Pneumococcal Competence Coordination Relies on a Cell-Contact Sensing Mechanism

Abstract

Bacteria have evolved various inducible genetic programs to face many types of stress that challenge their growth and survival. Competence is one such program. It enables genetic transformation, a major horizontal gene transfer process. Competence development in liquid cultures of Streptococcus pneumoniae is synchronized within the whole cell population. This collective behavior is known to depend on an exported signaling Competence Stimulating Peptide (CSP), whose action generates a positive feedback loop. However, it is unclear how this CSP-dependent population switch is coordinated. By monitoring spontaneous competence development in real time during growth of four distinct pneumococcal lineages, we have found that competence shift in the population relies on a self-activated cell fraction that arises via a growth time-dependent mechanism. We demonstrate that CSP remains bound to cells during this event, and conclude that the rate of competence development corresponds to the propagation of competence by contact between activated and quiescent cells. We validated this two-step cell-contact sensing mechanism by measuring competence development during co-cultivation of strains with altered capacity to produce or respond to CSP. Finally, we found that the membrane protein ComD retains the CSP, limiting its free diffusion in the medium. We propose that competence initiator cells originate stochastically in response to stress, to form a distinct subpopulation that then transmits the CSP by cell-cell contact.

Antibiotic-induced cell chaining triggers pneumococcal competence by reshaping quorum sensing to autocrine signaling

“…It is interesting to note that our observations reconcile observations made across different laboratories concerning the dynamics of pneumococcal competence. Recently we showed that unencapsulated strains are less synchronized in competence development, which would explain the difference between the findings reported by (Prudhomme et al 2016) and our previous study (Moreno-Gámez et al 2017). Strains R6 and D39 do not form chains under normal growth conditions, but this difference could be explained by the presence of a polysaccharide Figure 6.…”

“…Neighbor communication between cells is crucial in the modulation of the relationship between the number of cells and the CSP concentration, rather than the heterogeneity of the CSP production by single cells (Figure 7, left). However, several factors, such as pH or antibiotics, can modify the rates at which single cells produce and sense CSP (Moreno-Gámez et al 2017;Prudhomme et al 2016). Our results suggest that chain formation by the presence of AZT or CLA, modifies the balance between CSP production and sensing, increasing the self-sensing of CSP between cells in the same chain.…”

“…5× SDS-PAGE loading buffer was added to each and to synchronize competence once a critical CSP (green dots) threshold is reached. In addition, self-sensing CSP (blue arrows) plays a role, as part of the CSP pool is retained to the producer (Prudhomme et al 2016). Diplococci producing more CSP than average (highlighted in red) contribute to the increase of the extracellular CSP pool.…”

“…One of the genes regulated by ComE, comX, encodes a sigma factor, which activates the genes required for DNA repair, DNA uptake, and transformation. CSP can be retained by producing cells (Prudhomme et al, 2016), but CSP also diffuses and can induce competence in neighboring cells (Christie et al, 2016;Håvarstein et al, 1995;Moreno-Gámez et al, 2017). Other environmental factors such as pH, oxygen, phosphate and diffusibility of the growth medium also influence competence development (Chen and Morrison, 1987;Claverys et al, 2002;Echenique et al, 2000;Yang et al, 2010).…”

Mutation of the Streptococcus gordonii Thiol-Disulfide Oxidoreductase SdbA Leads to Enhanced Biofilm Formation Mediated by the CiaRH Two-Component Signaling System

“…First, it could be a direct effect of the CSP protein. Charged peptides can bind to the cell surface or to matrix components resulting in altered adhesion [44], and pneumococcal CSP was recently shown to be retained at the cell surface by ComD [45]. A second possible mechanism is that CSP activates Com signalling, which in turn affects expression of other factors that mediate biofilm formation.…”