Geometry of cell division in Staphylococcus aureus

FacZ is a GpsB-interacting protein that prevents aberrant division-site placement in Staphylococcus aureus

Staphylococcus aureus is a Gram-positive pathogen responsible for antibiotic-resistant infections. To identify vulnerabilities in cell envelope biogenesis that may overcome resistance, we enriched for S. aureus transposon mutants with defects in cell surface integrity or cell division by sorting for cells that stain with propidium iodide or have increased light-scattering properties, respectively. Transposon sequencing of the sorted populations identified more than 20 previously uncharacterized factors impacting these processes. Cells inactivated for one of these proteins, factor preventing extra Z-rings (FacZ, SAOUHSC_01855), showed aberrant membrane invaginations and multiple FtsZ cytokinetic rings. These phenotypes were suppressed in mutants lacking the conserved cell-division protein GpsB, which forms an interaction hub bridging envelope biogenesis factors with the cytokinetic ring in S. aureus. FacZ was found to interact directly with GpsB in vitro and in vivo. We therefore propose that FacZ is an envelope biogenesis factor that antagonizes GpsB function to prevent aberrant division events in S. aureus.

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.

The cell cycle of Staphylococcus aureus: An updated review

As bacteria proliferate, DNA replication, chromosome segregation, cell wall synthesis, and cytokinesis occur concomitantly and need to be tightly regulated and coordinated. Although these cell cycle processes have been studied for decades, several mechanisms remain elusive, specifically in coccus-shaped cells such as Staphylococcus aureus. In recent years, major progress has been made in our understanding of how staphylococci divide, including new, fundamental insights into the mechanisms of cell wall synthesis and division site selection. Furthermore, several novel proteins and mechanisms involved in the regulation of replication initiation or progression of the cell cycle have been identified and partially characterized. In this review, we will summarize our current understanding of the cell cycle processes in the spheroid model bacterium S. aureus, with a focus on recent advances in the understanding of how these processes are regulated.

Antibacterial nanopatterned coatings for dental implants

Dental implants, usually made of titanium, are exposed to hostile oral microflora that facilitate bacterial infections and subsequent inflammation. To mitigate these processes, we coated titanium substrates with block copolymer nanopatterns and investigated the bactericidal effect of these coatings against Gram-positive and Gram-negative bacteria. We found that the bactericidal efficacy of the coatings depends on their morphology and surface chemistry as well as on the bacterial strain: an optimal combination can lead to significant bacterial death for a short time, i.e. 90% for 90 min. Human gingival fibroblasts in contact with the nanopatterned coatings showed similar cell attachment and morphology as on bare Ti. Immunostaining assays showed similar levels of CCR7 and CD206 in macrophages cultured over the nanopatterns and bare Ti, demonstrating adequate properties for tissue integration. The nanopatterns induced a small increase in macrophage aspect ratio, which might indicate early states of M2 polarization, given the absence of CD206.

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.

eHooke: A tool for automated image analysis of spherical bacteria based on cell cycle progression

Fluorescence microscopy is a critical tool for cell biology studies on bacterial cell division and morphogenesis. Because the analysis of fluorescence microscopy images evolved beyond initial qualitative studies, numerous images analysis tools were developed to extract quantitative parameters on cell morphology and organization. To understand cellular processes required for bacterial growth and division, it is particularly important to perform such analysis in the context of cell cycle progression. However, manual assignment of cell cycle stages is laborious and prone to user bias. Although cell elongation can be used as a proxy for cell cycle progression in rod-shaped or ovoid bacteria, that is not the case for cocci, such as Staphylococcus aureus. Here, we describe eHooke, an image analysis framework developed specifically for automated analysis of microscopy images of spherical bacterial cells. eHooke contains a trained artificial neural network to automatically classify the cell cycle phase of individual S. aureus cells. Users can then apply various functions to obtain biologically relevant information on morphological features of individual cells and cellular localization of proteins, in the context of the cell cycle.

Peptidoglycan Contribution to the B Cell Superantigen Activity of Staphylococcal Protein A

Staphylococcus aureus causes reiterative and chronic persistent infections. This can be explained by the formidable ability of this pathogen to escape immune surveillance mechanisms. Cells of S. aureus display the abundant staphylococcal protein A (SpA). SpA binds to immunoglobulin (Ig) molecules and coats the bacterial surface to prevent phagocytic uptake. SpA also binds and cross-links variable heavy 3 (VH3) idiotype (IgM) B cell receptors, promoting B cell expansion and the secretion of nonspecific VH3-IgM via a mechanism requiring CD4+ T cell help. SpA binding to antibodies is mediated by the N-terminal Ig-binding domains (IgBDs). The so-called region X, uncharacterized LysM domain, and C-terminal LPXTG sorting signal for peptidoglycan attachment complete the linear structure of the protein. Here, we report that both the LysM domain and the LPXTG motif sorting signal are required for the B cell superantigen activity of SpA in a mouse model of infection. SpA molecules purified from staphylococcal cultures are sufficient to exert B cell superantigen activity and promote immunoglobulin secretion as long as they carry intact LysM and LPXTG motif domains with bound peptidoglycan fragments. The LysM domain binds the glycan chains of peptidoglycan fragments, whereas the LPXTG motif is covalently linked to wall peptides lacking glycan. These findings emphasize the complexity of SpA interactions with B cell receptors.IMPORTANCE The LysM domain is found in all kingdoms of life. While their function in mammals is not known, LysM domains of bacteria and their phage parasites are associated with enzymes that cleave or remodel peptidoglycan. Plants recognize microbe-associated molecular patterns such as chitin via receptors endowed with LysM-containing ectodomains. In plants, such receptors play equally important roles in defense and symbiosis signaling. SpA of S. aureus carries a LysM domain that binds glycan strands of peptidoglycan to influence defined B cell responses that divert pathogen-specific adaptive immune responses.

Ectopic positioning of the cell division plane is associated with single amino acid substitutions in the FtsZ-recruiting SsgB in Streptomyces

In most bacteria, cell division begins with the polymerization of the GTPase FtsZ at mid-cell, which recruits the division machinery to initiate cell constriction. In the filamentous bacterium Streptomyces, cell division is positively controlled by SsgB, which recruits FtsZ to the future septum sites and promotes Z-ring formation. Here, we show that various amino acid (aa) substitutions in the highly conserved SsgB protein result in ectopically placed septa that sever spores diagonally or along the long axis, perpendicular to the division plane. Fluorescence microscopy revealed that between 3.3% and 9.8% of the spores of strains expressing SsgB E120 variants were severed ectopically. Biochemical analysis of SsgB variant E120G revealed that its interaction with FtsZ had been maintained. The crystal structure of Streptomyces coelicolor SsgB was resolved and the key residues were mapped on the structure. Notably, residue substitutions (V115G, G118V, E120G) that are associated with septum misplacement localize in the α2-α3 loop region that links the final helix and the rest of the protein. Structural analyses and molecular simulation revealed that these residues are essential for maintaining the proper angle of helix α3. Our data suggest that besides altering FtsZ, aa substitutions in the FtsZ-recruiting protein SsgB also lead to diagonally or longitudinally divided cells in Streptomyces.

New insights in the coordinated amidase and glucosaminidase activity of the major autolysin (Atl) in Staphylococcus aureus

After bacterial cell division, the daughter cells are still covalently interlinked by the peptidoglycan network which is resolved by specific hydrolases (autolysins) to release the daughter cells. In staphylococci, the major autolysin (Atl) with its two domain enzymes, N-acetylmuramyl-L-alanine amidase (AmiA) and β-N-acetylglucosaminidase (GlcA), resolves the peptidoglycan to release the daughter cells. Internal deletions in each of the enzyme domains revealed defined morphological alterations such as cell cluster formation in ΔamiA, ΔglcA and Δatl, and asymmetric cell division in the ΔglcA. A most important finding was that GlcA activity requires the prior removal of the stem peptide by AmiA for its activity thus the naked glycan strand is its substrate. Furthermore, GlcA is not an endo-β-N-acetylglucosaminidase but an exo-enzyme that cuts the glycan backbone to disaccharides independent of its O-acetylation modification. Our results shed new light into the sequential peptidoglycan hydrolysis by AmiA and GlcA during cell division in staphylococci.

Nega et al. shed light on the interplay of the two domain enzymes of the major autolysin, AmiA and GlcA, in S. aureus for peptidoglycan hydrolysis during bacterial cell division. They show that GlcA requires the prior removal of the stem peptide by AmiA for its activity and that GlcA is not an endo-enzyme as previously thought, but an exo-enzyme that chops down the glycan backbone to disaccharides independent of its O-acetylation modification.

Reassessment of the distinctive geometry of Staphylococcus aureus cell division

Staphylococcus aureus is generally thought to divide in three alternating orthogonal planes over three consecutive division cycles. Although this mode of division was proposed over four decades ago, the molecular mechanism that ensures this geometry of division has remained elusive. Here we show, for three different strains, that S. aureus cells do not regularly divide in three alternating perpendicular planes as previously thought. Imaging of the divisome shows that a plane of division is always perpendicular to the previous one, avoiding bisection of the nucleoid, which segregates along an axis parallel to the closing septum. However, one out of the multiple planes perpendicular to the septum which divide the cell in two identical halves can be used in daughter cells, irrespective of its orientation in relation to the penultimate division plane. Therefore, division in three orthogonal planes is not the rule in S. aureus.

Genetic and Biochemical Analysis of CodY-Mediated Cell Aggregation in Staphylococcus aureus Reveals an Interaction between Extracellular DNA and Polysaccharide in the Extracellular Matrix

The global regulator CodY links nutrient availability to the regulation of virulence factor gene expression in Staphylococcus aureus, including many genes whose products affect biofilm formation. Antithetical phenotypes of both biofilm deficiency and accumulation have been reported for codY-null mutants; thus, the role of CodY in biofilm development remains unclear. codY mutant cells of a strain producing a robust biofilm elaborate proaggregation surface-associated features not present on codY mutant cells that do not produce a robust biofilm. Biochemical analysis of the clinical isolate SA564, which aggregates when deficient for CodY, revealed that these features are sensitive to nuclease treatment and are resistant to protease exposure. Genetic analyses revealed that disrupting lgt (the diacylglycerol transferase gene) in codY mutant cells severely weakened aggregation, indicating a role for lipoproteins in the attachment of the biofilm matrix to the cell surface. An additional and critical role of IcaB in producing functional poly-N-acetylglucosamine (PIA) polysaccharide in extracellular DNA (eDNA)-dependent biofilm formation was shown. Moreover, overproducing PIA is sufficient to promote aggregation in a DNA-dependent manner regardless of source of nucleic acids. Taken together, our results point to PIA synthesis as the primary determinant of biofilm formation when CodY activity is reduced and suggest a modified electrostatic net model for matrix attachment whereby PIA associates with eDNA, which interacts with the cell surface via covalently attached membrane lipoproteins. This work counters the prevailing view that polysaccharide- and eDNA/protein-based biofilms are mutually exclusive. Rather, we demonstrate that eDNA and PIA can work synergistically to form a biofilm.IMPORTANCE Staphylococcus aureus remains a global health concern and exemplifies the ability of an opportunistic pathogen to adapt and persist within multiple environments, including host tissue. Not only does biofilm contribute to persistence and immune evasion in the host environment, it also may aid in the transition to invasive disease. Thus, understanding how biofilms form is critical for developing strategies for dispersing biofilms and improving biofilm disease-related outcomes. Using biochemical, genetic, and cell biology approaches, we reveal a synergistic interaction between PIA and eDNA that promotes cell aggregation and biofilm formation in a CodY-dependent manner in S. aureus We also reveal that envelope-associated lipoproteins mediate attachment of the biofilm matrix to the cell surface.

The Staphylococcal Cell Wall

Dating back to the 1960s, initial studies on the staphylococcal cell wall were driven by the need to clarify the mode of action of the first antibiotics and the resistance mechanisms developed by the bacteria. During the following decades, the elucidation of the biosynthetic path and primary composition of staphylococcal cell walls was propelled by advances in microbial cell biology, specifically, the introduction of high-resolution analytical techniques and molecular genetic approaches. The field of staphylococcal cell wall gradually gained its own significance as the complexity of its chemical structure and involvement in numerous cellular processes became evident, namely its versatile role in host interactions, coordination of cell division and environmental stress signaling.This chapter includes an updated description of the anatomy of staphylococcal cell walls, paying particular attention to information from the last decade, under four headings: high-resolution analysis of the Staphylococcus aureus peptidoglycan; variations in peptidoglycan composition; genetic determinants and enzymes in cell wall synthesis; and complex functions of cell walls. The latest contributions to a more precise picture of the staphylococcal cell envelope were possible due to recently developed state-of-the-art microscopy and spectroscopy techniques and to a wide combination of -omics approaches, that are allowing to obtain a more integrative view of this highly dynamic structure.

Spo0J and SMC are required for normal chromosome segregation in Staphylococcus aureus

Bacterial chromosome segregation is an essential cellular process that is particularly elusive in spherical bacteria such as the opportunistic human pathogen Staphylococcus aureus. In this study, we examined the functional significance of a ParB homologue, Spo0J, in staphylococcal chromosome segregation and investigated the role of the structural maintenance of chromosomes (SMC) bacterial condensin in this process. We show that neither spo0J nor smc is essential in S. aureus; however, their absence causes abnormal chromosome segregation. We demonstrate that formation of complexes containing Spo0J and SMC is required for efficient S. aureus chromosome segregation and that SMC localization is dependent on Spo0J. Furthermore, we found that cell division and cell cycle progression are unaffected by the absence of spo0J or smc. Our results verify the role of Spo0J and SMC in ensuring accurate staphylococcal chromosome segregation and also imply functional redundancy or the involvement of additional mechanisms that might contribute to faithful chromosome inheritance.

SosA inhibits cell division in Staphylococcus aureus in response to DNA damage

Inhibition of cell division is critical for viability under DNA‐damaging conditions. DNA damage induces the SOS response that in bacteria inhibits cell division while repairs are being made. In coccoids, such as the human pathogen, Staphylococcus aureus, this process remains poorly studied. Here, we identify SosA as the staphylococcal SOS‐induced cell division inhibitor. Overproduction of SosA inhibits cell division, while sosA inactivation sensitizes cells to genotoxic stress. SosA is a small, predicted membrane protein with an extracellular C‐terminal domain in which point mutation of residues that are conserved in staphylococci and major truncations abolished the inhibitory activity. In contrast, a minor truncation led to SosA accumulation and a strong cell division inhibitory activity, phenotypically similar to expression of wild‐type SosA in a CtpA membrane protease mutant. This suggests that the extracellular C‐terminus of SosA is required both for cell division inhibition and for turnover of the protein. Microscopy analysis revealed that SosA halts cell division and synchronizes the cell population at a point where division proteins such as FtsZ and EzrA are localized at midcell, and the septum formation is initiated but unable to progress to closure. Thus, our findings show that SosA is central in cell division regulation in staphylococci.

Staphylococcal Protein Secretion and Envelope Assembly

The highly cross-linked peptidoglycan represents the rigid layer of the bacterial envelope and protects bacteria from osmotic lysis. In Gram-positive bacteria, peptidoglycan also functions as a scaffold for the immobilization of capsular polysaccharide, wall teichoic acid (WTA), and surface proteins. This chapter captures recent development on the assembly of the envelope of Staphylococcus aureus including mechanisms accounting for immobilization of molecules to peptidoglycan as well as hydrolysis of peptidoglycan for the specific release of bound molecules, facilitation of protein secretion across the envelope and cell division. Peptidoglycan, WTA and capsular polysaccharide are directly synthesized onto undecaprenol. Surface proteins are anchored by Sortase A, a membrane-embedded transpeptidase that scans secreted polypeptides for the C-terminal LPXTG motif of sorting signals. The resulting acyl enzyme intermediate is resolved by lipid II, the undecaprenol-bound peptidoglycan precursor. While these pathways share membrane diffusible undecaprenol, assembly of these molecules occurs either at the cross-walls or the cell poles. In S. aureus, the cross-wall represents the site of de novo peptidoglycan synthesis which is eventually split to complete the cell cycle yielding newly divided daughter cells. Peptidoglycan synthesized at the cross-wall is initially devoid of WTA. Conversely, lipoteichoic acid (LTA) synthesis which does not require bactoprenol is seemingly restricted to septal membranes. Similarly, S. aureus distinguishes two types of surface protein precursors. Polypeptides with canonical signal peptides are deposited at the cell poles, whereas precursors with conserved YSIRK-GXXS motif signal peptides traffic to the cross-wall. A model for protein trafficking in the envelope and uneven distribution of teichoic acids is discussed.

Septal secretion of protein A in Staphylococcus aureus requires SecA and lipoteichoic acid synthesis

Surface proteins of Staphylococcus aureus are secreted across septal membranes for assembly into the bacterial cross-wall. This localized secretion requires the YSIRK/GXXS motif signal peptide, however the mechanisms supporting precursor trafficking are not known. We show here that the signal peptide of staphylococcal protein A (SpA) is cleaved at the YSIRK/GXXS motif. A SpA signal peptide mutant defective for YSIRK/GXXS cleavage is also impaired for septal secretion and co-purifies with SecA, SecDF and LtaS. SecA depletion blocks precursor targeting to septal membranes, whereas deletion of secDF diminishes SpA secretion into the cross-wall. Depletion of LtaS blocks lipoteichoic acid synthesis and abolishes SpA precursor trafficking to septal membranes. We propose a model whereby SecA directs SpA precursors to lipoteichoic acid-rich septal membranes for YSIRK/GXXS motif cleavage and secretion into the cross-wall.

Fracture mechanics modeling of popping event during daughter cell separation

Most bacteria cells divide by binary fission which is part of a bacteria cell cycle and requires tight regulations and precise coordination. Fast separation of Staphylococcus Aureus (S. Aureus) daughter cells, named as popping event, has been observed in recent experiments. The popping event was proposed to be driven by mechanical crack propagation in the peripheral ring which connected two daughter cells before their separation. It has also been shown that after the fast separation, a small portion of the peripheral ring was left as a hinge. In the article, we develop a fracture mechanics model for the crack growth in the peripheral ring during S. Aureus daughter cell separation. In particular, using finite element analysis, we calculate the energy release rate associated with the crack growth in the peripheral ring, when daughter cells are inflated by a uniform turgor pressure inside. Our results show that with a fixed inflation of daughter cells, the energy release rate depends on the crack length non-monotonically. The energy release rate reaches a maximum value for a crack of an intermediate length. The non-monotonic relationship between the energy release rate and crack length clearly indicates that the crack propagation in the peripheral ring can be unstable. The computed energy release rate as a function of crack length can also be used to explain the existence of a small portion of peripheral ring remained as hinge after the popping event.

Nanodarts, nanoblades, and nanospikes: Mechano-bactericidal nanostructures and where to find them

Over the past ten years, a next-generation approach to combat bacterial contamination has emerged: one which employs nanostructure geometry to deliver lethal mechanical forces causing bacterial cell death. In this review, we first discuss advances in both colloidal and topographical nanostructures shown to exhibit such "mechano-bactericidal" mechanisms of action. Next, we highlight work from pioneering research groups in this area of antibacterials. Finally, we provide suggestions for unexplored research topics that would benefit the field of mechano-bactericidal nanostructures. Traditionally, antibacterial materials are loaded with antibacterial agents with the expectation that these agents will be released in a timely fashion to reach their intended bacterial metabolic target at a sufficient concentration. Such antibacterial approaches, generally categorized as chemical-based, face design drawbacks as compounds diffuse in all directions, leach into the environment, and require replenishing. In contrast, due to their mechanisms of action, mechano-bactericidal nanostructures can benefit from sustainable opportunities. Namely, mechano-bactericidal efficacy needs not replenishing since they are not consumed metabolically, nor are they designed to release or leach compounds. For this same reason, however, their action is limited to the bacterial cells that have made direct contact with mechano-bactericidal nanostructures. As suspended colloids, mechano-bactericidal nanostructures such as carbon nanotubes and graphene nanosheets can pierce or slice bacterial membranes. Alternatively, surface topography such as mechano-bactericidal nanopillars and nanospikes can inflict critical membrane damage to microorganisms perched upon them, leading to subsequent cell lysis and death. Despite the infancy of this area of research, materials constructed from these nanostructures show remarkable antibacterial potential worthy of further investigation.

Coordination of Chromosome Segregation and Cell Division in Staphylococcus aureus

Productive bacterial cell division and survival of progeny requires tight coordination between chromosome segregation and cell division to ensure equal partitioning of DNA. Unlike rod-shaped bacteria that undergo division in one plane, the coccoid human pathogen Staphylococcus aureus divides in three successive orthogonal planes, which requires a different spatial control compared to rod-shaped cells. To gain a better understanding of how this coordination between chromosome segregation and cell division is regulated in S. aureus, we investigated proteins that associate with FtsZ and the divisome. We found that DnaK, a well-known chaperone, interacts with FtsZ, EzrA and DivIVA, and is required for DivIVA stability. Unlike in several rod shaped organisms, DivIVA in S. aureus associates with several components of the divisome, as well as the chromosome segregation protein, SMC. This data, combined with phenotypic analysis of mutants, suggests a novel role for S. aureus DivIVA in ensuring cell division and chromosome segregation are coordinated.

Bacterial Cell Division: Nonmodels Poised to Take the Spotlight

The last three decades have witnessed an explosion of discoveries about the mechanistic details of binary fission in model bacteria such as Escherichia coli, Bacillus subtilis, and Caulobacter crescentus. This was made possible not only by advances in microscopy that helped answer questions about cell biology but also by clever genetic manipulations that directly and easily tested specific hypotheses. More recently, research using understudied organisms, or nonmodel systems, has revealed several alternate mechanistic strategies that bacteria use to divide and propagate. In this review, we highlight new findings and compare these strategies to cell division mechanisms elucidated in model organisms.

Long-term microfluidic tracking of coccoid cyanobacterial cells reveals robust control of division timing

Background

Cyanobacteria are important agents in global carbon and nitrogen cycling and hold great promise for biotechnological applications. Model organisms such as Synechocystis sp. and Synechococcus sp. have advanced our understanding of photosynthetic capacity and circadian behavior, mostly using population-level measurements in which the behavior of individuals cannot be monitored. Synechocystis sp. cells are small and divide slowly, requiring long-term experiments to track single cells. Thus, the cumulative effects of drift over long periods can cause difficulties in monitoring and quantifying cell growth and division dynamics.

Results

To overcome this challenge, we enhanced a microfluidic cell-culture device and developed an image analysis pipeline for robust lineage reconstruction. This allowed simultaneous tracking of many cells over multiple generations, and revealed that cells expand exponentially throughout their cell cycle. Generation times were highly correlated for sister cells, but not between mother and daughter cells. Relationships between birth size, division size, and generation time indicated that cell-size control was inconsistent with the “sizer” rule, where division timing is based on cell size, or the “timer” rule, where division occurs after a fixed time interval. Instead, single cell growth statistics were most consistent with the “adder” rule, in which division occurs after a constant increment in cell volume. Cells exposed to light-dark cycles exhibited growth and division only during the light period; dark phases pause but do not disrupt cell-cycle control.

Conclusions

Our analyses revealed that the “adder” model can explain both the growth-related statistics of single Synechocystis cells and the correlation between sister cell generation times. We also observed rapid phenotypic response to light-dark transitions at the single cell level, highlighting the critical role of light in cyanobacterial cell-cycle control. Our findings suggest that by monitoring the growth kinetics of individual cells we can build testable models of circadian control of the cell cycle in cyanobacteria.

Electronic supplementary material

The online version of this article (doi:10.1186/s12915-016-0344-4) contains supplementary material, which is available to authorized users.

Staphylococcus aureus requires at least one FtsK/SpoIIIE protein for correct chromosome segregation

Faithful coordination between bacterial cell division and chromosome segregation in rod-shaped bacteria, such as Escherichia coli and Bacillus subtilis, is dependent on the DNA translocase activity of FtsK/SpoIIIE proteins, which move DNA away from the division site before cytokinesis is completed. However, the role of these proteins in chromosome partitioning has not been well studied in spherical bacteria. Here, it was shown that the two Staphylococcus aureus FtsK/SpoIIIE homologues, SpoIIIE and FtsK, operate in independent pathways to ensure correct chromosome management during cell division. SpoIIIE forms foci at the centre of the closing septum in at least 50% of the cells that are close to complete septum synthesis. FtsK is a multifunctional septal protein with a C-terminal DNA translocase domain that is not required for correct chromosome management in the presence of SpoIIIE. However, lack of both SpoIIIE and FtsK causes severe nucleoid segregation and morphological defects, showing that the two proteins have partially redundant roles in S. aureus.

The Peptidoglycan Pattern of Staphylococcus carnosus TM300-Detailed Analysis and Variations Due to Genetic and Metabolic Influences

The Gram-positive bacterium Staphylococcus carnosus (S. carnosus) TM300 is an apathogenic staphylococcal species commonly used in meat starter cultures. As with all Gram-positive bacteria, its cytoplasmic membrane is surrounded by a thick peptidoglycan (PGN) or murein sacculus consisting of several layers of glycan strands cross-linked by peptides. In contrast to pathogenic staphylococci, mainly Staphylococcus aureus (S. aureus), the chemical composition of S. carnosus PGN is not well studied so far. UPLC/MS analysis of enzymatically digested S. carnosus TM300 PGN revealed substantial differences in its composition compared to the known pattern of S. aureus. While in S. aureus the uncross-linked stem peptide consists of a pentapeptide, in S. carnosus, this part of the PGN is shortened to tripeptides. Furthermore, we found the PGN composition to vary when cells were incubated under certain conditions. The collective overproduction of HlyD, FtsE and FtsX—a putative protein complex interacting with penicillin-binding protein 2 (PBP2)—caused the reappearance of classical penta stem peptides. In addition, under high sugar conditions, tetra stem peptides occur due to overflow metabolism. This indicates that S. carnosus TM300 cells adapt to various conditions by modification of their PGN.

Mechanical signatures of microbial biofilms in micropillar-embedded growth chambers

Biofilms are surface-attached communities of microorganisms embedded in an extracellular matrix and are essential for the cycling of organic matter in natural and engineered environments. They are also the leading cause of many infections, for example, those associated with chronic wounds and implanted medical devices. The extracellular matrix is a key biofilm component that determines its architecture and defines its physical properties. Herein, we used growth chambers embedded with micropillars to study the net mechanical forces (differential pressure) exerted during biofilm formation in situ. Pressure from the biofilm is transferred to the micropillars via the extracellular matrix, and reduction of major matrix components decreases the magnitude of micropillar deflections. The spatial arrangement of micropillar deflections caused by pressure differences in the different biofilm strains may potentially be used as mechanical signatures for biofilm characterization. Hence, we submit that micropillar-embedded growth chambers provide insights into the mechanical properties and dynamics of the biofilm and its matrix.

Supramolecular structure in the membrane of Staphylococcus aureus

All life demands the temporal and spatial control of essential biological functions. In bacteria, the recent discovery of coordinating elements provides a framework to begin to explain cell growth and division. Here we present the discovery of a supramolecular structure in the membrane of the coccal bacterium Staphylococcus aureus, which leads to the formation of a large-scale pattern across the entire cell body; this has been unveiled by studying the distribution of essential proteins involved in lipid metabolism (PlsY and CdsA). The organization is found to require MreD, which determines morphology in rod-shaped cells. The distribution of protein complexes can be explained as a spontaneous pattern formation arising from the competition between the energy cost of bending that they impose on the membrane, their entropy of mixing, and the geometric constraints in the system. Our results provide evidence for the existence of a self-organized and nonpercolating molecular scaffold involving MreD as an organizer for optimal cell function and growth based on the intrinsic self-assembling properties of biological molecules.

Cell shape dynamics during the staphylococcal cell cycle

Staphylococcus aureus is an aggressive pathogen and a model organism to study cell division in sequential orthogonal planes in spherical bacteria. However, the small size of staphylococcal cells has impaired analysis of changes in morphology during the cell cycle. Here we use super-resolution microscopy and determine that S. aureus cells are not spherical throughout the cell cycle, but elongate during specific time windows, through peptidoglycan synthesis and remodelling. Both peptidoglycan hydrolysis and turgor pressure are required during division for reshaping the flat division septum into a curved surface. In this process, the septum generates less than one hemisphere of each daughter cell, a trait we show is common to other cocci. Therefore, cell surface scars of previous divisions do not divide the cells in quadrants, generating asymmetry in the daughter cells. Our results introduce a need to reassess the models for division plane selection in cocci.

Staphylococci are spherical bacteria that divide in sequential orthogonal planes. Here, the authors use super-resolution microscopy to show that staphylococcal cells elongate before dividing, and that the division septum generates less than one hemisphere of each daughter cell, generating asymmetry.

Bacterial Cell Enlargement Requires Control of Cell Wall Stiffness Mediated by Peptidoglycan Hydrolases

Most bacterial cells are enclosed in a single macromolecule of the cell wall polymer, peptidoglycan, which is required for shape determination and maintenance of viability, while peptidoglycan biosynthesis is an important antibiotic target. It is hypothesized that cellular enlargement requires regional expansion of the cell wall through coordinated insertion and hydrolysis of peptidoglycan. Here, a group of (apparent glucosaminidase) peptidoglycan hydrolases are identified that are together required for cell enlargement and correct cellular morphology of Staphylococcus aureus, demonstrating the overall importance of this enzyme activity. These are Atl, SagA, ScaH, and SagB. The major advance here is the explanation of the observed morphological defects in terms of the mechanical and biochemical properties of peptidoglycan. It was shown that cells lacking groups of these hydrolases have increased surface stiffness and, in the absence of SagB, substantially increased glycan chain length. This indicates that, beyond their established roles (for example in cell separation), some hydrolases enable cellular enlargement by making peptidoglycan easier to stretch, providing the first direct evidence demonstrating that cellular enlargement occurs via modulation of the mechanical properties of peptidoglycan.

IMPORTANCE

Understanding bacterial growth and division is a fundamental problem, and knowledge in this area underlies the treatment of many infectious diseases. Almost all bacteria are surrounded by a macromolecule of peptidoglycan that encloses the cell and maintains shape, and bacterial cells must increase the size of this molecule in order to enlarge themselves. This requires not only the insertion of new peptidoglycan monomers, a process targeted by antibiotics, including penicillin, but also breakage of existing bonds, a potentially hazardous activity for the cell. Using Staphylococcus aureus, we have identified a set of enzymes that are critical for cellular enlargement. We show that these enzymes are required for normal growth and define the mechanism through which cellular enlargement is accomplished, i.e., by breaking bonds in the peptidoglycan, which reduces the stiffness of the cell wall, enabling it to stretch and expand, a process that is likely to be fundamental to many bacteria.

Bacterial division. Mechanical crack propagation drives millisecond daughter cell separation in Staphylococcus aureus

When Staphylococcus aureus undergoes cytokinesis, it builds a septum, generating two hemispherical daughters whose cell walls are only connected via a narrow peripheral ring. We found that resolution of this ring occurred within milliseconds ("popping"), without detectable changes in cell volume. The likelihood of popping depended on cell-wall stress, and the separating cells split open asymmetrically, leaving the daughters connected by a hinge. An elastostatic model of the wall indicated high circumferential stress in the peripheral ring before popping. Last, we observed small perforations in the peripheral ring that are likely initial points of mechanical failure. Thus, the ultrafast daughter cell separation in S. aureus appears to be driven by accumulation of stress in the peripheral ring and exhibits hallmarks of mechanical crack propagation.

Controlling mechanisms in directional growth of aggregated archaeal cells

Members of the family Methanosarcinaceae are important archaeal representatives due to their broad functionality, ubiquitous presence, and functionality in harsh environments. A key characteristic is their multicellular (packet) morphology represented by aggregates of spatially confined cells. This morphology is driven by directed growth of cells in confinement with sequential variation in growth direction. To further understand why spatially confined Methanosarcina cells (and in general, confined prokaryotes) change their direction of growth during consecutive growth-division stages, and how a particular cell senses its wall topology and responds to changes on it a theoretical model for stress dependent growth of aggregated archaeal cells was developed. The model utilizes a confined elastic shell representation of aggregated archaeal cell and is derived based on a work-energy principle. The growth law takes into account the fine structure of archaeal cell wall, polymeric nature of methanochondroitin layer, molecular-biochemical processes and is based on thermodynamic laws. The developed model has been applied to three typical configurations of aggregated cell in 3D. The developed model predicted a geometry response with delayed growth of aggregated archaeal cells explained from mechanistic principles, as well as continuous changes in direction of growth during the consecutive growth-division stages. This means that cell wall topology sensing and growth anisotropy can be predicted using simple cellular mechanisms without the need for dedicated cellular machinery.

Sec-secretion and sortase-mediated anchoring of proteins in Gram-positive bacteria

Signal peptide-driven secretion of precursor proteins directs polypeptides across the plasma membrane of bacteria. Two pathways, Sec- and SRP-dependent, converge at the SecYEG translocon to thread unfolded precursor proteins across the membrane, whereas folded preproteins are routed via the Tat secretion pathway. Gram-positive bacteria lack an outer membrane and are surrounded by a rigid layer of peptidoglycan. Interactions with their environment are mediated by proteins that are retained in the cell wall, often through covalent attachment to the peptidoglycan. In this review, we describe the mechanisms for both Sec-dependent secretion and sortase-dependent assembly of proteins in the envelope of Gram-positive bacteria. This article is part of a Special Issue entitled: Protein trafficking and secretion in bacteria. Guest Editors: Anastassios Economou and Ross Dalbey.

Structure-related antibacterial activity of a titanium nanostructured surface fabricated by glancing angle sputter deposition

The aim of this study was to reproduce the physico-mechanical antibacterial effect of the nanocolumnar cicada wing surface for metallic biomaterials by fabrication of titanium (Ti) nanocolumnar surfaces using glancing angle sputter deposition (GLAD). Nanocolumnar Ti thin films were fabricated by GLAD on silicon substrates. S. aureus as well as E. coli were incubated with nanostructured or reference dense Ti thin film test samples for one or three hours at 37 °C. Bacterial adherence, morphology, and viability were analyzed by fluorescence staining and scanning electron microscopy and compared to human mesenchymal stem cells (hMSCs).Bacterial adherence was not significantly different after short (1 h) incubation on the dense or the nanostructured Ti surface. In contrast to S. aureus the viability of E. coli was significantly decreased after 3 h on the nanostructured film compared to the dense film and was accompanied by an irregular morphology and a cell wall deformation. Cell adherence, spreading and viability of hMSCs were not altered on the nanostructured surface. The results show that the selective antibacterial effect of the cicada wing could be transferred to a nanostructured metallic biomaterial by mimicking the natural nanocolumnar topography.

Moenomycin resistance mutations in Staphylococcus aureus reduce peptidoglycan chain length and cause aberrant cell division

Staphylococcus aureus is a Gram-positive pathogen with an unusual mode of cell division in that it divides in orthogonal rather than parallel planes. Through selection using moenomycin, an antibiotic proposed to target peptidoglycan glycosyltransferases (PGTs), we have generated resistant mutants containing a single point mutation in the active site of the PGT domain of an essential peptidoglycan (PG) biosynthetic enzyme, PBP2. Using cell free polymerization assays, we show that this mutation alters PGT activity so that much shorter PG chains are made. The same mutation in another S. aureus PGT, SgtB, has a similar effect on glycan chain length. Moenomycin-resistant S. aureus strains containing mutated PGTs that make only short glycan polymers display major cell division defects, implicating PG chain length in determining bacterial cell morphology and division site placement.

Division site positioning in bacteria: one size does not fit all

Spatial regulation of cell division in bacteria has been a focus of research for decades. It has been well studied in two model rod-shaped organisms, Escherichia coli and Bacillus subtilis, with the general belief that division site positioning occurs as a result of the combination of two negative regulatory systems, Min and nucleoid occlusion. These systems influence division by preventing the cytokinetic Z ring from forming anywhere other than midcell. However, evidence is accumulating for the existence of additional mechanisms that are involved in controlling Z ring positioning both in these organisms and in several other bacteria. In some cases the decision of where to divide is solved by variations on a common evolutionary theme, and in others completely different proteins and mechanisms are involved. Here we review the different ways bacteria solve the problem of finding the right place to divide. It appears that a one-size-fits-all model does not apply, and that individual species have adapted a division-site positioning mechanism that best suits their lifestyle, environmental niche and mode of growth to ensure equal partitioning of DNA for survival of the next generation.

The giant protein Ebh is a determinant of Staphylococcus aureus cell size and complement resistance

Staphylococcus aureus USA300, the clonal type associated with epidemic community-acquired methicillin-resistant S. aureus (MRSA) infections, displays the giant protein Ebh on its surface. Mutations that disrupt the ebh reading frame increase the volume of staphylococcal cells and alter the cross wall, a membrane-enclosed peptidoglycan synthesis and assembly compartment. S. aureus ebh variants display increased sensitivity to oxacillin (methicillin) as well as susceptibility to complement-mediated killing. Mutations in ebh are associated with reduced survival of mutant staphylococci in blood and diminished virulence in mice. We propose that Ebh, following its secretion into the cross wall, contributes to the characteristic cell growth and envelope assembly pathways of S. aureus, thereby enabling complement resistance and the pathogenesis of staphylococcal infections.

A mutation in the promoter region of zipA, a component of the divisome, suppresses the shape defect of RodZ-deficient cells

RodZ is important for maintaining the rod shape of Escherichia coli. Loss of RodZ causes conversion of the rod shape to a round shape and a growth rate slower than that of wild-type cells. Suppressor mutations that simultaneously restore both the growth rates and the rod shape were isolated. Most of the suppressor mutations are found in mreB, mrdA, or mrdB. One of the mutations was in the promoter region of zipA, which encodes a crucial component of the cell division machinery. In this study, we investigated the mechanism of the suppression by this mutation. ZipA was slightly but significantly increased in the suppressor cells and led to a delay in cell division. While round-shaped mreB and mrdA mutants lose cell bipolarity, we found that round-shaped rodZ mutants retained cell bipolarity. Therefore, we concluded that a delay in the completion of septation provides extra time to elongate the cell laterally so that the zipA suppressor mutant is able to recover its ovoid or rod shape. The suppression by zipA demonstrates that the regulation of timing of septation potentially contributes to the conversion of morphology in bacterial cells.

Growth and laboratory maintenance of Staphylococcus aureus

Staphylococcus aureus is a facultative anaerobic Gram-positive coccus and a member of the normal skin flora as well as the nasal passages of humans. S. aureus is also the etiological agent of suppurative abscesses, as first described by Sir Alexander Ogston in 1880. Ever since, studies on S. aureus have focused on the complex battery of virulence factors and regulators that allow for its swift transition between commensalism and pathogenic states and escape from host immune defenses. The success of this pathogen is further evidenced by its ability to acquire antibiotic resistance traits through mechanisms that often remain poorly understood.

Biological consequences and advantages of asymmetric bacterial growth

Asymmetries in cell growth and division occur in eukaryotes and prokaryotes alike. Even seemingly simple and morphologically symmetric cell division processes belie inherent underlying asymmetries in the composition of the resulting daughter cells. We consider the types of asymmetry that arise in various bacterial cell growth and division processes, which include both conditionally activated mechanisms and constitutive, hardwired aspects of bacterial life histories. Although asymmetry disposes some cells to the deleterious effects of aging, it may also benefit populations by efficiently purging accumulated damage and rejuvenating newborn cells. Asymmetries may also generate phenotypic variation required for successful exploitation of variable environments, even when extrinsic changes outpace the capacity of cells to sense and respond to challenges. We propose specific experimental approaches to further develop our understanding of the prevalence and the ultimate importance of asymmetric bacterial growth.

Comparing contractile apparatus-driven cytokinesis mechanisms across kingdoms

Cytokinesis is the final stage of the cell cycle during which a cell physically divides into two daughters through the assembly of new membranes (and cell wall in some cases) between the forming daughters. New membrane assembly can either proceed centripetally behind a contractile apparatus, as in the case of prokaryotes, archaea, fungi, and animals or expand centrifugally, as in the case of higher plants. In this article, we compare the mechanisms of cytokinesis in diverse organisms dividing through the use of a contractile apparatus. While an actomyosin ring participates in cytokinesis in almost all centripetally dividing eukaryotes, the majority of bacteria and archaea (except Crenarchaea) divide using a ring composed of the tubulin-related protein FtsZ. Curiously, despite molecular conservation of the division machinery components, division site placement and its cell cycle regulation occur by a variety of unrelated mechanisms even among organisms from the same kingdom. While molecular motors and cytoskeletal polymer dynamics contribute to force generation during eukaryotic cytokinesis, cytoskeletal polymer dynamics alone appears to be sufficient for force generation during prokaryotic cytokinesis. Intriguingly, there are life forms on this planet that appear to lack molecules currently known to participate in cytokinesis and how these cells perform cytokinesis remains a mystery waiting to be unravelled.

Determinants of murein hydrolase targeting to cross-wall of Staphylococcus aureus peptidoglycan

Cells of eukaryotic or prokaryotic origin express proteins with LysM domains that associate with the cell wall envelope of bacteria. The molecular properties that enable LysM domains to interact with microbial cell walls are not yet established. Staphylococcus aureus, a spherical microbe, secretes two murein hydrolases with LysM domains, Sle1 and LytN. We show here that the LysM domains of Sle1 and LytN direct murein hydrolases to the staphylococcal envelope in the vicinity of the cross-wall, the mid-cell compartment for peptidoglycan synthesis. LysM domains associate with the repeating disaccharide β-N-acetylmuramic acid, (1→4)-β-N-acetylglucosamine of staphylococcal peptidoglycan. Modification of N-acetylmuramic acid with wall teichoic acid, a ribitol-phosphate polymer tethered to murein linkage units, prevents the LysM domain from binding to peptidoglycan. The localization of LytN and Sle1 to the cross-wall is abolished in staphylococcal tagO mutants, which are defective for wall teichoic acid synthesis. We propose a model whereby the LysM domain ensures septal localization of LytN and Sle1 followed by processive cleavage of peptidoglycan, thereby exposing new LysM binding sites in the cross-wall and separating bacterial cells.

LytN, a murein hydrolase in the cross-wall compartment of Staphylococcus aureus, is involved in proper bacterial growth and envelope assembly

Cell cycle progression for the spherical microbe Staphylococcus aureus requires the coordinated synthesis and remodeling of peptidoglycan. The majority of these rearrangements takes place at the mid-cell, in a compartment designated the cross-wall. Secreted polypeptides endowed with a YSIRK-G/S signal peptide are directly delivered to the cross-wall compartment. One such YSIRK-containing protein is the murein hydrolase LytN. lytN mutations precipitate structural damage to the cross-wall and interfere with staphylococcal growth. Overexpression of lytN also affects growth and triggers rupture of the cross-wall. The lytN phenotype can be reversed by the controlled expression of lytN but not by adding purified LytN to staphylococcal cultures. LytN harbors LysM and CHAP domains, the latter of which functions as both an N-acetylmuramoyl-L-alanine amidase and D-alanyl-glycine endopeptidase. Thus, LytN secretion into the cross-wall promotes peptidoglycan separation and completion of the staphylococcal cell cycle.

ABI domain-containing proteins contribute to surface protein display and cell division in Staphylococcus aureus

The human pathogen Staphylococcus aureus requires cell wall anchored surface proteins to cause disease. During cell division, surface proteins with YSIRK signal peptides are secreted into the cross-wall, a layer of newly synthesized peptidoglycan between separating daughter cells. The molecular determinants for the trafficking of surface proteins are, however, still unknown. We screened mutants with non-redundant transposon insertions by fluorescence-activated cell sorting for reduced deposition of protein A (SpA) into the staphylococcal envelope. Three mutants, each of which harboured transposon insertions in genes for transmembrane proteins, displayed greatly reduced envelope abundance of SpA and surface proteins with YSIRK signal peptides. Characterization of the corresponding mutations identified three transmembrane proteins with abortive infectivity (ABI) domains, elements first described in lactococci for their role in phage exclusion. Mutations in genes for ABI domain proteins, designated spdA, spdB and spdC (surface protein display), diminish the expression of surface proteins with YSIRK signal peptides, but not of precursor proteins with conventional signal peptides. spdA, spdB and spdC mutants display an increase in the thickness of cross-walls and in the relative abundance of staphylococci with cross-walls, suggesting that spd mutations may represent a possible link between staphylococcal cell division and protein secretion.

Peptidoglycan architecture can specify division planes in Staphylococcus aureus

Division in Staphylococci occurs equatorially and on specific sequentially orthogonal planes in three dimensions, resulting, after incomplete cell separation, in the 'bunch of grapes' cluster organization that defines the genus. The shape of Staphylococci is principally maintained by peptidoglycan. In this study, we use Atomic Force Microscopy (AFM) and fluorescence microscopy with vancomycin labelling to examine purified peptidoglycan architecture and its dynamics in Staphylococcus aureus and correlate these with the cell cycle. At the presumptive septum, cells were found to form a large belt of peptidoglycan in the division plane before the centripetal formation of the septal disc; this often had a 'piecrust' texture. After division, the structures remain as orthogonal ribs, encoding the location of past division planes in the cell wall. We propose that this epigenetic information is used to enable S. aureus to divide in sequentially orthogonal planes, explaining how a spherical organism can maintain division plane localization with fidelity over many generations.

Fluorescent reporters for studies of cellular localization of proteins in Staphylococcus aureus

We have constructed a set of plasmids that allow expression, from their native chromosomal loci, of Staphylococcus aureus proteins fused to one of four different fluorescent proteins (green fluorescent protein [GFP], cyan fluorescent protein [CFP], yellow fluorescent protein [YFP], and mCherry), using two different resistance markers (kanamycin and erythromycin). We have also constructed a plasmid that allows expression of proteins from the ectopic spa locus in the S. aureus chromosome. This toolbox can be used for studies of the localization of proteins in S. aureus, a prominent pathogen in both health care and community settings.

Characterization of the Synechocystis strain PCC 6803 penicillin-binding proteins and cytokinetic proteins FtsQ and FtsW and their network of interactions with ZipN

Because very little is known about cell division in noncylindrical bacteria and cyanobacteria, we investigated 10 putative cytokinetic proteins in the unicellular spherical cyanobacterium Synechocystis strain PCC 6803. Concerning the eight penicillin-binding proteins (PBPs), which define three classes, we found that Synechocystis can survive in the absence of one but not two PBPs of either class A or class C, whereas the unique class B PBP (also termed FtsI) is indispensable. Furthermore, we showed that all three classes of PBPs are required for normal cell size. Similarly, the putative FtsQ and FtsW proteins appeared to be required for viability and normal cell size. We also used a suitable bacterial two-hybrid system to characterize the interaction web among the eight PBPs, FtsQ, and FtsW, as well as ZipN, the crucial FtsZ partner that occurs only in cyanobacteria and plant chloroplasts. We showed that FtsI, FtsQ, and ZipN are self-interacting proteins and that both FtsI and FtsQ interact with class A PBPs, as well as with ZipN. Collectively, these findings indicate that ZipN, in interacting with FtsZ and both FtsI and FtQ, plays a similar role to the Escherichia coli FtsA protein, which is missing in cyanobacteria and chloroplasts.

Evidence that CT694 is a novel Chlamydia trachomatis T3S substrate capable of functioning during invasion or early cycle development

Chlamydia trachomatis is an obligate intracellular parasite, occupies a membrane-bound vacuole throughout development and is capable of manipulating the eukaryotic host by translocating effector molecules via a type III secretion system (T3SS). The infectious chlamydial elementary body (EB) is metabolically inactive yet possesses a functional T3S apparatus capable of translocating effector proteins into the host cell to facilitate invasion and other early cycle events. We present evidence here that the C. trachomatis protein CT694 represents an early cycle-associated effector protein. CT694 is secreted by the Yersinia T3SS and immunodetection studies of infected HeLa cultures indicate that CT694-specific signal accumulates directly adjacent to, but not completely overlapping with EBs during invasion. Yeast two-hybrid analyses revealed an interaction of CT694 with the repeat region and C-terminus of human AHNAK. Immunolocalization studies of CT694 ectopically expressed in HeLa cells were consistent with an interaction with endogenous AHNAK. Additionally, expression of CT694 in HeLa cells resulted in alterations in the detection of stress fibres that correlated with the ability of CT694 to interact with AHNAK. These data indicate that CT694 is a novel T3S-dependent substrate unique to C. trachomatis, and that its interaction with host proteins such as AHNAK may be important for aspects of invasion or development particular to this species.