Abstract
BackgroundIn streptococci of the mitis group, competence for natural transformation is a transient physiological state triggered by competence stimulating peptides (CSPs). Although low transformation yields and the absence of a widespread functional competence system have been reported for Streptococcus mitis, recent studies revealed that, at least for some strains, high efficiencies can be achieved following optimization protocols. To gain a deeper insight into competence in this species, we used RNA-seq, to map the global CSP response of two transformable strains: the type strain NCTC12261T and SK321.ResultsAll known genes induced by ComE in Streptococcus pneumoniae, including sigX, were upregulated in the two strains. Likewise, all sets of streptococcal SigX core genes involved in extracellular DNA uptake, recombination, and fratricide were upregulated. No significant differences in the set of induced genes were observed when the type strain was grown in rich or semi-defined media. Five upregulated operons unique to S. mitis with a SigX-box in the promoter region were identified, including two specific to SK321, and one specific to NCTC12261T. Two of the strain-specific operons coded for different bacteriocins. Deletion of the unique S. mitis sigX regulated genes had no effect on transformation.ConclusionsOverall, comparison of the global transcriptome in response to CSP shows the conservation of the ComE and SigX-core regulons in competent S. mitis isolates, as well as species and strain-specific genes. Although some S. mitis exhibit truncations in key competence genes, this study shows that in transformable strains, competence seems to depend on the same core genes previously identified in S. pneumoniae.
Highlights
In streptococci of the mitis group, competence for natural transformation is a transient physiological state triggered by competence stimulating peptides (CSPs)
Competence for genetic transformation in pneumococcus occurs during a brief period of highly specialized protein synthesis (10–20 min), coordinated among many or all cells of an actively growing culture
The global transcriptome responses during competence have been studied in only three oral streptococci, S. mutans [29, 34, 35], S. sanguinis [31] and S. gordonii [36], and despite a few reports of competence in S. mitis [22, 30, 37], little or nothing is known about its regulation or response specificity in different strains
Summary
In streptococci of the mitis group, competence for natural transformation is a transient physiological state triggered by competence stimulating peptides (CSPs). In several streptococci intercellular coordination of gene expression mediated by peptide pheromones is associated with development of competence for transformation [1]. The pheromones activate a signal transduction pathway that regulates natural transformation. This physiological feature provides a selective advantage by allowing competent cells to acquire new characteristics, such as antibiotic resistance, by incorporation of DNA from other cells. In Streptococcus pneumoniae, natural competence is a tightly controlled transient state: it spontaneously arises during the early exponential growth phase at a certain cell density and reaches its peak after approximately 20 min, before it Salvadori et al BMC Genomics (2018) 19:453. Fratricide is not unique to pneumococci, as it has been demonstrated in closely related species such as Streptococcus mitis and Streptococcus oralis [10]
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