Mobilization of Bacteriocins during Competence in Streptococci

Abstract

Many streptococci have evolved the ability for natural genetic competence. Recent studies have uncovered regulatory links between competence and the production of antimicrobial peptides called bacteriocins in multiple streptococcal species. This reveals a broadly distributed strategy among streptococci to exploit bacteriocin-mediated killing during competence for adaptive gain.

The streptococci are a group of Gram-positive bacteria that includes many human commensals and clinically significant pathogens. Many streptococcal species naturally reside in competitive, polymicrobial niches within the bodies of humans and other mammals. To exploit the large pools of genetic material in these niches, streptococci have evolved the ability to undergo natural genetic transformation, a process in which cells take up extracellular DNA and incorporate it into their genomes via recombination.

Transformation requires that cells enter a special physiological state termed competence. In the streptococci, competence is a tightly regulated transient state [1]. The master streptococcal regulator of competence is ComX (SigX or σ^X), an alternative sigma factor that in association with RNA polymerase activates transcription of genes involved in competence and transformation. In all known cases the production of ComX is regulated by a quorum sensing-like system, but the specific details differ from species to species. In most clades of streptococci, including the Pyogenic, Mutans, Bovis, and Salivarius groups, ComX is regulated by the ComRS system. Here, ComS is a peptide pre-pheromone that is processed and exported via an unknown mechanism to yield XIP (ComX-inducing peptide). XIP is then re-imported into the cell via a non-specific oligopeptide import system, after which it associates with the transcription factor ComR to induce expression of ComS (forming a positive feedback loop) as well as ComX. In contrast, the Mitis group streptococci regulate ComX using the ComCDE system. Here, ComC is another peptide pre-pheromone (with no homology to ComS) that is processed and exported via a C39 peptidase domain-containing ABC transporter (classically ComAB) to yield CSP (competence-stimulating peptide). Instead of re-import, CSP binds to and activates the membrane-bound receptor histidine kinase ComD. ComD then phosphorylates its cognate response regulator ComE, which proceeds to induce expression of ComCDE and ComAB (forming a positive feedback loop) as well as ComX.

A recent paper by Mignolet et al. [2] reports the characterization of the ComRS regulatory system in Streptococcus salivarius. The mechanistic details of ComRS regulation of competence in this bacterium were found to fit nicely into the canonical ComRS paradigm described above. Critically, the authors also found that ComR directly upregulates the blp bacteriocin locus. Many streptococci produce bacteriocins – antimicrobial peptides that target closely related bacteria – to inhibit competing strains and species. The blp bacteriocin locus is canonically regulated by the BlpHR two-component system (a paralog of ComDE) which responds to the pheromone BlpC. A series of reports have described links between competence and bacteriocin regulation in different streptococci. In S. mutans, ComX directly upregulates expression of BlpR (formerly named ComE) [3]. In the Mitis group member Streptococcus pneumoniae, ComE directly upregulates the expression of BlpC and its canonical transporter BlpAB while ComAB secretes BlpC in addition to ComC/CSP [4, 5]. In fellow Mitis group member Streptococcus gordonii, microarray analysis shows that bacteriocin genes are upregulated in response to CSP [6], though the mechanism remains undefined.

The newest report from Mignolet et al. [2] continues this trend and introduces a new mode of competence-bacteriocin regulation. In S. salivarius the competence regulatory machinery directly controls expression of the bacteriocins, in contrast to the situation in S. mutans and S. pneumoniae, where competence indirectly controls bacteriocin production through its effects on the BlpHR system (Fig. 1). The “short-circuiting” of competence-bacteriocin signaling in S. salivarius may be an adaptation unique to that species. Mignolet et al. show that even in the closely related species S. thermophilus, competence-induced bacteriocin expression most likely still works through BlpHR.

Figure 1.

Competence-Bacteriocin Cross-regulation in Streptococci. Left: phylogeny of the major species groups of the genus Streptococcus [7]. Branch lengths are not to scale. Right: regulation of competence and competence-to-bacteriocin crosstalk in (from top to bottom) S. mutans [3], S. salivarius [2], and S. pneumoniae [4, 5]. In all cases, the alternative sigma factor ComX is the master regulator of competence. The mechanism of ComS processing and export remains unknown. Note that in S. mutans, MIP, BlpH, and BlpR were formerly named CSP, ComD, and ComE, respectively. XIP: ComX-inducing peptide; CSP: competence-stimulating peptide; MIP: mutacin-inducing peptide.

The coupling of bacteriocin production to competence has been suggested to be a method to increase access to DNA during competence through bacteriocin-mediated lysis of target cells. An alternative proposed role for bacteriocins in this context is to inhibit competing strains during competence, perhaps as a general defense mechanism for competent cells to protect themselves while they are in a physiologically vulnerable state. The various competence-bacteriocin signaling mechanisms found in different species may reflect different prioritization for bacteriocin use in each species. In S. mutans and S. pneumoniae, bacteriocin production can be delayed as much as 90 minutes following the onset of competence. Therefore, these two species may couple bacteriocin production with competence for use mainly in a defensive role; it would be inefficient for cells to enter competence only then to be forced to wait long periods of time for bacteriocin-mediated DNA release. The case for this is especially strong with respect to S. pneumoniae, in which competence lasts for no more than 30 minutes [1] and so would already have ended by the time bacteriocin production begins. Meanwhile, the direct ComR-mediated regulation of bacteriocins in S. salivarius should allow for bacteriocin production concomitant with competence development. This suggests that S. salivarius has perhaps adapted its regulatory pathways to favor the use of bacteriocins for predation and DNA acquisition. At the same time, losing competence-independent, BlpHR-mediated bacteriocin regulation prevents S. salivarius from deploying bacteriocins in other contexts. Ostensibly, the benefits of the short circuit outweigh the costs for S. salivarius but not for other species that maintain BlpHR control of bacteriocin production.

The recruitment of bacteriocins during competence is a recurring theme among the streptococci. The fact that the Mitis group completely replaced ComRS-driven regulation of competence with the non-homologous ComCDE system but nonetheless preserved the regulatory link between competence and bacteriocins argues for the positive adaptive value of such a link. However, it is also evident that different streptococci have evolved distinct mechanisms to facilitate this regulatory crosstalk. This likely represents each species’ efforts to fine-tune the properties of its own crosstalk mechanism (e.g. strength and kinetics) to provide it the greatest benefit in its respective ecological niche.