Subcellular Dynamics of a Conserved Bacterial Polar Scaffold Protein

Cell wall synthesizing complexes in Mycobacteriales

Members of the order Mycobacteriales are distinguished by a characteristic diderm cell envelope, setting them apart from other Actinobacteria species. In addition to the conventional peptidoglycan cell wall, these organisms feature an extra polysaccharide polymer composed of arabinose and galactose, termed arabinogalactan. The nonreducing ends of arabinose are covalently linked to mycolic acids (MAs), forming the immobile inner leaflet of the highly hydrophobic MA membrane. The contiguous outer leaflet of the MA membrane comprises trehalose mycolates and various lipid species. Similar to all actinobacteria, Mycobacteriales exhibit apical growth, facilitated by a polar localized elongasome complex. A septal cell envelope synthesis machinery, the divisome, builds instead of the cell wall structures during cytokinesis. In recent years, a growing body of knowledge has emerged regarding the cell wall synthesizing complexes of Mycobacteriales., focusing particularly on three model species: Corynebacterium glutamicum, Mycobacterium smegmatis, and Mycobacterium tuberculosis.

Effects of benzothiazinone and ethambutol on the integrity of the corynebacterial cell envelope

The mycomembrane (MM) is a mycolic acid layer covering the surface of Mycobacteria and related species. This group includes important pathogens such as Mycobacterium tuberculosis, Corynebacterium diphtheriae, but also the biotechnologically important strain Corynebacterium glutamicum. Biosynthesis of the MM is an attractive target for antibiotic intervention. The first line anti-tuberculosis drug ethambutol (EMB) and the new drug candidate, benzothiazinone 043 (BTZ) interfere with the synthesis of the arabinogalactan (AG), which is a structural scaffold for covalently attached mycolic acids that form the inner leaflet of the MM. We previously showed that C. glutamicum cells treated with a sublethal concentration of EMB lose the integrity of the MM. In this study we examined the effects of BTZ on the cell envelope. Our work shows that BTZ efficiently blocks the apical growth machinery, however effects in combinatorial treatment with β-lactam antibiotics are only additive, not synergistic. Transmission electron microscopy (TEM) analysis revealed a distinct middle layer in the septum of control cells considered to be the inner leaflet of the MM covalently attached to the AG. This layer was not detectable in the septa of BTZ or EMB treated cells. In addition, we observed that EMB treated cells have a thicker and less electron dense peptidoglycan (PG). While EMB and BTZ both effectively block elongation growth, BTZ also strongly reduces septal cell wall synthesis, slowing down growth effectively. This renders BTZ treated cells likely more tolerant to antibiotics that act on growing bacteria.

The roles of GpsB and DivIVA in Staphylococcus aureus growth and division

The spheroid bacterium Staphylococcus aureus is often used as a model of morphogenesis due to its apparently simple cell cycle. S. aureus has many cell division proteins that are conserved across bacteria alluding to common functions. However, despite intensive study, we still do not know the roles of many of these components. Here, we have examined the functions of the paralogues DivIVA and GpsB in the S. aureus cell cycle. Cells lacking gpsB display a more spherical phenotype than the wild-type cells, which is associated with a decrease in peripheral cell wall peptidoglycan synthesis. This correlates with increased localization of penicillin-binding proteins at the developing septum, notably PBPs 2 and 3. Our results highlight the role of GpsB as an apparent regulator of cell morphogenesis in S. aureus.

Mycobacterial IHF is a highly dynamic nucleoid-associated protein that assists HupB in organizing chromatin

Nucleoid-associated proteins (NAPs) crucially contribute to organizing bacterial chromatin and regulating gene expression. Among the most highly expressed NAPs are the HU and integration host factor (IHF) proteins, whose functional homologues, HupB and mycobacterial integration host factor (mIHF), are found in mycobacteria. Despite their importance for the pathogenicity and/or survival of tubercle bacilli, the role of these proteins in mycobacterial chromosome organization remains unknown. Here, we used various approaches, including super-resolution microscopy, to perform a comprehensive analysis of the roles of HupB and mIHF in chromosome organization. We report that HupB is a structural agent that maintains chromosome integrity on a local scale, and that the lack of this protein alters chromosome morphology. In contrast, mIHF is a highly dynamic protein that binds DNA only transiently, exhibits susceptibility to the chromosomal DNA topology changes and whose depletion leads to the growth arrest of tubercle bacilli. Additionally, we have shown that depletion of Mycobacterium smegmatis integration host factor (msIHF) leads to chromosome shrinkage and replication inhibition.

Insights Into the Helical Shape Complex of Helicobacter pylori

One important factor that promotes the colonization of the upper digestive system of the human pathogen Helicobacter pylori is its helical cell shape. The bacteria cell shape is predominantly defined by its peptidoglycan cell wall. In rod-shaped species, PG synthesis is mediated by two dynamic molecular machines that facilitate growth along the perpendicular axis and the septum, called the elongasome and the divisome, respectively. Furthermore, many bacteria evolved additional mechanisms to locally change PG synthesis patterns to generate diverse cell shapes. Recent work characterizing cell shape mutants of Helicobacter pylori revealed a novel mechanism for the generation of a twisted helix from a rod, including PG-modifying enzymes as well as additional proteins such as the bactofilin homolog CcmA or the membrane proteins Csd5 and Csd7. In this study, we investigate the localization and dynamics of CcmA and Csd7 using live-cell imaging. We also address the question of how these change in the presence or absence of the putative interaction partners.

Localized Production of Cell Wall Precursors May Be Critical for Regulating the Mycobacterial Cell Wall

The paper "Cell wall damage reveals spatial flexibility in peptidoglycan synthesis and a nonredundant role for RodA in mycobacteria" by Melzer et al. (E. S. Melzer, T. Kado, A. Garcia-Heredia, K. R. Gupta, et al., J Bacteriol 204:e00540-21, 2022, https://doi.org/10.1128/JB.00540-21) presents several new observations about the localization and function of cell wall enzymes in Mycobacterium smegmatis and their responses to stress. This work illustrates some important aspects of cell wall physiology in mycobacteria and also points to a new model for how peptidoglycan synthesis may be organized in pole-growing bacteria.