early and late cell wall inhibitors, such as cycloserine, bacitracin, and vancomycin, and were also less susceptible to these drugs. In contrast, loss-of-function CiaRH mutants were hypersusceptible to these drugs and were apparently unable to maintain a stationary growth phase in normal growth medium and under choline deprivation as well. Moreover, disruption of CiaR in penicillin-resistant mutants with an altered pbp2x gene encoding low-affinity PBP2x resulted in severe growth defects and rapid lysis. This phenotype was observed with pbp2x genes containing point mutations selected in the laboratory and with highly altered mosaic pbp2x genes from penicillin-resistant clinical isolates as well. This documents for the first time that PBP2x mutations required for development of beta-lactam resistance are functionally not neutral and are tolerated only in the presence of the CiaRH system. This might explain why cia mutations have not been observed in penicillin-resistant clinical isolates. The results document that the CiaRH system is required for maintenance of the stationary growth phase and for prevention of autolysis triggered under many different conditions, suggesting a major role for this system in ensuring cell wall integrity.