Tetracycline hypersensitivity of an ezrA mutant links GalE and TseB (YpmB) to cell division

The morphogenic protein CopD controls the spatio-temporal dynamics of PBP1a and PBP2b in Streptococcus pneumoniae

IMPORTANCE

Penicillin-binding proteins (PBPs) are essential for proper bacterial cell division and morphogenesis. The genome of Streptococcus pneumoniae encodes for two class B PBPs (PBP2x and 2b), which are required for the assembly of the peptidoglycan framework and three class A PBPs (PBP1a, 1b and 2a), which remodel the peptidoglycan mesh during cell division. Therefore, their activities should be finely regulated in space and time to generate the pneumococcal ovoid cell shape. To date, two proteins, CozE and MacP, are known to regulate the function of PBP1a and PBP2a, respectively. In this study, we describe a novel regulator (CopD) that acts on both PBP1a and PBP2b. These findings provide valuable information for understanding bacterial cell division. Furthermore, knowing that ß-lactam antibiotic resistance often arises from PBP mutations, the characterization of such a regulator represents a promising opportunity to develop new strategies to resensitize resistant strains.

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.

Characterization of TseB: A new actor in cell wall elongation in Bacillus subtilis

Penicillin-binding proteins (PBPs) are crucial enzymes of peptidoglycan assembly and targets of β-lactam antibiotics. However, little is known about their regulation. Recently, membrane proteins were shown to regulate the bifunctional transpeptidases/glycosyltransferases aPBPs in some bacteria. However, up to now, regulators of monofunctional transpeptidases bPBPs have yet to be revealed. Here, we propose that TseB could be such a PBP regulator. This membrane protein was previously found to suppress tetracycline sensitivity of a Bacillus subtilis strain deleted for ezrA, a gene encoding a regulator of septation ring formation. In this study, we show that TseB is required for B. subtilis normal cell shape, tseB mutant cells being shorter and wider than wild-type cells. We observed that TseB interacts with PBP2A, a monofunctional transpeptidase. While TseB is not required for PBP2A activity, stability, and localization, we show that the overproduction of PBP2A is deleterious in the absence of TseB. In addition, we showed that TseB is necessary not only for efficient cell wall elongation during exponential phase but also during spore outgrowth, as it was also observed for PBP2A. Altogether, our results suggest that TseB is a new member of the elongasome that regulates PBP2A function during cell elongation and spore germination.

A flat embedding method for transmission electron microscopy reveals an unknown mechanism of tetracycline

Transmission electron microscopy of cell sample sections is a popular technique in microbiology. Currently, ultrathin sectioning is done on resin-embedded cell pellets, which consumes milli- to deciliters of culture and results in sections of randomly orientated cells. This is problematic for rod-shaped bacteria and often precludes large-scale quantification of morphological phenotypes due to the lack of sufficient numbers of longitudinally cut cells. Here we report a flat embedding method that enables observation of thousands of longitudinally cut cells per single section and only requires microliter culture volumes. We successfully applied this technique to Bacillus subtilis, Escherichia coli, Mycobacterium bovis, and Acholeplasma laidlawii. To assess the potential of the technique to quantify morphological phenotypes, we monitored antibiotic-induced changes in B. subtilis cells. Surprisingly, we found that the ribosome inhibitor tetracycline causes membrane deformations. Further investigations showed that tetracycline disturbs membrane organization and localization of the peripheral membrane proteins MinD, MinC, and MreB. These observations are not the result of ribosome inhibition but constitute a secondary antibacterial activity of tetracycline that so far has defied discovery.

Cell division protein FtsZ: from structure and mechanism to antibiotic target

Antimicrobial resistance to virtually all clinically applied antibiotic classes severely limits the available options to treat bacterial infections. Hence, there is an urgent need to develop and evaluate new antibiotics and targets with resistance-breaking properties. Bacterial cell division has emerged as a new antibiotic target pathway to counteract multidrug-resistant pathogens. New approaches in antibiotic discovery and bacterial cell biology helped to identify compounds that either directly interact with the major cell division protein FtsZ, thereby perturbing the function and dynamics of the cell division machinery, or affect the structural integrity of FtsZ by inducing its degradation. The impressive antimicrobial activities and resistance-breaking properties of certain compounds validate the inhibition of bacterial cell division as a promising strategy for antibiotic intervention.

Aspartate deficiency limits peptidoglycan synthesis and sensitizes cells to antibiotics targeting cell wall synthesis in Bacillus subtilis

Peptidoglycan synthesis is an important target for antibiotics and relies on intermediates derived from central metabolism. As a result, alterations of metabolism may affect antibiotic sensitivity. An aspB mutant is auxotrophic for aspartate (Asp) and asparagine (Asn) and lyses when grown in Difco sporulation medium (DSM), but not in LB medium. Genetic and physiological studies, supported by amino acid analysis, reveal that cell lysis in DSM results from Asp limitation due to a relatively low Asp and high glutamate (Glu) concentrations, with Glu functioning as a competitive inhibitor of Asp uptake by the major Glu/Asp transporter GltT. Lysis can be specifically suppressed by supplementation with 2,6-diaminopimelate (DAP), which is imported by two different cystine uptake systems. These studies suggest that aspartate limitation depletes the peptidoglycan precursor meso-2,6-diaminopimelate (mDAP), inhibits peptidoglycan synthesis, upregulates the cell envelope stress response mediated by σ^M and eventually leads to cell lysis. Aspartate limitation sensitizes cells to antibiotics targeting late steps of PG synthesis, but not steps prior to the addition of mDAP into the pentapeptide sidechain. This work highlights the ability of perturbations of central metabolism to sensitize cells to peptidoglycan synthesis inhibitors.

Interaction of the Morphogenic Protein RodZ with the Bacillus subtilis Min System

Vegetative cell division in Bacillus subtilis takes place precisely at the middle of the cell to ensure that two viable daughter cells are formed. The first event in cell division is the positioning of the FtsZ Z-ring at the correct site. This is controlled by the coordinated action of both negative and positive regulators. The existence of positive regulators has been inferred, but none have presently been identified in B. subtilis. Noc and the Min system belong to negative regulators; Noc prevents division from occurring over the chromosomes, and the Min system inhibits cell division at the poles. Here we report that the morphogenic protein, RodZ, an essential cell shape determinant, is also required for proper septum positioning during vegetative growth. In rodZ mutant cells, the vegetative septum is positioned off center, giving rise to small, round, DNA-containing cells. Searching for the molecular mechanism giving rise to this phenotype led us to discover that RodZ directly interacts with MinJ. We hypothesize that RodZ may aid the Min system in preventing non-medial vegetative division.