MapZ marks the division sites and positions FtsZ rings in Streptococcus pneumoniae

Molecular dissection of the chromosome partitioning protein RocS and regulation by phosphorylation

Chromosome segregation in bacteria is a critical process ensuring that each daughter cell receives an accurate copy of the genetic material during cell division. Active segregation factors, such as the ParABS system or SMC complexes, are usually essential for this process, but they are surprisingly dispensable in Streptococcus pneumoniae. Rather, chromosome segregation in S. pneumoniae relies on the protein Regulator of Chromosome Segregation (RocS), although the molecular mechanisms involved remain elusive. By combining genetics, in vivo imaging, and biochemical approaches, we dissected the molecular features of RocS involved in chromosome segregation. We investigated the respective functions of the three RocS domains, specifically the C-terminal amphipathic helix (AH), the N-terminal DNA-binding domain (DBD), and the coiled-coil domain (CCD) separating the AH and the DBD. Notably, we found that a single AH is not sufficient for membrane binding and that RocS requires prior oligomerization to interact with the membrane. We further demonstrated that this self-interaction was driven by the N-terminal part of the CCD. On the other hand, we revealed that the C-terminal part of the CCD corresponds to a domain of unknown function (DUF 536) and is defined by three conserved glutamines, which play a crucial role in RocS-mediated chromosome segregation. Finally, we showed that the DBD is phosphorylated by the unique serine-threonine kinase of S. pneumoniae StkP and that mimicking this phosphorylation abrogated RocS binding to DNA. Overall, this study offers new insights into chromosome segregation in Streptococci and paves the way for a deeper understanding of RocS-like proteins in other bacteria.IMPORTANCEBacteria have evolved a variety of mechanisms to properly segregate their genetic material during cell division. In this study, we performed a molecular dissection of the chromosome partitioning protein Regulator of Chromosome Segregation (RocS), a pillar element of chromosome segregation in S. pneumoniae that is also generally conserved in the Streptococcaceae family. Our systematic investigation sheds light on the molecular features required for successful pneumococcal chromosome segregation and the regulation of RocS by phosphorylation. In addition, our study also revealed that RocS shares functional domains with the Par protein, involved in an atypical plasmid segregation system. Therefore, we expect that our findings may serve to extend our understanding of RocS and RocS-like proteins while broadening the repertoire of partitioning systems used in bacteria.

GpsB Coordinates StkP Signaling as a PASTA Kinase Adaptor in Streptococcus pneumoniae Cell Division

StkP, the Ser/Thr protein kinase of the major human pathogen Streptococcus pneumoniae, monitors cell wall signals and regulates growth and division in response. In vivo, StkP interacts with GpsB, a cell division protein required for septal ring formation and closure, that affects StkP-dependent phosphorylation. Here, we report that although StkP has basal intrinsic kinase activity, GpsB promotes efficient autophosphorylation of StkP and phosphorylation of StkP substrates. Phosphoproteomic analyzes showed that GpsB is phosphorylated at several Ser and Thr residues. We confirmed that StkP directly phosphorylates GpsB in vitro and in vivo, with T79 and T83 being the major phosphorylation sites. In vitro, phosphoablative GpsB substitutions had a lower potential to stimulate StkP activity, whereas phosphomimetic substitutions were functional in terms of StkP activation. In vivo, substitutions of GpsB phosphoacceptor residues, either phosphoablative or mimetic, had a negative effect on GpsB function, resulting in reduced StkP-dependent phosphorylation and impaired cell division. The bacterial two-hybrid assay and co-immunoprecipitation of GpsB from cells with differentially active StkP indicated that increased phosphorylation of GpsB resulted in a more efficient interaction of GpsB with StkP. Our data suggest that GpsB acts as an adaptor that directly promotes StkP activity by mediating interactions within the StkP signaling hub, ensuring StkP recruitment into the complex and substrate specificity. We present a model that interaction of StkP with GpsB and its phosphorylation and dephosphorylation dynamically modulate kinase activity during exponential growth and under cell wall stress of S. pneumoniae, ensuring the proper functioning of the StkP signaling pathway.

An untargeted cultivation approach revealed Pseudogemmatithrix spongiicola gen. nov., sp. nov., and sheds light on the gemmatimonadotal mode of cell division: binary fission

Members of the phylum Gemmatimonadota can account for up to 10% of the phylogenetic diversity in bacterial communities. However, a detailed investigation of their cell biology and ecological roles is restricted by currently only six characterized species. By combining low-nutrient media, empirically determined inoculation volumes and long incubation times in a 96-well plate cultivation platform, we isolated two strains from a limnic sponge that belong to this under-studied phylum. The characterization suggests that the two closely related strains constitute a novel species of a novel genus, for which we introduce the name Pseudogemmatithrix spongiicola. The here demonstrated isolation of novel members from an under-studied bacterial phylum substantiates that the cultivation platform can provide access to axenic bacterial cultures from various environmental samples. Similar to previously described members of the phylum, the novel isolates form spherical appendages at the cell poles that were believed to be daughter cells resulting from asymmetric cell division by budding. However, time-lapse microscopy experiments and quantitative image analysis showed that the spherical appendages never grew or divided. Although the role of these spherical cells remains enigmatic, our data suggests that cells of the phylum Gemmatimonadota divide via FtsZ-based binary fission with different division plane localization patterns than in other bacterial phyla.

O-glycosylation of intrinsically disordered regions regulates homeostasis of membrane proteins in streptococci

Proteins harboring intrinsically disordered regions (IDRs) lacking stable secondary or tertiary structures are abundant across the three domains of life. These regions have not been systematically studied in prokaryotes. Our genome-wide analysis identifies extracytoplasmic serine/threonine-rich IDRs in several biologically important membrane proteins in streptococci. We demonstrate that these IDRs are O-glycosylated with glucose by glycosyltransferases GtrB and PgtC2 in Streptococcus pyogenes and Streptococcus pneumoniae, and with N-acetylgalactosamine by a Pgf-dependent mechanism in Streptococcus mutans. Absence of glycosylation leads to a defect in biofilm formation under ethanol-stressed conditions in S. mutans. We link this phenotype to the C-terminal IDR of a post-translocation secretion chaperone PrsA. O-glycosylation of the IDR protects this region from proteolytic degradation. The IDR length attenuates the efficiency of glycosylation and, consequently, the expression level of PrsA. Taken together, our data reveal that O-glycosylation of IDRs functions as a dynamic switch of protein homeostasis in streptococci.

Elongasome core proteins and class A PBP1a display zonal, processive movement at the midcell of Streptococcus pneumoniae

Ovoid-shaped bacteria, such as Streptococcus pneumoniae (pneumococcus), have two spatially separated peptidoglycan (PG) synthase nanomachines that locate zonally to the midcell of dividing cells. The septal PG synthase bPBP2x:FtsW closes the septum of dividing pneumococcal cells, whereas the elongasome located on the outer edge of the septal annulus synthesizes peripheral PG outward. We showed previously by sm-TIRFm that the septal PG synthase moves circumferentially at midcell, driven by PG synthesis and not by FtsZ treadmilling. The pneumococcal elongasome consists of the PG synthase bPBP2b:RodA, regulators MreC, MreD, and RodZ, but not MreB, and genetically associated proteins Class A aPBP1a and muramidase MpgA. Given its zonal location separate from FtsZ, it was of considerable interest to determine the dynamics of proteins in the pneumococcal elongasome. We found that bPBP2b, RodA, and MreC move circumferentially with the same velocities and durations at midcell, driven by PG synthesis. However, outside of the midcell zone, the majority of these elongasome proteins move diffusively over the entire surface of cells. Depletion of MreC resulted in loss of circumferential movement of bPBP2b, and bPBP2b and RodA require each other for localization and circumferential movement. Notably, a fraction of aPBP1a molecules also moved circumferentially at midcell with velocities similar to those of components of the core elongasome, but for shorter durations. Other aPBP1a molecules were static at midcell or diffusing over cell bodies. Last, MpgA displayed nonprocessive, subdiffusive motion that was largely confined to the midcell region and less frequently detected over the cell body.

Elongasome core proteins and class A PBP1a display zonal, processive movement at the midcell of Streptococcus pneumoniae

Ovoid-shaped bacteria, such as Streptococcus pneumoniae (pneumococcus), have two spatially separated peptidoglycan (PG) synthase nanomachines that locate zonally to the midcell of dividing cells. The septal PG synthase bPBP2x:FtsW closes the septum of dividing pneumococcal cells, whereas the elongasome located on the outer edge of the septal annulus synthesizes peripheral PG outward. We showed previously by sm-TIRFm that the septal PG synthase moves circumferentially at midcell, driven by PG synthesis and not by FtsZ treadmilling. The pneumococcal elongasome consists of the PG synthase bPBP2b:RodA, regulators MreC, MreD, and RodZ, but not MreB, and genetically associated proteins Class A aPBP1a and muramidase MpgA. Given its zonal location separate from FtsZ, it was of considerable interest to determine the dynamics of proteins in the pneumococcal elongasome. We found that bPBP2b, RodA, and MreC move circumferentially with the same velocities and durations at midcell, driven by PG synthesis. However, outside of the midcell zone, the majority of these elongasome proteins move diffusively over the entire surface of cells. Depletion of MreC resulted in loss of circumferential movement of bPBP2b, and bPBP2b and RodA require each other for localization and circumferential movement. Notably, a fraction of aPBP1a molecules also moved circumferentially at midcell with velocities similar to those of components of the core elongasome, but for shorter durations. Other aPBP1a molecules were static at midcell or diffusing over cell bodies. Last, MpgA displayed non-processive, subdiffusive motion that was largely confined to the midcell region and less frequently detected over the cell body.

Noninvasive Analysis of Peptidoglycan from Living Animals

The role of the intestinal microbiota in host health is increasingly revealed in its contributions to disease states. The host-microbiome interaction is multifactorial and dynamic. One of the factors that has recently been strongly associated with host physiological responses is peptidoglycan from bacterial cell walls. Peptidoglycan from gut commensal bacteria activates peptidoglycan sensors in human cells, including the nucleotide-binding oligomerization domain-containing protein 2. When present in the gastrointestinal tract, both the polymeric form (sacculi) and depolymerized fragments can modulate host physiology, including checkpoint anticancer therapy efficacy, body temperature and appetite, and postnatal growth. To utilize this growing area of biology toward therapeutic prescriptions, it will be critical to directly analyze a key feature of the host-microbiome interaction from living hosts in a reproducible and noninvasive way. Here we show that metabolically labeled peptidoglycan/sacculi can be readily isolated from fecal samples collected from both mice and humans. Analysis of fecal samples provided a noninvasive route to probe the gut commensal community including the metabolic synchronicity with the host circadian clock. Together, these results pave the way for noninvasive diagnostic tools to interrogate the causal nature of peptidoglycan in host health and disease.

Insights into the assembly and regulation of the bacterial divisome

The ability to split one cell into two is fundamental to all life, and many bacteria can accomplish this feat several times per hour with high accuracy. Most bacteria call on an ancient homologue of tubulin, called FtsZ, to localize and organize the cell division machinery, the divisome, into a ring-like structure at the cell midpoint. The divisome includes numerous other proteins, often including an actin homologue (FtsA), that interact with each other at the cytoplasmic membrane. Once assembled, the protein complexes that comprise the dynamic divisome coordinate membrane constriction with synthesis of a division septum, but only after overcoming checkpoints mediated by specialized protein-protein interactions. In this Review, we summarize the most recent evidence showing how the divisome proteins of Escherichia coli assemble at the cell midpoint, interact with each other and regulate activation of septum synthesis. We also briefly discuss the potential of divisome proteins as novel antibiotic targets.

Anchors: A way for FtsZ filaments to stay membrane bound

Most bacteria use the tubulin homolog FtsZ to organize their cell division. FtsZ polymers initially assemble into mobile complexes that circle around a ring-like structure at the cell midpoint, followed by the recruitment of other proteins that will constrict the cytoplasmic membrane and synthesize septal peptidoglycan to divide the cell. Despite the need for FtsZ polymers to associate with the membrane, FtsZ lacks intrinsic membrane binding ability. Consequently, FtsZ polymers have evolved to interact with the membrane through adaptor proteins that both bind FtsZ and the membrane. Here, we discuss recent progress in understanding the functions of these FtsZ membrane tethers. Some, such as FtsA and SepF, are widely conserved and assemble into varied oligomeric structures bound to the membrane through an amphipathic helix. Other less-conserved proteins, such as EzrA and ZipA, have transmembrane domains, make extended structures, and seem to bind to FtsZ through two separate interactions. This review emphasizes that most FtsZs use multiple membrane tethers with overlapping functions, which not only attach FtsZ polymers to the membrane but also organize them in specific higher-order structures that can optimize cell division activity. We discuss gaps in our knowledge of these concepts and how future studies can address them.

Protein coopted from a phage restriction system dictates orthogonal cell division plane selection in Staphylococcus aureus

The spherical bacterium Staphylococcus aureus, a leading cause of nosocomial infections, undergoes binary fission by dividing in two alternating orthogonal planes, but the mechanism by which S. aureus correctly selects the next cell division plane is not known. To identify cell division placement factors, we performed a chemical genetic screen that revealed a gene which we termed pcdA. We show that PcdA is a member of the McrB family of AAA+ NTPases that has undergone structural changes and a concomitant functional shift from a restriction enzyme subunit to an early cell division protein. PcdA directly interacts with the tubulin-like central divisome component FtsZ and localizes to future cell division sites before membrane invagination initiates. This parallels the action of another McrB family protein, CTTNBP2, which stabilizes microtubules in animals. We show that PcdA also interacts with the structural protein DivIVA and propose that the DivIVA/PcdA complex recruits unpolymerized FtsZ to assemble along the proper cell division plane. Deletion of pcdA conferred abnormal, non-orthogonal division plane selection, increased sensitivity to cell wall-targeting antibiotics, and reduced virulence in a murine infection model. Targeting PcdA could therefore highlight a treatment strategy for combatting antibiotic-resistant strains of S. aureus.

Non-invasive Analysis of Peptidoglycan from Living Animals

The role of the intestinal microbiota in host health is increasingly revealed in its contributions to disease states. The host-microbiome interaction is multifactorial and dynamic. One of the factors that has recently been strongly associated with host physiological responses is peptidoglycan from bacterial cell walls. Peptidoglycan from gut commensal bacteria activate peptidoglycan sensors in human cells, including the Nucleotide-binding oligomerization domain containing protein 2 (NOD2). When present in the gastrointestinal tract, both the polymeric form (sacculi) and de-polymerized fragments can modulate host physiology, including checkpoint anticancer therapy efficacy, body temperature and appetite, and postnatal growth. To leverage this growing area of biology towards therapeutic prescriptions, it will be critical to directly analyze a key feature of the host-microbiome interaction from living hosts in a reproducible and non-invasive way. Here we show that metabolically labeled peptidoglycan/sacculi can be readily isolated from fecal samples collected from both mice and humans. Analysis of fecal samples provided a non-invasive route to probe the gut commensal community including the metabolic synchronicity with the host circadian clock. Together, these results pave the way for non-invasive diagnostic tools to interrogate the causal nature of peptidoglycan in host health and disease.

Molecular basis of the final step of cell division in Streptococcus pneumoniae

Bacterial cell-wall hydrolases must be tightly regulated during bacterial cell division to prevent aberrant cell lysis and to allow final separation of viable daughter cells. In a multidisciplinary work, we disclose the molecular dialogue between the cell-wall hydrolase LytB, wall teichoic acids, and the eukaryotic-like protein kinase StkP in Streptococcus pneumoniae. After characterizing the peptidoglycan recognition mode by the catalytic domain of LytB, we further demonstrate that LytB possesses a modular organization allowing the specific binding to wall teichoic acids and to the protein kinase StkP. Structural and cellular studies notably reveal that the temporal and spatial localization of LytB is governed by the interaction between specific modules of LytB and the final PASTA domain of StkP. Our data collectively provide a comprehensive understanding of how LytB performs final separation of daughter cells and highlights the regulatory role of eukaryotic-like kinases on lytic machineries in the last step of cell division in streptococci.

PBP2b Mutations Improve the Growth of Phage-Resistant Lactococcus cremoris Lacking Polysaccharide Pellicle

Lactococcus lactis and Lactococcus cremoris are Gram-positive lactic acid bacteria widely used as starter in milk fermentations. Lactococcal cells are covered with a polysaccharide pellicle (PSP) that was previously shown to act as the receptor for numerous bacteriophages of the Caudoviricetes class. Thus, mutant strains lacking PSP are phage resistant. However, because PSP is a key cell wall component, PSP-negative mutants exhibit dramatic alterations of cell shape and severe growth defects, which limit their technological value. In the present study, we isolated spontaneous mutants with improved growth, from L. cremoris PSP-negative mutants. These mutants grow at rates similar to the wild-type strain, and based on transmission electron microscopy analysis, they exhibit improved cell morphology compared to their parental PSP-negative mutants. In addition, the selected mutants maintain their phage resistance. Whole-genome sequencing of several such mutants showed that they carried a mutation in pbp2b, a gene encoding a penicillin-binding protein involved in peptidoglycan biosynthesis. Our results indicate that lowering or turning off PBP2b activity suppresses the requirement for PSP and ameliorates substantially bacterial fitness and morphology. IMPORTANCE Lactococcus lactis and Lactococcus cremoris are widely used in the dairy industry as a starter culture. As such, they are consistently challenged by bacteriophage infections which may result in reduced or failed milk acidification with associated economic losses. Bacteriophage infection starts with the recognition of a receptor at the cell surface, which was shown to be a cell wall polysaccharide (the polysaccharide pellicle [PSP]) for the majority of lactococcal phages. Lactococcal mutants devoid of PSP exhibit phage resistance but also reduced fitness, since their morphology and division are severely impaired. Here, we isolated spontaneous, food-grade non-PSP-producing L. cremoris mutants resistant to bacteriophage infection with a restored fitness. This study provides an approach to isolate non-GMO phage-resistant L. cremoris and L. lactis strains, which can be applied to strains with technological functionalities. Also, our results highlight for the first time the link between peptidoglycan and cell wall polysaccharide biosynthesis.

Biomolecular condensate drives polymerization and bundling of the bacterial tubulin FtsZ to regulate cell division

Cell division is spatiotemporally precisely regulated, but the underlying mechanisms are incompletely understood. In the social bacterium Myxococcus xanthus, the PomX/PomY/PomZ proteins form a single megadalton-sized complex that directly positions and stimulates cytokinetic ring formation by the tubulin homolog FtsZ. Here, we study the structure and mechanism of this complex in vitro and in vivo. We demonstrate that PomY forms liquid-like biomolecular condensates by phase separation, while PomX self-assembles into filaments generating a single large cellular structure. The PomX structure enriches PomY, thereby guaranteeing the formation of precisely one PomY condensate per cell through surface-assisted condensation. In vitro, PomY condensates selectively enrich FtsZ and nucleate GTP-dependent FtsZ polymerization and bundle FtsZ filaments, suggesting a cell division site positioning mechanism in which the single PomY condensate enriches FtsZ to guide FtsZ-ring formation and division. This mechanism shares features with microtubule nucleation by biomolecular condensates in eukaryotes, supporting this mechanism's ancient origin.

DivIVA Interacts with the Cell Wall Hydrolase MltG To Regulate Peptidoglycan Synthesis in Streptococcus suis

Bacterial morphology is largely determined by the spatial and temporal regulation of peptidoglycan (PG) biosynthesis. Ovococci possess a unique pattern of PG synthesis different from the well studied Bacillus, and the mechanism of the coordination of PG synthesis remains poorly understood. Several regulatory proteins have been identified to be involved in the regulation of ovococcal morphogenesis, among which DivIVA is an important one to regulate PG synthesis in streptococci, while its mechanism is largely unknown. Here, the zoonotic pathogen Streptococcus suis was used to investigate the regulation of DivIVA on PG synthesis. Fluorescent d-amino acid probing and 3D-structured illumination microscopy found that DivIVA deletion caused abortive peripheral PG synthesis, resulting in a decreased aspect ratio. The phosphorylation-depleted mutant (DivIVA3A) cells displayed a longer nascent PG and became longer, whereas the phosphorylation-mimicking mutant (DivIVA3E) cells showed a shorter nascent PG and became shorter, suggesting that DivIVA phosphorylation is involved in regulating peripheral PG synthesis. Several DivIVA-interacting proteins were identified, and the interaction was confirmed between DivIVA and MltG, a cell wall hydrolase essential for cell elongation. DivIVA did not affect the PG hydrolysis activity of MltG, while the phosphorylation state of DivIVA affected its interaction with MltG. MltG was mislocalized in the ΔdivIVA and DivIVA3E cells, and both ΔmltG and DivIVA3E cells formed significantly rounder cells, indicating an important role of DivIVA phosphorylation in regulating PG synthesis through MltG. These findings highlight the regulatory mechanism of PG synthesis and morphogenesis of ovococci. IMPORTANCE The peptidoglycan (PG) biosynthesis pathway provides a rich source of novel antimicrobial drug targets. However, bacterial PG synthesis and its regulation is a very complex process involving dozens of proteins. Moreover, unlike the well studied Bacillus, ovococci undergo unusual PG synthesis with unique mechanisms of coordination. DivIVA is an important regulator of PG synthesis in ovococci, while its exact role in regulating PG synthesis remains poorly understood. In this study, we determined the role of DivIVA in regulating lateral PG synthesis of Streptococcus suis and identified a critical interacting partner, MltG, in which DivIVA influenced the subcellular localizations of MltG through its phosphorylation. Our study characterizes the detailed role of DivIVA in regulating bacterial PG synthesis, which is very helpful for understanding the process of PG synthesis in streptococci.

The divisome but not the elongasome organizes capsule synthesis in Streptococcus pneumoniae

The bacterial cell envelope consists of multiple layers, including the peptidoglycan cell wall, one or two membranes, and often an external layer composed of capsular polysaccharides (CPS) or other components. How the synthesis of all these layers is precisely coordinated remains unclear. Here, we identify a mechanism that coordinates the synthesis of CPS and peptidoglycan in Streptococcus pneumoniae. We show that CPS synthesis initiates from the division septum and propagates along the long axis of the cell, organized by the tyrosine kinase system CpsCD. CpsC and the rest of the CPS synthesis complex are recruited to the septum by proteins associated with the divisome (a complex involved in septal peptidoglycan synthesis) but not the elongasome (involved in peripheral peptidoglycan synthesis). Assembly of the CPS complex starts with CpsCD, then CpsA and CpsH, the glycosyltransferases, and finally CpsJ. Remarkably, targeting CpsC to the cell pole is sufficient to reposition CPS synthesis, leading to diplococci that lack CPS at the septum. We propose that septal CPS synthesis is important for chain formation and complement evasion, thereby promoting bacterial survival inside the host.

StkP- and PhpP-Mediated Posttranslational Modifications Modulate the S. pneumoniae Metabolism, Polysaccharide Capsule, and Virulence

Pneumococcal Ser/Thr kinase (StkP) and its cognate phosphatase (PhpP) play a crucial role in bacterial cytokinesis. However, their individual and reciprocal metabolic and virulence regulation-related functions have yet to be adequately investigated in encapsulated pneumococci. Here, we demonstrate that the encapsulated pneumococcal strain D39-derived D39ΔPhpP and D39ΔStkP mutants displayed differential cell division defects and growth patterns when grown in chemically defined media supplemented with glucose or nonglucose sugars as the sole carbon source. Microscopic and biochemical analyses supported by RNA-seq-based global transcriptomic analyses of these mutants revealed significantly down- and upregulated polysaccharide capsule formation and cps2 genes in D39ΔPhpP and D39ΔStkP mutants, respectively. While StkP and PhpP individually regulated several unique genes, they also participated in sharing the regulation of the same set of differentially regulated genes. Cps2 genes were reciprocally regulated in part by the StkP/PhpP-mediated reversible phosphorylation but independent of the MapZ-regulated cell division process. StkP-mediated dose-dependent phosphorylation of CcpA proportionately inhibited CcpA-binding to Pcps2A, supporting increased cps2 gene expression and capsule formation in D39ΔStkP. While the attenuation of the D39ΔPhpP mutant in two mouse infection models corroborated with several downregulated capsules-, virulence-, and phosphotransferase systems (PTS)-related genes, the D39ΔStkP mutant with increased amounts of polysaccharide capsules displayed significantly decreased virulence in mice compared to the D39 wild-type, but more virulence compared to D39ΔPhpP. NanoString technology-based inflammation-related gene expression and Meso Scale Discovery-based multiplex chemokine analysis of human lung cells cocultured with these mutants confirmed their distinct virulence phenotypes. StkP and PhpP may, therefore, serve as critical therapeutic targets.

Models versus pathogens: how conserved is the FtsZ in bacteria?

Combating anti-microbial resistance by developing alternative strategies is the need of the hour. Cell division, particularly FtsZ, is being extensively studied for its potential as an alternative target for anti-bacterial therapy. Bacillus subtilis and Escherichia coli are the two well-studied models for research on FtsZ, the leader protein of the cell division machinery. As representatives of gram-positive and gram-negative bacteria, respectively, these organisms have provided an extensive outlook into the process of cell division in rod-shaped bacteria. However, research on other shapes of bacteria, like cocci and ovococci, lags behind that of model rods. Even though most regions of FtsZ show sequence and structural conservation throughout bacteria, the differences in FtsZ functioning and interacting partners establish several different modes of division in different bacteria. In this review, we compare the features of FtsZ and cell division in the model rods B. subtilis and E. coli and the four pathogens: Staphylococcus aureus, Streptococcus pneumoniae, Mycobacterium tuberculosis, and Pseudomonas aeruginosa. Reviewing several recent articles on these pathogenic bacteria, we have highlighted the functioning of FtsZ, the unique roles of FtsZ-associated proteins, and the cell division processes in them. Further, we provide a detailed look at the anti-FtsZ compounds discovered and their target bacteria, emphasizing the need for elucidation of the anti-FtsZ mechanism of action in different bacteria. Current challenges and opportunities in the ongoing journey of identifying potent anti-FtsZ drugs have also been described.

The involvement of CiaR and the CiaR-regulated serine protease HtrA in thermal adaptation of Streptococcus pneumoniae

The in vivo temperature can vary according to the host tissue and the response to infection. Streptococcus pneumoniae has evolved mechanisms to survive these temperature differences, but neither the consequences of different temperatures for pneumococcal phenotype nor the genetic basis of thermal adaptation are known in detail. In our previous study [16], we found that CiaR, which is a part of two-component regulatory system CiaRH, as well as 17 genes known to be controlled by CiaRH, were identified to be differentially expressed with temperature. One of the CiaRH-regulated genes shown to be differentially regulated by temperature is for the high-temperature requirement protein (HtrA), coded by SPD_2068 (htrA). In this study, we hypothesized that the CiaRH system plays an important role in pneumococcal thermal adaptation through its control over htrA. This hypothesis was evaluated by testing strains mutated or overexpressing ciaR and/or htrA, in in vitro and in vivo assays. The results showed that in the absence of ciaR, the growth, haemolytic activity, amount of capsule and biofilm formation were considerably diminished at 40 °C only, while the cell size and virulence were affected at both 34 and 40 °C. The overexpression of htrA in the ∆ciaR background reconstituted the growth at all temperatures, and the haemolytic activity, biofilm formation and virulence of ∆ciaR partially at 40 °C. We also showed that overexpression of htrA in the wild-type promoted pneumococcal virulence at 40 °C, while the increase of capsule was observed at 34 °C, suggesting that the role of htrA changes at different temperatures. Our data suggest that CiaR and HtrA play an important role in pneumococcal thermal adaptation.

Real-time non-invasive fluorescence imaging of gut commensal bacteria to detect dynamic changes in the microbiome of live mice

In mammals, gut commensal microbiota interact extensively with the host, and the same interactions can be dysregulated in diseased states. Animal imaging is a powerful technique that is widely used to diagnose, measure, and track biological changes in model organisms such as laboratory mice. Several imaging techniques have been discovered and adopted by the research community that provide dynamic, non-invasive assessment of live animals, but these gains have not been universal across all fields of biology. Herein, we describe a method to non-invasively image commensal bacteria based on the specific metabolic labeling of bacterial cell walls to illuminate the gut bacteria of live mice. This tagging strategy may additionally provide unprecedented insight into cell wall turnover of gut commensals, which has implications for bacterial cellular growth and division, in a live animal.

Roles of RodZ and class A PBP1b in the assembly and regulation of the peripheral peptidoglycan elongasome in ovoid-shaped cells of Streptococcus pneumoniae D39

RodZ of rod-shaped bacteria functions to link MreB filaments to the Rod peptidoglycan (PG) synthase complex that moves circumferentially perpendicular to the long cell axis, creating hoop-like sidewall PG. Ovoid-shaped bacteria, such as Streptococcus pneumoniae (pneumococcus; Spn) that lack MreB, use a different modality for peripheral PG elongation that emanates from the midcell of dividing cells. Yet, S. pneumoniae encodes a RodZ homolog similar to RodZ in rod-shaped bacteria. We show here that the helix-turn-helix and transmembrane domains of RodZ(Spn) are essential for growth at 37°C. ΔrodZ mutations are suppressed by Δpbp1a, mpgA(Y488D), and ΔkhpA mutations that suppress ΔmreC, but not ΔcozE. Consistent with a role in PG elongation, RodZ(Spn) co-localizes with MreC and aPBP1a throughout the cell cycle and forms complexes and interacts with PG elongasome proteins and regulators. Depletion of RodZ(Spn) results in aberrantly shaped, non-growing cells and mislocalization of elongasome proteins MreC, PBP2b, and RodA. Moreover, Tn-seq reveals that RodZ(Spn), but not MreCD(Spn), displays a specific synthetic-viable genetic relationship with aPBP1b, whose function is unknown. We conclude that RodZ(Spn) acts as a scaffolding protein required for elongasome assembly and function and that aPBP1b, like aPBP1a, plays a role in elongasome regulation and possibly peripheral PG synthesis.

Structural variations and roles of rhamnose-rich cell wall polysaccharides in Gram-positive bacteria

Rhamnose-rich cell wall polysaccharides (Rha-CWPSs) have emerged as crucial cell wall components of numerous Gram-positive, ovoid-shaped bacteria—including streptococci, enterococci, and lactococci—of which many are of clinical or biotechnological importance. Rha-CWPS are composed of a conserved polyrhamnose backbone with side-chain substituents of variable size and structure. Because these substituents contain phosphate groups, Rha-CWPS can also be classified as polyanionic glycopolymers, similar to wall teichoic acids, of which they appear to be functional homologs. Recent advances have highlighted the critical role of these side-chain substituents in bacterial cell growth and division, as well as in specific interactions between bacteria and infecting bacteriophages or eukaryotic hosts. Here, we review the current state of knowledge on the structure and biosynthesis of Rha-CWPS in several ovoid-shaped bacterial species. We emphasize the role played by multicomponent transmembrane glycosylation systems in the addition of side-chain substituents of various sizes as extracytoplasmic modifications of the polyrhamnose backbone. We provide an overview of the contribution of Rha-CWPS to cell wall architecture and biogenesis and discuss current hypotheses regarding their importance in the cell division process. Finally, we sum up the critical roles that Rha-CWPS can play as bacteriophage receptors or in escaping host defenses, roles that are mediated mainly through their side-chain substituents. From an applied perspective, increased knowledge of Rha-CWPS can lead to advancements in strategies for preventing phage infection of lactococci and streptococci in food fermentation and for combating pathogenic streptococci and enterococci.

Structure and kinase activity of bacterial cell cycle regulator CcrZ

CcrZ is a recently discovered cell cycle regulator that connects DNA replication initiation with cell division in pneumococci and may have a similar function in related bacteria. CcrZ is also annotated as a putative kinase, suggesting that CcrZ homologs could represent a novel family of bacterial kinase-dependent cell cycle regulators. Here, we investigate the CcrZ homolog in Bacillus subtilis and show that cells lacking ccrZ are sensitive to a broad range of DNA damage. We demonstrate that increased expression of ccrZ results in over-initiation of DNA replication. In addition, increased expression of CcrZ activates the DNA damage response. Using sensitivity to DNA damage as a proxy, we show that the negative regulator for replication initiation (yabA) and ccrZ function in the same pathway. We show that CcrZ interacts with replication initiation proteins DnaA and DnaB, further suggesting that CcrZ is important for replication timing. To understand how CcrZ functions, we solved the crystal structure bound to AMP-PNP to 2.6 Å resolution. The CcrZ structure most closely resembles choline kinases, consisting of a bilobal structure with a cleft between the two lobes for binding ATP and substrate. Inspection of the structure reveals a major restructuring of the substrate-binding site of CcrZ relative to the choline-binding pocket of choline kinases, consistent with our inability to detect activity with choline for this protein. Instead, CcrZ shows activity on D-ribose and 2-deoxy-D-ribose, indicating adaptation of the choline kinase fold in CcrZ to phosphorylate a novel substrate. We show that integrity of the kinase active site is required for ATPase activity in vitro and for function in vivo. This work provides structural, biochemical, and functional insight into a newly identified, and conserved group of bacterial kinases that regulate DNA replication initiation.

What Is Motion? Recent Advances in the Study of Molecular Movement Patterns of the Peptidoglycan Synthesis Machines

How proteins move through space and time is a fundamental question in biology. While great strides have been made toward a mechanistic understanding of protein movement, many questions remain. We discuss the biological implications of motion in the context of the peptidoglycan (PG) synthesis machines. We reviewed systems in several bacteria, including Escherichia coli, Bacillus subtilis, and Streptococcus pneumoniae, and present a comprehensive view of our current knowledge regarding movement dynamics. Discrepancies are also addressed because "one size does not fit all". For bacteria to divide, new PG is synthesized and incorporated into the growing cell wall by complex multiprotein nanomachines consisting of PG synthases (transglycosylases [TG] and/or transpeptidases [TP]) as well as a variety of regulators and cytoskeletal factors. Advances in imaging capabilities and labeling methods have revealed that these machines are not static but rather circumferentially transit the cell via directed motion perpendicular to the long axis of model rod-shaped bacteria such as E. coli and B. subtilis. The enzymatic activity of the TG:TPs drives motion in some species while motion is mediated by FtsZ treadmilling in others. In addition, both directed and diffusive motion of the PG synthases have been observed using single-particle tracking technology. Here, we examined the biological role of diffusion regarding transit. Lastly, findings regarding the monofunctional transglycosylases (RodA and FtsW) as well as the Class A PG synthases are discussed. This minireview serves to showcase recent advances, broach mechanistic unknowns, and stimulate future areas of study.

Streptococcus mutans cell division protein FtsZ has higher GTPase and polymerization activities in acidic environment

The acid tolerance of Streptococcus mutans plays an important role in its cariogenic process. S. mutans initiates a powerful transcriptional and physiological adaptation mechanism, eventually shielding the cellular machinery from acid damage and contributing to bacterial survival under acidic stress conditions. Although S. mutans contains complex regulatory systems, existing studies have shown that S. mutans, unlike Escherichia coli, cannot maintain a neutral intracellular environment. As the pH of the extracellular environment decreases, the intracellular pH decreases in parallel. There is insufficient knowledge regarding the acid resistance of the intracellular proteins of S. mutans, particularly when it comes to the key cytoskeletal division protein FtsZ. In this study, the data showed that S. mutans had similar cell division progress in acidic and neutral environments. The splitting position was in the middle of cells, and the cytoplasm were divided evenly in the acidic environment. Additionally, the treadmilling velocity of S. mutans FtsZ in the middle of cells was not affected by the acidic environment. S. mutans FtsZ had higher GTPase activity in pH 6.0 buffer than in the neutral environment. Furthermore, the polymerization of S. mutans FtsZ in the acidic environment was more robust than that in the neutral environment. After two particular amino acids of S. mutans FtsZ amino acids were mutated (E88K, L269K), the polymerization of S. mutans FtsZ in the acidic environment was significantly reduced. Overall, S. mutans FtsZ exhibited higher functional activity in pH 6.0 buffer in vitro. The acid resistance of S. mutans FtsZ is affected by its particular amino acids. This article is protected by copyright. All rights reserved.

Regulation of Lytic Machineries by the FtsEX Complex in the Bacterial Divisome

The essential membrane complex FtsE/FtsX (FtsEX), belonging to the ABC transporter superfamily and widespread among bacteria, plays a relevant function in some crucial cell wall remodeling processes such as cell division, elongation, or sporulation. FtsEX plays a double role by recruiting proteins to the divisome apparatus and by regulating lytic activity of the cell wall hydrolases required for daughter cell separation. Interestingly, FtsEX does not act as a transporter but uses the ATPase activity of FtsE to mechanically transmit a signal from the cytosol, through the membrane, to the periplasm that activates the attached hydrolases. While the complete molecular details of such mechanism are not yet known, evidence has been recently reported that clarify essential aspects of this complex system. In this chapter we will present recent structural advances on this topic. The three-dimensional structure of FtsE, FtsX, and some of the lytic enzymes or their cognate regulators revealed an unexpected scenario in which a delicate set of intermolecular interactions, conserved among different bacterial genera, could be at the core of this regulatory mechanism providing exquisite control in both space and time of this central process to assist bacterial survival.

FtsZ-Ring Regulation and Cell Division Are Mediated by Essential EzrA and Accessory Proteins ZapA and ZapJ in Streptococcus pneumoniae

The bacterial FtsZ-ring initiates division by recruiting a large repertoire of proteins (the divisome; Z-ring) needed for septation and separation of cells. Although FtsZ is essential and its role as the main orchestrator of cell division is conserved in most eubacteria, the regulators of Z-ring presence and positioning are not universal. This study characterizes factors that regulate divisome presence and placement in the ovoid-shaped pathogen, Streptococcus pneumoniae (Spn), focusing on FtsZ, EzrA, SepF, ZapA, and ZapJ, which is reported here as a partner of ZapA. Epi-fluorescence microscopy (EFm) and high-resolution microscopy experiments showed that FtsZ and EzrA co-localize during the entire Spn cell cycle, whereas ZapA and ZapJ are late-arriving divisome proteins. Depletion and conditional mutants demonstrate that EzrA is essential in Spn and required for normal cell growth, size, shape homeostasis, and chromosome segregation. Moreover, EzrA(Spn) is required for midcell placement of FtsZ-rings and PG synthesis. Notably, overexpression of EzrA leads to the appearance of extra Z-rings in Spn. Together, these observations support a role for EzrA as a positive regulator of FtsZ-ring formation in Spn. Conversely, FtsZ is required for EzrA recruitment to equatorial rings and for the organization of PG synthesis. In contrast to EzrA depletion, which causes a bacteriostatic phenotype in Spn, depletion of FtsZ results in enlarged spherical cells that are subject to LytA-dependent autolysis. Co-immunoprecipitation and bacterial two-hybrid assays show that EzrA(Spn) is in complexes with FtsZ, Z-ring regulators (FtsA, SepF, ZapA, MapZ), division proteins (FtsK, StkP), and proteins that mediate peptidoglycan synthesis (GpsB, aPBP1a), consistent with a role for EzrA at the interface of cell division and PG synthesis. In contrast to the essentiality of FtsZ and EzrA, ZapA and SepF have accessory roles in regulating pneumococcal physiology. We further show that ZapA interacts with a non-ZapB homolog, named here as ZapJ, which is conserved in Streptococcus species. The absence of the accessory proteins, ZapA, ZapJ, and SepF, exacerbates growth defects when EzrA is depleted or MapZ is deleted. Taken together, these results provide new information about the spatially and temporally distinct proteins that regulate FtsZ-ring organization and cell division in Spn.

Protein phosphatase SppA regulates apical growth and dephosphorylates cell polarity determinant DivIVA in Streptomyces coelicolor

Members of the Actinobacteria, including mycobacteria and streptomycetes, exhibit a distinctive mode of polar growth, with cell wall synthesis occurring in zones at cell poles and directed by the essential cell polarity determinant DivIVA. Streptomyces coelicolor modulates polar growth via the Ser/Thr protein kinase AfsK, which phosphorylates DivIVA. Here, we show that the phosphoprotein phosphatase SppA has strong effects on polar growth and cell shape and that it reverses the AfsK-mediated phosphorylation of DivIVA. SppA affects hyphal branching and the rate of tip extension. The sppA mutant hyphae also exhibit a high frequency of spontaneous growth arrests, indicating problems with maintenance of tip extension. The phenotypic effects are partially suppressed in an afsK sppA double mutant, indicating that AfsK and SppA to some extent share target proteins. Strains with a nonphosphorylatable mutant DivIVA confirm that the effect of afsK on hyphal branching during normal growth is mediated by DivIVA phosphorylation. However, the phenotypic effects of sppA deletion are independent of DivIVA phosphorylation and must be mediated via other substrates. This study adds a PPP-family protein phosphatase to the proteins involved in the control of polar growth and cell shape determination in S. coelicolor.

Cell Wall Stress Stimulates the Activity of the Protein Kinase StkP of Streptococcus pneumoniae, Leading to Multiple Phosphorylation

Streptococcus pneumoniae is an opportunistic human pathogen that encodes a single eukaryotic-type Ser/Thr protein kinase StkP and its functional counterpart, the protein phosphatase PhpP. These signaling enzymes play critical roles in coordinating cell division and growth in pneumococci. In this study, we determined the proteome and phosphoproteome profiles of relevant mutants. Comparison of those with the wild-type provided a representative dataset of novel phosphoacceptor sites and StkP-dependent substrates. StkP phosphorylates key proteins involved in cell division and cell wall biosynthesis in both the unencapsulated laboratory strain Rx1 and the encapsulated virulent strain D39. Furthermore, we show that StkP plays an important role in triggering an adaptive response induced by a cell wall-directed antibiotic. Phosphorylation of the sensor histidine kinase WalK and downregulation of proteins of the WalRK core regulon suggest crosstalk between StkP and the WalRK two-component system. Analysis of proteomic profiles led to the identification of gene clusters regulated by catabolite control mechanisms, indicating a tight coupling of carbon metabolism and cell wall homeostasis. The imbalance of steady-state protein phosphorylation in the mutants as well as after antibiotic treatment is accompanied by an accumulation of the global Spx regulator, indicating a Spx-mediated envelope stress response. In summary, StkP relays the perceived signal of cell wall status to key cell division and regulatory proteins, controlling the cell cycle and cell wall homeostasis.

Comparative Phenotypic, Proteomic, and Phosphoproteomic Analysis Reveals Different Roles of Serine/Threonine Phosphatase and Kinase in the Growth, Cell Division, and Pathogenicity of Streptococcus suis

Eukaryote-like serine/threonine kinases (STKs) and cognate phosphatases (STPs) comprise an important regulatory system in many bacterial pathogens. The complexity of this regulatory system has not been fully understood due to the presence of multiple STKs/STPs in many bacteria and their multiple substrates involved in many different physiological and pathogenetic processes. Streptococci are the best materials for the study due to a single copy of the gene encoding STK and its cognate STP. Although several studies have been done to investigate the roles of STK and STP in zoonotic Streptococcus suis, respectively, few studies were performed on the coordinated regulatory roles of this system. In this study, we carried out a systemic study on STK/STP in S. suis by using a comparative phenotypic, proteomic, and phosphoproteomic analysis. Mouse infection assays revealed that STK played a much more important role in S. suis pathogenesis than STP. The ∆stk and ∆stp∆stk strains, but not ∆stp, showed severe growth retardation. Moreover, both ∆stp and ∆stk strains displayed defects in cell division, but they were abnormal in different ways. The comparative proteomics and phosphoproteomics revealed that deletion of stk or stp had a significant influence on protein expression. Interestingly, more virulence factors were found to be downregulated in ∆stk than ∆stp. In ∆stk strain, a substantial number of the proteins with a reduced phosphorylation level were involved in cell division, energy metabolism, and protein translation. However, only a few proteins showed increased phosphorylation in ∆stp, which also included some proteins related to cell division. Collectively, our results show that both STP and STK are critical regulatory proteins for S. suis and that STK seems to play more important roles in growth, cell division, and pathogenesis.

Recent advances in bacterial signaling by serine/threonine protein kinases

It has been nearly three decades since the discovery of the first bacterial serine/threonine protein kinase (STPK). Since then, a blend of technological advances has led to the characterization of a multitude of STPKs and phosphorylation substrates in several bacterial species that finely regulate intricate signaling cascades. Years of intense research from several laboratories have demonstrated unexpected roles for serine/threonine phosphorylation, regulating not only bacterial growth and cell division but also antibiotic persistence, virulence and infection, metabolism, chromosomal biology, and cellular differentiation. This review aims to provide an account of the most recent and significant developments in this up and growing field in microbiology.

Pivotal Roles for Ribonucleases in Streptococcus pneumoniae Pathogenesis

RNases perform indispensable functions in regulating gene expression in many bacterial pathogens by processing and/or degrading RNAs. Despite the pivotal role of RNases in regulating bacterial virulence factors, the functions of RNases have not yet been studied in the major human respiratory pathogen Streptococcus pneumoniae (pneumococcus). Here, we sought to determine the impact of two conserved RNases, the endoribonuclease RNase Y and exoribonuclease polynucleotide phosphorylase (PNPase), on the physiology and virulence of S. pneumoniae serotype 2 strain D39. We report that RNase Y and PNPase are essential for pneumococcal pathogenesis, as both deletion mutants showed strong attenuation of virulence in murine models of invasive pneumonia. Genome-wide transcriptomic analysis revealed that the abundances of nearly 200 mRNA transcripts were significantly increased, whereas those of several pneumococcal small regulatory RNAs (sRNAs), including the Ccn (CiaR-controlled noncoding RNA) sRNAs, were altered in the Δrny mutant relative to the wild-type strain. Additionally, lack of RNase Y resulted in pleiotropic phenotypes that included defects in pneumococcal cell morphology and growth in vitro. In contrast, Δpnp mutants showed no growth defect in vitro but differentially expressed a total of 40 transcripts, including the tryptophan biosynthesis operon genes and numerous 5' cis-acting regulatory RNAs, a majority of which were previously shown to impact pneumococcal disease progression in mice using the serotype 4 strain TIGR4. Together, our data suggest that RNase Y exerts a global impact on pneumococcal physiology, while PNPase mediates virulence phenotypes, likely through sRNA regulation. IMPORTANCE Streptococcus pneumoniae is a notorious human pathogen that adapts to conditions in distinct host tissues and responds to host cell interactions by adjusting gene expression. RNases are key players that modulate gene expression by mediating the turnover of regulatory and protein-coding transcripts. Here, we characterized two highly conserved RNases, RNase Y and PNPase, and evaluated their impact on the S. pneumoniae transcriptome for the first time. We show that PNPase influences the levels of a narrow set of mRNAs but a large number of regulatory RNAs primarily implicated in virulence control, whereas RNase Y has a more sweeping effect on gene expression, altering levels of transcripts involved in diverse cellular processes, including cell division, metabolism, stress response, and virulence. This study further reveals that RNase Y regulates expression of genes governing competence by mediating the turnover of CiaR-controlled noncoding (Ccn) sRNAs.

The Pneumococcal Divisome: Dynamic Control of Streptococcus pneumoniae Cell Division

Cell division in Streptococcus pneumoniae (pneumococcus) is performed and regulated by a protein complex consisting of at least 14 different protein elements; known as the divisome. Recent findings have advanced our understanding of the molecular events surrounding this process and have provided new understanding of the mechanisms that occur during the division of pneumococcus. This review will provide an overview of the key protein complexes and how they are involved in cell division. We will discuss the interaction of proteins in the divisome complex that underpin the control mechanisms for cell division and cell wall synthesis and remodelling that are required in S. pneumoniae, including the involvement of virulence factors and capsular polysaccharides.

Identification and characterization of the cell division protein MapZ from Streptococcus suis

Streptococcus suis, an emerging zoonotic pathogen, causes invasive diseases in pigs, including sepsis, meningitis, endocarditis, pneumonia, and arthritis. Importantly, similar pathologies are reported in human S. suis infections. In previous work, the locus SSU0375 of S. suis strain P1.7 had been identified as a conditionally essential gene by intrathecal experimental infection of pigs with a transposon library of S. suis. This study aimed to identify the function of the corresponding gene product. Bioinformatics analysis and homology modeling revealed sequence and structural homologies with the Streptococcus pneumoniae mid-cell-anchored protein Z (MapZ) that is involved in cell division in different bacterial species. Indeed, depletion of this locus in S. suis strain 10 revealed a growth defect as compared to the wild type. Electron microscopy analysis of the corresponding mutant demonstrated morphological growth defects as compared to the wild-type strain, including an irregular cell shape and size as well as mispositioned division septa. Light microscopy and subsequent quantitative image analysis confirmed these morphological alterations. In the genetic rescue strain, the wild-type phenotype was completely restored. In summary, we proposed that SSU0375 or the corresponding locus in strain 10 encode for a S. suis MapZ homolog that guides septum positioning as evidenced for other members of the Streptococci family.

MapZ deficiency leads to defects in the envelope structure and changes stress tolerance of Streptococcus mutans

Cell division is a central process in bacteria and a prerequisite for pathogenicity. Several proteins are involved in this process to ensure the accurate localization and proper function of the division machinery. In Streptococcus mutans, MapZ marks the division sites and position of the Z-ring to regulate cell division; however, whether MapZ deficiency can impair the cariogenic virulence of S. mutans remains unclear. Here, using a phenotypic assay and RNA-seq, we investigated the role of MapZ in cell envelope maintenance, biofilm formation, and stress tolerance in S. mutans. The results show that MapZ is important for normal cell shape and envelope structure, and its deletion causes abnormal septum structure and a thin cell wall. Subsequently, we found that the absence of MapZ leads to a greater level of cell death within 12 h biofilms, but it does not seem to affect biofilm architecture and accumulation. mapZ deletion also results in a decreased acid and osmotic stress tolerance. Furthermore, RNA-seq data reveal that MapZ deficiency causes changes in the expression levels of genes involved in transport systems, sugar metabolism, nature competence, and bacteriocin synthesis. Interestingly, we found that mapZ mutation renders S. mutans more sensitive to chlorhexidine. Taken together, our study suggests that MapZ plays a role in maintaining cell envelope structure and stress tolerance in S. mutans, showing a potential application as a drug target for caries prevention.

A Comprehensive Evolutionary Scenario of Cell Division and Associated Processes in the Firmicutes

The cell cycle is a fundamental process that has been extensively studied in bacteria. However, many of its components and their interactions with machineries involved in other cellular processes are poorly understood. Furthermore, most knowledge relies on the study of a few models, but the real diversity of the cell division apparatus and its evolution are largely unknown. Here, we present a massive in-silico analysis of cell division and associated processes in around 1,000 genomes of the Firmicutes, a major bacterial phylum encompassing models (i.e. Bacillus subtilis, Streptococcus pneumoniae, and Staphylococcus aureus), as well as many important pathogens. We analyzed over 160 proteins by using an original approach combining phylogenetic reconciliation, phylogenetic profiles, and gene cluster survey. Our results reveal the presence of substantial differences among clades and pinpoints a number of evolutionary hotspots. In particular, the emergence of Bacilli coincides with an expansion of the gene repertoires involved in cell wall synthesis and remodeling. We also highlight major genomic rearrangements at the emergence of Streptococcaceae. We establish a functional network in Firmicutes that allows identifying new functional links inside one same process such as between FtsW (peptidoglycan polymerase) and a previously undescribed Penicilin-Binding Protein or between different processes, such as replication and cell wall synthesis. Finally, we identify new candidates involved in sporulation and cell wall synthesis. Our results provide a previously undescribed view on the diversity of the bacterial cell cycle, testable hypotheses for further experimental studies, and a methodological framework for the analysis of any other biological system.

Undermodification cues division

Extracellular peptidoglycan-linked polysaccharide modifications mediate cell morphology, division, and autolysis in some Gram-positive bacterial pathogens. A new study shows that the degree and location of a specific modification controls peptidoglycan hydrolysis and placement of the axis of cell division.

HetF Protein Is a New Divisome Component in a Filamentous and Developmental Cyanobacterium

Bacterial cell division, with a few exceptions, is driven by FtsZ through a treadmilling mechanism to remodel and constrict the rigid peptidoglycan (PG) layer. Yet different organisms may differ in the composition of the cell division complex (divisome). In the filamentous cyanobacterium Anabaena sp. strain PCC 7120, hetF is required for the initiation of the differentiation of heterocysts, cells specialized in N₂ fixation under combined-nitrogen deprivation. In this study, we demonstrate that hetF is expressed in vegetative cells and necessary for cell division under certain conditions. Under nonpermissive conditions, cells of a ΔhetF mutant stop dividing, consistent with increased levels of HetF under similar conditions in the wild type. Furthermore, HetF is a membrane protein located at midcell and cell-cell junctions. In the absence of HetF, FtsZ rings are still present in the elongated cells; however, PG remodeling is abolished. This phenotype is similar to that observed with the inhibition of the septal PG synthase FtsI. We further reveal that HetF is recruited to or stabilized at the divisome by interacting with FtsI and that this interaction is necessary for HetF function in cell division. Our results indicate that HetF is a member of the divisome depending mainly on light intensity and reveal distinct features of the cell division machinery in cyanobacteria that are of high ecological and environmental importance.

DivIVA Regulates Its Expression and the Orientation of New Septum Growth in Deinococcus radiodurans

In rod-shaped Gram-negative bacteria, FtsZ localization at midcell position is regulated by the gradient of MinCDE complex across the poles. In round-shaped bacteria, which lack predefined poles, the next plane of cell division is perpendicular to the previous plane, and determination of the FtsZ assembly site is still intriguing. Deinococcus radiodurans, a coccus bacterium, is characterized by its extraordinary resistance to DNA damage. DivIVA, a putative component of the Min system in this bacterium, interacts with cognate cell division and genome segregation proteins. Here, we report that deletion of a chromosomal copy of DivIVA was possible only when the wild-type copy of DivIVA was expressed in trans on a plasmid. However, deletion of the C-terminal domain (CTD) of DivIVA (CTD mutant) was possible but produced distinguishable phenotypes, like smaller cells, slower growth, and tilted septum orientation, in D. radiodurans. In trans expression of DivIVA in the CTD mutant could restore these features of the wild type. Interestingly, the overexpression of DivIVA led to delayed separation of tetrads from an octet state in both trans-complemented divIVA-mutant and wild-type cells. The CTD mutant showed upregulation of the yggS-divIVAN operon. Both the wild type and CTD mutant formed FtsZ foci; however, unlike wild type, the position of foci in the mutant cells was found to be away from conjectural midcell position in cocci. Notably, DivIVA-red fluorescent protein (DivIVA-RFP) localizes to the septum during cell division at the new division site. These results suggested that DivIVA is an essential protein in D. radiodurans, and its C-terminal domain plays an important role in the regulation of its expression and orientation of new septal growth in this bacterium. IMPORTANCE In rod-shaped Gram-negative bacteria, the midcell position for binary fission is relatively easy to model. In cocci that do not have predefined poles, the plane of next cell division is shown to be perpendicular to the previous plane. However, the molecular basis of perpendicularity is not known in cocci. The DivIVA protein of Deinococcus radiodurans, a coccus bacterium, physically interacts with the septum and establishes macromolecular interactions with genome segregation proteins through its N-terminal domain and with MinC through the C-terminal domain. Here, we have brought forth some evidence to suggest that DivIVA is essential for growth and plays an important role in cell polarity determination, and its C-terminal domain plays a crucial role in the growth of new septa in the correct orientation as well as in the regulation of DivIVA expression.

SepF is the FtsZ anchor in archaea, with features of an ancestral cell division system

Most archaea divide by binary fission using an FtsZ-based system similar to that of bacteria, but they lack many of the divisome components described in model bacterial organisms. Notably, among the multiple factors that tether FtsZ to the membrane during bacterial cell constriction, archaea only possess SepF-like homologs. Here, we combine structural, cellular, and evolutionary analyses to demonstrate that SepF is the FtsZ anchor in the human-associated archaeon Methanobrevibacter smithii. 3D super-resolution microscopy and quantitative analysis of immunolabeled cells show that SepF transiently co-localizes with FtsZ at the septum and possibly primes the future division plane. M. smithii SepF binds to membranes and to FtsZ, inducing filament bundling. High-resolution crystal structures of archaeal SepF alone and in complex with the FtsZ C-terminal domain (FtsZCTD) reveal that SepF forms a dimer with a homodimerization interface driving a binding mode that is different from that previously reported in bacteria. Phylogenetic analyses of SepF and FtsZ from bacteria and archaea indicate that the two proteins may date back to the Last Universal Common Ancestor (LUCA), and we speculate that the archaeal mode of SepF/FtsZ interaction might reflect an ancestral feature. Our results provide insights into the mechanisms of archaeal cell division and pave the way for a better understanding of the processes underlying the divide between the two prokaryotic domains.

Modification of cell wall polysaccharide guides cell division in Streptococcus mutans

In ovoid-shaped, Gram-positive bacteria, MapZ guides FtsZ-ring positioning at cell equators. The cell wall of the ovococcus Streptococcus mutans contains peptidoglycan decorated with serotype c carbohydrates (SCCs). In the present study, we identify the major cell separation autolysin AtlA as an SCC-binding protein. AtlA binding to SCC is attenuated by the glycerol phosphate (GroP) modification. Using fluorescently labeled AtlA constructs, we mapped SCC distribution on the streptococcal surface, revealing enrichment of GroP-deficient immature SCCs at the cell poles and equators. The immature SCCs co-localize with MapZ at the equatorial rings throughout the cell cycle. In GroP-deficient mutants, AtlA is mislocalized, resulting in dysregulated cellular autolysis. These mutants display morphological abnormalities associated with MapZ mislocalization, leading to FtsZ-ring misplacement. Altogether, our data support a model in which maturation of a cell wall polysaccharide provides the molecular cues for the recruitment of cell division machinery, ensuring proper daughter cell separation and FtsZ-ring positioning.

Super-resolved live-cell imaging using random illumination microscopy

Current super-resolution microscopy (SRM) methods suffer from an intrinsic complexity that might curtail their routine use in cell biology. We describe here random illumination microscopy (RIM) for live-cell imaging at super-resolutions matching that of 3D structured illumination microscopy, in a robust fashion. Based on speckled illumination and statistical image reconstruction, easy to implement and user-friendly, RIM is unaffected by optical aberrations on the excitation side, linear to brightness, and compatible with multicolor live-cell imaging over extended periods of time. We illustrate the potential of RIM on diverse biological applications, from the mobility of proliferating cell nuclear antigen (PCNA) in U2OS cells and kinetochore dynamics in mitotic S. pombe cells to the 3D motion of myosin minifilaments deep inside Drosophila tissues. RIM's inherent simplicity and extended biological applicability, particularly for imaging at increased depths, could help make SRM accessible to biology laboratories.

Ser/Thr Kinase-Dependent Phosphorylation of the Peptidoglycan Hydrolase CwlA Controls Its Export and Modulates Cell Division in Clostridioides difficile

Cell growth and division require a balance between synthesis and hydrolysis of the peptidoglycan (PG). Inhibition of PG synthesis or uncontrolled PG hydrolysis can be lethal for the cells, making it imperative to control peptidoglycan hydrolase (PGH) activity. The synthesis or activity of several key enzymes along the PG biosynthetic pathway is controlled by the Hanks-type serine/threonine kinases (STKs). In Gram-positive bacteria, inactivation of genes encoding STKs is associated with a range of phenotypes, including cell division defects and changes in cell wall metabolism, but only a few kinase substrates and associated mechanisms have been identified. We previously demonstrated that STK-PrkC plays an important role in cell division, cell wall metabolism, and resistance to antimicrobial compounds in the human enteropathogen Clostridioides difficile In this work, we characterized a PG hydrolase, CwlA, which belongs to the NlpC/P60 family of endopeptidases and hydrolyses cross-linked PG between daughter cells to allow cell separation. We identified CwlA as the first PrkC substrate in C. difficile We demonstrated that PrkC-dependent phosphorylation inhibits CwlA export, thereby controlling hydrolytic activity in the cell wall. High levels of CwlA at the cell surface led to cell elongation, whereas low levels caused cell separation defects. Thus, we provided evidence that the STK signaling pathway regulates PGH homeostasis to precisely control PG hydrolysis during cell division.IMPORTANCE Bacterial cells are encased in a PG exoskeleton that helps to maintain cell shape and confers physical protection. To allow bacterial growth and cell separation, PG needs to be continuously remodeled by hydrolytic enzymes that cleave PG at critical sites. How these enzymes are regulated remains poorly understood. We identify a new PG hydrolase involved in cell division, CwlA, in the enteropathogen C. difficile Lack or accumulation of CwlA at the bacterial surface is responsible for a division defect, while its accumulation in the absence of PrkC also increases susceptibility to antimicrobial compounds targeting the cell wall. CwlA is a substrate of the kinase PrkC in C. difficile PrkC-dependent phosphorylation controls the export of CwlA, modulating its levels and, consequently, its activity in the cell wall. This work provides a novel regulatory mechanism by STK in tightly controlling protein export.

EloR interacts with the lytic transglycosylase MltG at midcell in Streptococcus pneumoniae R6

The ellipsoid shape of Streptococcus pneumoniae is determined by the synchronized actions of the elongasome and the divisome, which have the task of creating a protective layer of peptidoglycan (PG) enveloping the cell membrane. The elongasome is necessary for expanding PG in the longitudinal direction whereas the divisome synthesizes the PG that divides one cell into two. Although there is still little knowledge about how these two modes of PG synthesis are coordinated, it was recently discovered that two RNA-binding proteins called EloR and KhpA are part of a novel regulatory pathway controlling elongation in S. pneumoniae EloR and KhpA form a complex that work closely with the Ser/Thr kinase StkP to regulate cell elongation. Here, we have further explored how this regulation occur. EloR/KhpA is found at midcell, a localization fully dependent on EloR. Using a bacterial two-hybrid assay we probed EloR against several elongasome proteins and found an interaction with the lytic transglycosylase homolog MltG. By using EloR as bait in immunoprecipitation assays, MltG was pulled down confirming that they are part of the same protein complex. Fluorescent microscopy demonstrated that the Jag domain of EloR is essential for EloR's midcell localization and its interaction with MltG. Since MltG is found at midcell independent of EloR, our results suggest that MltG is responsible for recruitment of the EloR/KhpA complex to the division zone to regulate cell elongation.Importance Bacterial cell division has been a successful target for antimicrobial agents for decades. How different pathogens regulate cell division is, however, poorly understood. To fully exploit the potential for future antibiotics targeting cell division, we need to understand the details of how the bacteria regulate and construct cell wall during this process. Here we have revealed that the newly identified EloR/KhpA complex, regulating cell elongation in S. pneumoniae, forms a complex with the essential peptidoglycan transglycosylase MltG at midcell. EloR, KhpA and MltG are conserved among many bacterial species and the EloR/KhpA/MltG regulatory pathway is most likely a common mechanism employed by many Gram-positive bacteria to coordinate cell elongation and septation.

Metabolic labeling probes for interrogation of the host-pathogen interaction

Bacterial infections are still one of the leading causes of death worldwide; despite the near-ubiquitous availability of antibiotics. With antibiotic resistance on the rise, there is an urgent need for novel classes of antibiotic drugs. One particularly troublesome class of bacteria are those that have evolved highly efficacious mechanisms for surviving inside the host. These contribute to their virulence by immune evasion, and make them harder to treat with antibiotics due to their residence inside intracellular membrane-limited compartments. This has sparked the development of new chemical reporter molecules and bioorthogonal probes that can be metabolically incorporated into bacteria to provide insights into their activity status. In this review, we provide an overview of several classes of metabolic labeling probes capable of targeting either the peptidoglycan cell wall, the mycomembrane of mycobacteria and corynebacteria, or specific bacterial proteins. In addition, we highlight several important insights that have been made using these metabolic labeling probes.

Bacterial cell proliferation: from molecules to cells

Bacterial cell proliferation is highly efficient, both because bacteria grow fast and multiply with a low failure rate. This efficiency is underpinned by the robustness of the cell cycle and its synchronization with cell growth and cytokinesis. Recent advances in bacterial cell biology brought about by single-cell physiology in microfluidic chambers suggest a series of simple phenomenological models at the cellular scale, coupling cell size and growth with the cell cycle. We contrast the apparent simplicity of these mechanisms based on the addition of a constant size between cell cycle events (e.g. two consecutive initiation of DNA replication or cell division) with the complexity of the underlying regulatory networks. Beyond the paradigm of cell cycle checkpoints, the coordination between the DNA and division cycles and cell growth is largely mediated by a wealth of other mechanisms. We propose our perspective on these mechanisms, through the prism of the known crosstalk between DNA replication and segregation, cell division and cell growth or size. We argue that the precise knowledge of these molecular mechanisms is critical to integrate the diverse layers of controls at different time and space scales into synthetic and verifiable models.

CcrZ is a pneumococcal spatiotemporal cell cycle regulator that interacts with FtsZ and controls DNA replication by modulating the activity of DnaA

Most bacteria replicate and segregate their DNA concomitantly while growing, before cell division takes place. How bacteria synchronize these different cell cycle events to ensure faithful chromosome inheritance by daughter cells is poorly understood. Here, we identify Cell Cycle Regulator protein interacting with FtsZ (CcrZ) as a conserved and essential protein in pneumococci and related Firmicutes such as Bacillus subtilis and Staphylococcus aureus. CcrZ couples cell division with DNA replication by controlling the activity of the master initiator of DNA replication, DnaA. The absence of CcrZ causes mis-timed and reduced initiation of DNA replication, which subsequently results in aberrant cell division. We show that CcrZ from Streptococcus pneumoniae interacts directly with the cytoskeleton protein FtsZ, which places CcrZ in the middle of the newborn cell where the DnaA-bound origin is positioned. This work uncovers a mechanism for control of the bacterial cell cycle in which CcrZ controls DnaA activity to ensure that the chromosome is replicated at the right time during the cell cycle.

Bacterial Vivisection: How Fluorescence-Based Imaging Techniques Shed a Light on the Inner Workings of Bacteria

The rise in fluorescence-based imaging techniques over the past 3 decades has improved the ability of researchers to scrutinize live cell biology at increased spatial and temporal resolution. In microbiology, these real-time vivisections structurally changed the view on the bacterial cell away from the "watery bag of enzymes" paradigm toward the perspective that these organisms are as complex as their eukaryotic counterparts. Capitalizing on the enormous potential of (time-lapse) fluorescence microscopy and the ever-extending pallet of corresponding probes, initial breakthroughs were made in unraveling the localization of proteins and monitoring real-time gene expression. However, later it became clear that the potential of this technique extends much further, paving the way for a focus-shift from observing single events within bacterial cells or populations to obtaining a more global picture at the intra- and intercellular level. In this review, we outline the current state of the art in fluorescence-based vivisection of bacteria and provide an overview of important case studies to exemplify how to use or combine different strategies to gain detailed information on the cell's physiology. The manuscript therefore consists of two separate (but interconnected) parts that can be read and consulted individually. The first part focuses on the fluorescent probe pallet and provides a perspective on modern methodologies for microscopy using these tools. The second section of the review takes the reader on a tour through the bacterial cell from cytoplasm to outer shell, describing strategies and methods to highlight architectural features and overall dynamics within cells.

A CozE Homolog Contributes to Cell Size Homeostasis of Streptococcus pneumoniae

Control of peptidoglycan assembly is critical to maintain bacterial cell size and morphology. Penicillin-binding proteins (PBPs) are crucial enzymes for the polymerization of the glycan strand and/or their cross-linking via peptide branches. Over the last few years, it has become clear that PBP activity and localization can be regulated by specific cognate regulators. The first regulator of PBP activity in Gram-positive bacteria was discovered in the human pathogen Streptococcus pneumoniae This regulator, named CozE, controls the activity of the bifunctional PBP1a to promote cell elongation and achieve a proper cell morphology. In this work, we studied a previously undescribed CozE homolog in the pneumococcus, which we named CozEb. This protein displays the same membrane organization as CozE but is much more widely conserved among Streptococcaceae genomes. Interestingly, cozEb deletion results in cells that are smaller than their wild-type counterparts, which is the opposite effect of cozE deletion. Furthermore, double deletion of cozE and cozEb results in poor viability and exacerbated cell shape defects. Coimmunoprecipitation further showed that CozEb is part of the same complex as CozE and PBP1a. However, although we confirmed that CozE is required for septal localization of PBP1a, the absence of CozEb has no effect on PBP1a localization. Nevertheless, we found that the overexpression of CozEb can compensate for the absence of CozE in all our assays. Altogether, our results show that the interplay between PBP1a and the cell size regulators CozE and CozEb is required for the maintenance of pneumococcal cell size and shape.IMPORTANCE Penicillin-binding proteins (PBPs), the proteins catalyzing the last steps of peptidoglycan assembly, are critical for bacteria to maintain cell size, shape, and integrity. PBPs are consequently attractive targets for antibiotics. Resistance to antibiotics in Streptococcus pneumoniae (the pneumococcus) are often associated with mutations in the PBPs. In this work, we describe a new protein, CozEb, controlling the cell size of pneumococcus. CozEb is a highly conserved integral membrane protein that works together with other proteins to regulate PBPs and peptidoglycan synthesis. Deciphering the intricate mechanisms by which the pneumococcus controls peptidoglycan assembly might allow the design of innovative anti-infective strategies, for example, by resensitizing resistant strains to PBP-targeting antibiotics.

A key bacterial cytoskeletal cell division protein FtsZ as a novel therapeutic antibacterial drug target

Nowadays, the emergence of multidrug-resistant bacterial strains initiates the urgent need for the elucidation of the new drug targets for the discovery of antimicrobial drugs. Filamenting temperature-sensitive mutant Z (FtsZ), a eukaryotic tubulin homolog, is a GTP-dependent prokaryotic cytoskeletal protein and is conserved among most bacterial strains. In vitro studies revealed that FtsZ self-assembles into dynamic protofilaments or bundles and forms a dynamic Z-ring at the center of the cell in vivo, leading to septation and consequent cell division. Speculations on the ability of FtsZ in the blockage of cell division make FtsZ a highly attractive target for developing novel antibiotics. Researchers have been working on synthetic molecules and natural products as inhibitors of FtsZ. Accumulating data suggest that FtsZ may provide the platform for the development of novel antibiotics. In this review, we summarize recent advances in the properties of FtsZ protein and bacterial cell division, as well as in the development of FtsZ inhibitors.