Coordination of capsule assembly and cell wall biosynthesis in Staphylococcus aureus

Structural basis for the coordination of cell division with the synthesis of the bacterial cell envelope

Bacteria are surrounded by a complex cell envelope made up of one or two membranes supplemented with a layer of peptidoglycan (PG). The envelope is responsible for the protection of bacteria against lysis in their oft-unpredictable environments and it contributes to cell integrity, morphology, signaling, nutrient/small-molecule transport, and, in the case of pathogenic bacteria, host-pathogen interactions and virulence. The cell envelope requires considerable remodeling during cell division in order to produce genetically identical progeny. Several proteinaceous machines are responsible for the homeostasis of the cell envelope and their activities must be kept coordinated in order to ensure the remodeling of the envelope is temporally and spatially regulated correctly during multiple cycles of cell division and growth. This review aims to highlight the complexity of the components of the cell envelope, but focusses specifically on the molecular apparatuses involved in the synthesis of the PG wall, and the degree of cross talk necessary between the cell division and the cell wall remodeling machineries to coordinate PG remodeling during division. The current understanding of many of the proteins discussed here has relied on structural studies, and this review concentrates particularly on this structural work.

MgrA Negatively Impacts Staphylococcus aureus Invasion by Regulating Capsule and FnbA

Virulence genes are regulated by a complex regulatory network in Staphylococcus aureus Some of the regulators are global in nature and affect many downstream genes. MgrA is a multiple-gene regulator that has been shown to activate genes involved in capsule biosynthesis and repress surface protein genes. The goal of this study was to demonstrate the biological significance of MgrA regulation of capsule and surface proteins. We found that strain Becker possessed one fibronectin-binding protein, FnbA, and that FnbA was the predominant protein involved in invasion of nonphagocytic HeLa cells. By genetic analysis of strains with different amounts of capsule, we demonstrated that capsule impeded invasion of HeLa cells by masking the bacterial cell wall-anchored protein FnbA. Using variants with different levels of mgrA transcription, we further demonstrated that MgrA negatively impacted invasion by activating the cap genes involved in capsule biosynthesis and repressing the fnbA gene. Thus, we conclude that MgrA negatively impacts cell invasion of S. aureus Becker by promoting capsule and repressing FnbA.

¡vIVA la DivIVA!

Reproduction in the bacterial kingdom predominantly occurs through binary fission-a process in which one parental cell is divided into two similarly sized daughter cells. How cell division, in conjunction with cell elongation and chromosome segregation, is orchestrated by a multitude of proteins has been an active area of research spanning the past few decades. Together, the monumental endeavors of multiple laboratories have identified several cell division and cell shape regulators as well as their underlying regulatory mechanisms in rod-shaped Escherichia coli and Bacillus subtilis, which serve as model organisms for Gram-negative and Gram-positive bacteria, respectively. Yet our understanding of bacterial cell division and morphology regulation is far from complete, especially in noncanonical and non-rod-shaped organisms. In this review, we focus on two proteins that are highly conserved in Gram-positive organisms, DivIVA and its homolog GpsB, and attempt to summarize the recent advances in this area of research and discuss their various roles in cell division, cell growth, and chromosome segregation in addition to their interactome and posttranslational regulation.

Cell Wall Deficiency as a Coping Strategy for Stress

The cell wall is a surface layer located outside the cell membrane of almost all bacteria; it protects cells from environmental stresses and gives them their typical shape. The cell wall is highly conserved in bacteria and is the target for some of our best antibiotics. Surprisingly, some bacteria are able to shed their wall under the influence of stress, yielding cells that are cell-wall-deficient. Notably, wall-deficient cells are flexible and are able to maneuver through narrow spaces, insensitive to wall-targeting antibiotics, and capable of taking up and exchanging DNA. Moreover, given that wall-associated epitopes are often recognized by host defense systems, wall deficiency provides a plausible explanation for how some bacteria may hide in their host. In this review we focus on this paradoxical stress response, which provides cells with unique opportunities that are unavailable to walled cells.

Revisiting the regulation of the capsular polysaccharide biosynthesis gene cluster in Staphylococcus aureus

Capsular polysaccharide (CP) biosynthesis in Staphylococcus aureus is tightly controlled resulting in a heterogeneous phenotype within a population and CP being mainly detectable in nongrowing cells. Expression of the corresponding biosynthesis gene cluster is driven by one promoter element (P_cap ). Here, we demonstrate that P_cap contains a main SigB-dependent promoter. The SigB consensus motif overlaps with a previously described inverted repeat (IR) that is crucial for cap expression. The essentiality of the IR is derived from this region acting as a SigB binding site rather than as an operator site for the proposed cap activators RbsR and MsaB. Furthermore, P_cap contains an extensive upstream region harboring a weak SigA-dependent promoter and binding sites for cap repressors such as SaeR, CodY and Rot. Heterogeneous CP synthesis is determined by SigB activity and repressor binding to the upstream region. SigB dependency and regulation by the upstream repressors are also sufficient to explain the temporal gene expression pattern at the transcriptional level. However, CP synthesis remains growth phase-dependent even when transcription is rendered constitutive, suggesting additional posttranscriptional regulatory circuits. Thus, the interference of multiple repressors with SigB-dependent promoter activity as well as post-transcriptional mechanisms ensure the appropriate regulation of CP synthesis.