Structure of the heterotrimeric membrane protein complex FtsB-FtsL-FtsQ of the bacterial divisome

Plasmid-Stabilizing Strains for Antibiotic-Free Chemical Fermentation

Plasmid-mediated antibiotic-free fermentation holds significant industrial potential. However, the requirements for host elements and energy during plasmid inheritance often cause cell burden, leading to plasmid loss and reduced production. The stable maintenance of plasmids is primarily achieved through a complex mechanism, making it challenging to rationally design plasmid-stabilizing strains and characterize the associated genetic factors. In this study, we introduced a fluorescence-based high-throughput method and successfully screened plasmid-stabilizing strains from the genomic fragment-deletion strains of Escherichia coli MG1655 and Bacillus subtilis 168. The application of EcΔ50 in antibiotic-free fermentation increased the alanine titer 2.9 times. The enhanced plasmid stability in EcΔ50 was attributed to the coordinated deletion of genes involved in plasmid segregation and replication control, leading to improved plasmid maintenance and increased plasmid copy number. The increased plasmid stability of BsΔ44 was due to the deletion of the phage SPP1 surface receptor gene yueB, resulting in minimized sporulation, improved plasmid segregational stability and host adaptation. Antibiotic-free fermentation results showed that strain BsΔyueB exhibited a 61.99% higher acetoin titer compared to strain Bs168, reaching 3.96 g/L. When used for the fermentation of the downstream product, 2,3-butanediol, strain BsΔyueB achieved an 80.63% higher titer than Bs168, reaching 14.94 g/L using rich carbon and nitrogen feedstocks. Overall, our work provided a plasmid-stabilizing chassis for E. coli and B. subtilis, highlighting their potential for antibiotic-free fermentation of valuable products and metabolic engineering applications.

E. coli FtsN coordinates synthesis and degradation of septal peptidoglycan by partitioning between a synthesis track and a denuded glycan track

The E. coli cell division protein FtsN was proposed to coordinate septal peptidoglycan (sPG) synthesis and degradation to ensure robust cell wall constriction without lethal lesions. Although the precise mechanism remains unclear, previous work highlights the importance of two FtsN domains: the E domain, which interacts with and activates the sPG synthesis complex FtsWIQLB, and the SPOR domain, which binds to denuded glycan (dnG) strands, key intermediates in sPG degradation. Here, we used single-molecule tracking of FtsN and FtsW (a proxy for the sPG synthesis complex FtsWIQLB) to investigate how FtsN coordinates the two opposing processes. We observed dynamic behaviors indicating that FtsN's SPOR domain binds to dnGs cooperatively, which both sequesters the sPG synthesis complex on dnG (termed as the dnG-track) and protects dnGs from degradation by lytic transglycosylases (LTs). The release of the SPOR domain from dnGs leads to activating the sPG synthesis complex on the sPG-track and simultaneously exposing those same dnGs to degradation. Furthermore, FtsN's SPOR domain self-interacts and facilitates the formation of a multimeric sPG synthesis complex on both tracks. The cooperative self-interaction of the SPOR domain creates a sensitive switch to regulate the partitioning of FtsN between the dnG- and sPG-tracks, thereby controlling the balance between sequestered and active populations of the sPG synthesis complex. As such, FtsN coordinates sPG synthesis and degradation in space and time.

Role of the antiparallel double-stranded filament form of FtsA in activating the Escherichia coli divisome

The actin-like FtsA protein is essential for function of the cell division machinery, or divisome, in many bacteria including Escherichia coli. Previous in vitro studies demonstrated that purified wild-type FtsA assembles into closed mini-rings on lipid membranes, but oligomeric variants of FtsA such as FtsAR286W and FtsAG50E can bypass certain divisome defects and form arc and double-stranded (DS) oligomeric states, respectively, which may reflect conversion of an inactive to an active form of FtsA. However, it remains unproven which oligomeric forms of FtsA are responsible for assembling and activating the divisome. Here, we used an in vivo crosslinking assay for FtsA DS filaments to show that they largely depend on proper divisome assembly and are prevalent at later stages of cell division. We also used a previously reported variant that fails to assemble DS filaments, FtsAM96E R153D, to investigate the roles of FtsA oligomeric states in divisome assembly and activation. We show that FtsAM96E R153D cannot form DS filaments in vivo, fails to replace native FtsA, and confers a dominant negative phenotype, underscoring the importance of the DS filament stage for FtsA function. Surprisingly, however, activation of the divisome through the ftsL* or ftsW* superfission alleles suppressed the dominant negative phenotype and rescued the functionality of FtsAM96E R153D. Our results suggest that FtsA DS filaments are needed for divisome activation once it is assembled, but they are not essential for divisome assembly or guiding septum synthesis.IMPORTANCECell division is fundamental for cellular duplication. In simple cells like Escherichia coli bacteria, the actin homolog FtsA is essential for cell division and assembles into a variety of protein filaments at the cytoplasmic membrane. These filaments not only help tether polymers of the tubulin-like FtsZ to the membrane at early stages of cell division but also play crucial roles in recruiting other cell division proteins to a complex called the divisome. Once assembled, the E. coli divisome subsequently activates synthesis of the division septum that splits the cell in two. One recently discovered oligomeric conformation of FtsA is an antiparallel double-stranded filament. Using a combination of in vivo crosslinking and genetics, we provide evidence suggesting that these FtsA double filaments have a crucial role in activating the septum synthesis enzymes.

Role of the antiparallel double-stranded filament form of FtsA in activating the Escherichia coli divisome

The actin-like FtsA protein is essential for function of the cell division machinery, or divisome, in many bacteria including Escherichia coli. Previous in vitro studies demonstrated that purified wild-type FtsA assembles into closed mini-rings on lipid membranes, but oligomeric variants of FtsA such as FtsAR286W and FtsAG50E can bypass certain divisome defects and form arc and double-stranded (DS) oligomeric states, respectively, which may reflect conversion of an inactive to an active form of FtsA. Yet, it remains unproven which oligomeric forms of FtsA are responsible for assembling and activating the divisome. Here we used an in vivo crosslinking assay for FtsA DS filaments to show that they largely depend on proper divisome assembly and are prevalent at later stages of cell division. We also used a previously reported variant that fails to assemble DS filaments, FtsAM96E R153D, to investigate the roles of FtsA oligomeric states in divisome assembly and activation. We show that FtsAM96E R153D cannot form DS filaments in vivo, fails to replace native FtsA, and confers a dominant negative phenotype, underscoring the importance of the DS filament stage for FtsA function. Surprisingly, however, activation of the divisome through the ftsL* or ftsW* superfission alleles suppressed the dominant negative phenotype and rescued the functionallity of FtsAM96E R153D. Our results suggest that FtsA DS filaments are needed for divisome activation once it is assembled, but they are not essential for divisome assembly or guiding septum synthesis.

Stay on track - revelations of bacterial cell wall synthesis enzymes and things that go by single-molecule imaging

In this review, we explore the regulation of septal peptidoglycan (sPG) synthesis in bacterial cell division, a critical process for cell viability and proper morphology. Recent single-molecule imaging studies have revealed the processive movement of the FtsW:bPBP synthase complex along the septum, shedding light on the spatiotemporal dynamics of sPG synthases and their regulators. In diderm bacteria (E. coli and C. crescentus), the movement occurs at two distinct speeds, reflecting active synthesis or inactivity driven by FtsZ-treadmilling. In monoderm bacteria (B. subtilis, S. pneumoniae, and S. aureus), however, these enzymes exhibit only the active sPG-track-coupled processive movement. By comparing the dynamics of sPG synthases in these organisms and that of class-A penicillin-binding proteins in vivo and in vitro, we propose a unifying model for septal cell wall synthesis regulation across species, highlighting the roles of the sPG- and Z-tracks in orchestrating a robust bacterial cell wall constriction process.

Coevolutionary analysis of Pseudomonas syringae-phage interactions to help with rational design of phage treatments

Treating plant bacterial diseases is notoriously difficult because of the lack of available antimicrobials. Pseudomonas syringae pathovar syringae (Pss) is a major pathogen of cherry (Prunus avium) causing bacterial canker of the stem, leaf and fruit, impacting productivity and leading to a loss of trees. In an attempt to find a treatment for this disease, naturally occurring bacteriophage (phage) that specifically target Pss is being investigated as a biocontrol strategy. However, before using them as a biocontrol treatment, it is important to both understand their efficacy in reducing the bacterial population and determine if the bacterial pathogens can evolve resistance to evade phage infection. To investigate this, killing curve assays of five MR phages targeting Pss showed that phage resistance rapidly emerges in vitro, even when using a cocktail of the five phages together. To gain insight to the changes occurring, Pss colonies were collected three times during a 66-h killing curve assay and separately, Pss and phage were also coevolved over 10 generations, enabling the measurement of genomic and fitness changes in bacterial populations. Pss evolved resistance to phages through modifications in lipopolysaccharide (LPS) synthesis pathways. Bacterial fitness (growth) and virulence were affected in only a few mutants. Deletion of LPS-associated genes suggested that LPS was the main target receptor for all five MR phages. Later generations of coevolved phages from the coevolution experiment were more potent at reducing the bacterial density and when used with wild-type phages could reduce the emergence of phage-resistant mutants. This study shows that understanding the genetic mechanisms of bacterial pathogen resistance to phages is important for helping to design a more effective approach to kill the bacteria while minimizing the opportunity for phage resistance to manifest.

Antibacterial insights into alternariol and its derivative alternariol monomethyl ether produced by a marine fungus

Alternaria alternata FB1 is a marine fungus identified as a candidate for plastic degradation in our previous study. This fungus has been recently shown to produce secondary metabolites with significant antimicrobial activity against various pathogens, including methicillin-resistant Staphylococcus aureus (MRSA) and the notorious aquaculture pathogen Vibrio anguillarum. The antibacterial compounds were purified and identified as alternariol (AOH) and its derivative, alternariol monomethyl ether (AME). We found that AOH and AME primarily inhibited pathogenic bacteria (MRSA or V. anguillarum) by disordering cell division and some other key physiological and biochemical processes. We further demonstrated that AOH could effectively inhibit the unwinding activity of MRSA topoisomerases, which are closely related to cell division and are the potential action target of AOH. The antibacterial activities of AOH and AME were verified by using zebrafish as the in vivo model. Notably, AOH and AME did not significantly affect the viability of normal human liver cells at concentrations that effectively inhibited MRSA or V. anguillarum. Finally, we developed the genetic operation system of A. alternata FB1 and blocked the biosynthesis of AME by knocking out omtI (encoding an O-methyl transferase), which facilitated A. alternata FB1 to only produce AOH. The development of this system in the marine fungus will accelerate the discovery of novel natural products and further bioactivity study.IMPORTANCEMore and more scientific reports indicate that alternariol (AOH) and its derivative alternariol monomethyl ether (AME) exhibit antibacterial activities. However, limited exploration of their detailed antibacterial mechanisms has been performed. In the present study, the antibacterial mechanisms of AOH and AME produced by the marine fungus Alternaria alternata FB1 were disclosed in vitro and in vivo. Given their low toxicity on the normal human liver cell line under the concentrations exhibiting significant antibacterial activity against different pathogens, AOH and AME are proposed to be good candidates for developing promising antibiotics against methicillin-resistant Staphylococcus aureus and Vibrio anguillarum. We also succeeded in blocking the biosynthesis of AME, which facilitated us to easily obtain pure AOH. Moreover, based on our previous results, A. alternata FB1 was shown to enable polyethylene degradation.

A dynamic duo: Understanding the roles of FtsZ and FtsA for Escherichia coli cell division through in vitro approaches

Bacteria divide by binary fission. The protein machine responsible for this process is the divisome, a transient assembly of more than 30 proteins in and on the surface of the cytoplasmic membrane. Together, they constrict the cell envelope and remodel the peptidoglycan layer to eventually split the cell into two. For Escherichia coli, most molecular players involved in this process have probably been identified, but obtaining the quantitative information needed for a mechanistic understanding can often not be achieved from experiments in vivo alone. Since the discovery of the Z-ring more than 30 years ago, in vitro reconstitution experiments have been crucial to shed light on molecular processes normally hidden in the complex environment of the living cell. In this review, we summarize how rebuilding the divisome from purified components - or at least parts of it - have been instrumental to obtain the detailed mechanistic understanding of the bacterial cell division machinery that we have today.