Abstract
BackgroundPili were recently recognized in Streptococcus pneumoniae and implicated in the virulence of this bacterium, which led to the proposal of using these antigens in a future pneumococcal vaccine. However, pili were found to be encoded by the rlrA islet that was not universally distributed in the species. We examined the distribution of the pilus islet, using the presence of the rlrA gene as a marker for the locus, among a collection of invasive isolates recovered in Portugal and analyzed its association with capsular serotypes, clusters defined by the pulsed-field gel electrophoretic profiles (PFGE) and multilocus sequence types.ResultsOnly a minority of the isolates were positive for the presence of the rlrA gene (27%). There was a high correspondence between the serotype and the presence or absence of rlrA (Wallace coefficient, W = 0.778). In particular, there was an association between the presence of rlrA and the vaccine serotypes 4, 6B, 9V and 14 whereas the gene was significantly absent from other serotypes, namely 1, 7F, 8, 12B and 23F, a group that included a vaccine serotype (23F) and serotype 1 associated with enhanced invasiveness. Even within serotypes, there was variation in the presence of the pilus islet between PFGE clones and a higher Wallace coefficient (W = 0.939) indicates that carriage of the islet is a clonal property of pneumococci. Analysis of rlrA negative isolates revealed heterogeneity in the genomic region downstream of the rfl gene, the region where the islet is found in other isolates, compatible with recent loss of the islet in some lineages.ConclusionThe pilus islet is present in a minority of pneumococcal isolates recovered from human invasive infections and is therefore not an essential virulence factor in these infections. Carriage of the pilus islet is a clonal property of pneumococci that may vary between isolates expressing the same serotype and loss and acquisition of the islet may be ongoing.
Highlights
Pili were recently recognized in Streptococcus pneumoniae and implicated in the virulence of this bacterium, which led to the proposal of using these antigens in a future pneumococcal vaccine
Distribution of the rlrA islet among invasive pneumococci Only a small portion (27%) of our invasive isolates was rlrA positive and a total of 355 (73%) isolates were found to lack the pilus islet by Southern blot hybridization and PCR amplification of the rlrA gene (Additional file 1: Southern hybridization of a representative set of isolates with the rlrA gene probe.)
All isolates yielded fragments larger than 14 kb compatible with the presence of the entire pilus locus, confirming that the presence of the rlrA gene is a good marker for the presence of the entire pilus islet
Summary
Pili were recently recognized in Streptococcus pneumoniae and implicated in the virulence of this bacterium, which led to the proposal of using these antigens in a future pneumococcal vaccine. Pilus-like surface structures have been described in Gram-positive bacteria like Corynebacterium spp., Actinomyces spp. and several streptococcal species [1], but only recently were pili identified in the major pathogenic species of the genus: Streptococcus pyogenes, Lancefield (page number not for citation purposes). In S. pneumoniae pili are encoded by the rlrA pathogenicity islet, a 14.2 kb region composed of 7 genes encoding a putative transcriptional regulator (RlrA), 3 LPXTG surface proteins with weak homology to microbial surface components recognizing adhesive matrix molecules – MSCRAMMs (RrgA, RrgB and RrgC) and 3 sortases (SrtB, SrtC and SrtD) [4,5,6]. Later studies confirmed that the RlrA protein acted as a transcription factor recognizing several promoters within the rlrA islet and showed it to be essential for wild-type levels of expression of the pili structural genes and associated sortases [6]. The product of the mgrA gene, located outside the rlrA islet, was shown to act as a transcriptional repressor of the islet genes, including rlrA, being responsible for the silencing of the locus in the absence of RlrA [8]
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