LuxS Mediates Iron-Dependent Biofilm Formation, Competence, and Fratricide in Streptococcus pneumoniae

Abstract

During infection, Streptococcus pneumoniae exists mainly in sessile biofilms rather than in planktonic form, except during sepsis. The capacity to form biofilms is believed to be important for nasopharyngeal colonization as well as disease pathogenesis, but relatively little is known about the regulation of this process. Here, we investigated the effect of exogenous iron [Fe(III)] as well as the role of luxS (encoding S-ribosylhomocysteine lyase) on biofilm formation by S. pneumoniae D39. Fe(III) strongly enhanced biofilm formation at concentrations of >50 M, while Fe(III) chelation with deferoxamine was inhibitory. Importantly, Fe(III) also upregulated the expression of luxS in wild-type D39. A luxS-deficient mutant (D39luxS) failed to form a biofilm, even with Fe(III) supplementation, whereas a derivative overexpressing luxS (D39luxS؉) exhibited enhanced biofilm formation capacity and could form a biofilm without added Fe(III). D39luxS exhibited reduced expression of the major Fe(III) transporter PiuA, and the cellular [Fe(III)] was significantly lower than that in D39; in contrast, D39luxS؉ had a significantly higher cellular [Fe(III)] than the wild type. The release of extracellular DNA, which is an important component of the biofilm matrix, also was directly related to luxS expression. Similarly, genetic competence, as measured by transformation frequency as well as the expression of competence genes comD, comX, comW, cglA, and dltA and the murein hydrolase cbpD, which is associated with fratricide-dependent DNA release, all were directly related to luxS expression levels and were further upregulated by Fe(III). Moreover, mutagenesis of cbpD blocked biofilm formation. We propose that competence, fratricide, and biofilm formation are closely linked in pneumococci, and that luxS is a central regulator of these processes. We also propose that the stimulatory effects of Fe(III) on all of these parameters are due to the upregulation of luxS expression, and that LuxS provides for a positive Fe(III)-dependent amplification loop by increasing iron uptake.

Heterologous Expression of sahH Reveals That Biofilm Formation Is Autoinducer-2-independent in Streptococcus sanguinis but Is Associated with an Intact Activated Methionine Cycle

“…This has been demonstrated for S. mutans and is consistent with the findings of the present study (47). An influence on expression of late competence genes was also found in LuxS mutants of S. pneumonia (22,62); however, in those studies the direct influence of AI-2 was not investigated.…”

Quorum-Sensing Systems LuxS/Autoinducer 2 and Com Regulate Streptococcus pneumoniae Biofilms in a Bioreactor with Living Cultures of Human Respiratory Cells

“…We and others have independently discovered that a quorumsensing (QS) system, LuxS/autoinducer 2 (AI-2), regulates S. pneumoniae biofilm production on abiotic surfaces (i.e., glass and polystyrene), a finding consistent with the observation that this system regulates pneumococcal persistence in the mouse nasopharynx (18)(19)(20). In this signaling network, the enzyme LuxS synthesizes AI-2, which is required for QS-regulated gene expression (21).…”

Pneumococcal Interactions with Epithelial Cells Are Crucial for Optimal Biofilm Formation and Colonization In Vitro and In Vivo

“…There is a vast literature of pneumococcal biofilm studies on abiotic surfaces that show the importance of virulence factors, autoinducers, extracellular DNA, and extracellular matrix production in pneumococcal biofilm formation in vitro (15,50,56,59,(67)(68)(69). The in vivo relevance of these studies is, however, not clear, as most of these studies were conducted over too short a time span to actually produce biofilms, defined as aggregated bacterial communities producing extracellular matrix.…”