Fluorescent phospholipid analogs as microscopic probes for detection of the mycolic acid-containing layer in Corynebacterium glutamicum: detecting alterations in the mycolic acid-containing layer following ethambutol treatment

Ethambutol and meropenem/clavulanate synergy promotes enhanced extracellular and intracellular killing of Mycobacterium tuberculosis

Increasing evidence supports the repositioning of beta-lactams for tuberculosis (TB) therapy, but further research on their interaction with conventional anti-TB agents is still warranted. Moreover, the complex cell envelope of Mycobacterium tuberculosis (Mtb) may pose an additional obstacle to beta-lactam diffusion. In this context, we aimed to identify synergies between beta-lactams and anti-TB drugs ethambutol (EMB) and isoniazid (INH) by assessing antimicrobial effects, intracellular activity, and immune responses. Checkerboard assays with H37Rv and eight clinical isolates, including four drug-resistant strains, exposed that only treatments containing EMB and beta-lactams achieved synergistic effects. Meanwhile, the standard EMB and INH association failed to produce any synergy. In Mtb-infected THP-1 macrophages, combinations of EMB with increasing meropenem (MEM) concentrations consistently displayed superior killing activities over the individual antibiotics. Flow cytometry with BODIPY FL vancomycin, which binds directly to the peptidoglycan (PG), confirmed an increased exposure of this layer after co-treatment. This was reinforced by the high IL-1β secretion levels found in infected macrophages after incubation with MEM concentrations above 5 mg/L, indicating an exposure of the host innate response sensors to pathogen-associated molecular patterns in the PG. Our findings show that the proposed impaired access of beta-lactams to periplasmic transpeptidases is counteracted by concomitant administration with EMB. The efficiency of this combination may be attributed to the synchronized inhibition of arabinogalactan and PG synthesis, two key cell wall components. Given that beta-lactams exhibit a time-dependent bactericidal activity, a more effective pathogen recognition and killing prompted by this association may be highly beneficial to optimize TB regimens containing carbapenems.IMPORTANCEAddressing drug-resistant tuberculosis with existing therapies is challenging and the treatment success rate is lower when compared to drug-susceptible infection. This study demonstrates that pairing beta-lactams with ethambutol (EMB) significantly improves their efficacy against Mycobacterium tuberculosis (Mtb). The presence of EMB enhances beta-lactam access through the cell wall, which may translate into a prolonged contact between the drug and its targets at a concentration that effectively kills the pathogen. Importantly, we showed that the effects of the EMB and meropenem (MEM)/clavulanate combination were maintained intracellularly. These results are of high significance considering that the time above the minimum inhibitory concentration is the main determinant of beta-lactam efficacy. Moreover, a correlation was established between incubation with higher MEM concentrations during macrophage infection and increased IL-1β secretion. This finding unveils a previously overlooked aspect of carbapenem repurposing against tuberculosis, as certain Mtb strains suppress the secretion of this key pro-inflammatory cytokine to evade host surveillance.

Requirement of the LtsA Protein for Formation of the Mycolic Acid-Containing Layer on the Cell Surface of Corynebacterium glutamicum

The ltsA gene of Corynebacterium glutamicum encodes a purF-type glutamine-dependent amidotransferase, and mutations in this gene result in increased susceptibility to lysozyme. Recently, it was shown that the LtsA protein catalyzes the amidation of diaminopimelate residues in the lipid intermediates of peptidoglycan biosynthesis. In this study, intracellular localization of wild-type and mutant LtsA proteins fused with green fluorescent protein (GFP) was investigated. The GFP-fused wild-type LtsA protein showed a peripheral localization pattern characteristic of membrane-associated proteins. The GFP-fusions with a mutation in the N-terminal domain of LtsA, which is necessary for the glutamine amido transfer reaction, exhibited a similar localization to the wild type, whereas those with a mutation or a truncation in the C-terminal domain, which is not conserved among the purF-type glutamine-dependent amidotransferases, did not. These results suggest that the C-terminal domain is required for peripheral localization. Differential staining of cell wall structures with fluorescent dyes revealed that formation of the mycolic acid-containing layer at the cell division planes was affected in the ltsA mutant cells. This was also confirmed by observation that bulge formation was induced at the cell division planes in the ltsA mutant cells upon lysozyme treatment. These results suggest that the LtsA protein function is required for the formation of a mycolic acid-containing layer at the cell division planes and that this impairment results in increased susceptibility to lysozyme.

The role of transport proteins in the production of microbial glycolipid biosurfactants

Several microorganisms are currently being used as production platform for glycolipid biosurfactants, providing a greener alternative to chemical biosurfactants. One of the reasons why these processes are commercially competitive is the fact that microbial producers can efficiently export their product to the extracellular environment, reaching high product titers. Glycolipid biosynthetic genes are often found in a dedicated cluster, amidst which genes encoding a dedicated transporter committed to shuttle the glycolipid to the extracellular environment are often found, as is the case for many other secondary metabolites. Knowing this, one can rely on gene clustering features to screen for novel putative transporters, as described and performed in this review. The above strategy proves to be very powerful to identify glycolipid transporters in fungi but is less valid for bacterial systems. Indeed, the genetics of these export systems are currently largely unknown, but some hints are given. Apart from the direct export of the glycolipid, several other transport systems have an indirect effect on glycolipid production. Specific importers dictate which hydrophilic and hydrophobic substrates can be used for production and influence the final yields. In eukaryotes, cellular compartmentalization allows the assembly of glycolipid building blocks in a highly specialized and efficient way. Yet, this requires controlled transport across intracellular membranes. Next to the direct export of glycolipids, the current state of the art regarding this indirect involvement of transporter systems in microbial glycolipid synthesis is summarized in this review. KEY POINTS: • Transporters are directly and indirectly involved in microbial glycolipid synthesis. • Yeast glycolipid transporters are found in their biosynthetic gene cluster. • Hydrophilic and hydrophobic substrate uptake influence microbial glycolipid synthesis.

Functional analysis of putative transporters involved in oligotrophic growth of Rhodococcus erythropolis N9T-4

Rhodococcus erythropolis N9T-4, which is an extremely oligotrophic bacterium, can survive in a completely inorganic medium with no additional carbon source. This bacterium utilizes atmospheric CO₂, but does not require any additional energy source such as light and hydrogen gas, required by autotrophic microorganisms. However, its CO₂ fixation and energy-acquisition systems in the oligotrophic growth remain unrevealed. We expected N9T-4 to have the transporter(s) that imports essential compound(s) for its oligotrophic growth. Three putative ATP-binding cassette (ABC) transporters were found to be highly upregulated under oligotrophic conditions. We constructed the gene-deletion mutants of a gene encoding the substrate-binding protein for each ABC transporter (∆sbp1, ∆sbp2, and ∆sbp3). Among these mutants, ∆sbp1 showed growth defects on oligotrophic medium without carbon source. We examined the growth of the mutants on the oligotrophic medium containing 1% trehalose as a sole carbon source. The results exhibited worse growth of ∆sbp3 than that of the control strain (∆ligD), whereas intracellular trehalose content of all mutants decreased compared with that of ∆ligD. It was reported that trehalose functions as the mycolate carrier to the arabinogalactan layer in the cell wall of Mycobacterium tuberculosis. Transmission electron microscopic analysis of ∆sbp1 cells showed that an outermost envelope of the ∆sbp1 cell diminished, which was expected to be mycolate layer. From these results, we suggest that the same trehalose-recycling system as that in a Mycobacterium cell functions in the oligotrophic growth of N9T-4, and the ABC transporter comprising Sbp1 as the substrate-binding protein is strongly involved in the oligotrophic growth of N9T-4.

Sequential assembly of the septal cell envelope prior to V snapping in Corynebacterium glutamicum

Members of the Corynebacterineae, including Corynebacterium and Mycobacterium, have an atypical cell envelope characterized by an additional mycomembrane outside of the peptidoglycan layer. How this multilayered cell envelope is assembled remains unclear. Here, we tracked the assembly dynamics of different envelope layers in Corynebacterium glutamicum and Mycobacterium smegmatis by using metabolic labeling and found that the septal cell envelope is assembled sequentially in both species. Additionally, we demonstrate that in C. glutamicum, the peripheral peptidoglycan layer at the septal junction remains contiguous throughout septation, forming a diffusion barrier for the fluid mycomembrane. This diffusion barrier is resolved through perforations in the peripheral peptidoglycan, thus leading to the confluency of the mycomembrane before daughter cell separation (V snapping). Furthermore, the same junctional peptidoglycan also serves as a mechanical link holding the daughter cells together and undergoes mechanical fracture during V snapping. Finally, we show that normal V snapping in C. glutamicum depends on complete assembly of the septal cell envelope.

The Antituberculosis Drug Ethambutol Selectively Blocks Apical Growth in CMN Group Bacteria

Members of the genus Mycobacterium are the most prevalent cause of infectious diseases. Mycobacteria have a complex cell envelope containing a peptidoglycan layer and an additional arabinogalactan polymer to which a mycolic acid bilayer is linked; this complex, multilayered cell wall composition (mAGP) is conserved among all CMN group bacteria. The arabinogalactan and mycolic acid synthesis pathways constitute effective drug targets for tuberculosis treatment. Ethambutol (EMB), a classical antituberculosis drug, inhibits the synthesis of the arabinose polymer. Although EMB acts bacteriostatically, its underlying molecular mechanism remains unclear. Here, we used Corynebacterium glutamicum and Mycobacterium phlei as model organisms to study the effects of EMB at the single-cell level. Our results demonstrate that EMB specifically blocks apical cell wall synthesis, but not cell division, explaining the bacteriostatic effect of EMB. Furthermore, the data suggest that members of the family Corynebacterineae have two dedicated machineries for cell elongation (elongasome) and cytokinesis (divisome).

Antibiotic treatment of bacterial pathogens has contributed enormously to the increase in human health. Despite the apparent importance of antibiotic treatment of bacterial infections, surprisingly little is known about the molecular functions of antibiotic actions in the bacterial cell. Here, we analyzed the molecular effects of ethambutol, a first-line antibiotic against infections caused by members of the genus Mycobacterium. We find that this drug selectively blocks apical cell growth but still allows for effective cytokinesis. As a consequence, cells survive ethambutol treatment and adopt a pneumococcal cell growth mode with cell wall synthesis only at the site of cell division. However, combined treatment of ethambutol and beta-lactam antibiotics acts synergistically and effectively stops cell proliferation.

Extracellular Streptomyces lividans vesicles: composition, biogenesis and antimicrobial activity

We selected Streptomyces lividans to elucidate firstly the biogenesis and antimicrobial activities of extracellular vesicles that a filamentous and highly differentiated Gram-positive bacterium produces. Vesicle types range in diameter from 110 to 230 nm and 20 to 60 nm, respectively; they assemble to clusters, and contain lipids and phospholipids allowing their in situ imaging by specific fluorescent dyes. The presence of the identified secondary metabolite undecylprodigiosin provokes red fluorescence of a portion of the heterogeneous vesicle populations facilitating in vivo monitoring. Protuberances containing vesicles generate at tips, and alongside of substrate hyphae, and enumerate during late vegetative growth to droplet-like exudates. Owing to in situ imaging in the presence and absence of a green fluorescent vancomycin derivative, we conclude that protuberances comprising vesicles arise at sites with enhanced levels of peptidoglycan subunits [pentapeptide of lipid II (C55)-linked disaccharides], and reduced levels of polymerized and cross-linked peptidoglycan within hyphae. These sites correlate with enhanced levels of anionic phospholipids and lipids. Vesicles provoke pronounced damages of Aspergillus proliferans, Verticillium dahliae and induced clumping and distortion of Escherichia coli. These harmful effects are likely attributable to the action of the identified vesicular compounds including different enzyme types, components of signal transduction cascades and undecylprodigiosin. Based on our pioneering findings, we highlight novel clues with environmental implications and application potential.

Reconstitution experiments and gene deletions reveal the existence of two-component major cell wall channels in the genus Corynebacterium

Two small polypeptides, PorA and PorH, are known to form cell wall channels in Corynebacterium glutamicum and in Corynebacterium efficiens. The genes coding for both polypeptides are localized in close proximity to one another between the genes coding for GroEl2 and a polyphosphate kinase (PKK2). In this study, we investigated the relationship of PorA and PorH to one another. The results suggested that the major cell wall channels of Corynebacterium glutamicum, Corynebacterium efficiens, and Corynebacterium diphtheriae need the obligatory presence of two distinct polypeptides, one of class PorA and one of class PorH, to form an active cell wall channel. Identification of genes coding for homologous proteins in the chromosome of Corynebacterium callunae suggested a similar result for this strain. Contrary to our previous reports on channel-forming proteins in these strains, a heterooligomeric structure composed of PorA and PorH is needed in all of them to form the major cell wall channel. This was concluded from complementation experiments using a porH- and porA-deficient C. glutamicum strain. The stringent necessity of proteins of either class to recover the wild-type channels was demonstrated by black lipid bilayer experiments using detergent or organic solvent extracts of the complemented porH- and porA-deficient C. glutamicum strain. The channel-forming capability of recombinant expressed, affinity-purified PorA and PorH proteins of C. glutamicum revealed that the channels consisted solely of these two components. This agreed with results obtained from a transcript coding for both channel-forming components identified in C. glutamicum by Northern blot analysis and reverse transcription-PCR analysis. The transcription start point of the genes was determined by the rapid amplification of cDNA ends approach, allowing the prediction of the -35 and -10 regions of the promoter. The results demonstrate that the cell wall channels within the genus Corynebacterium may be formed by two-component oligomers.

Deletion of cgR_1596 and cgR_2070, encoding NlpC/P60 proteins, causes a defect in cell separation in Corynebacterium glutamicum R

In previous work, random genome deletion mutants of Corynebacterium glutamicum R were generated using the insertion sequence (IS) element IS31831 and the Cre/loxP excision system. One of these mutants, C. glutamicum strain RD41, resulting from the deletion of a 10.1-kb genomic region (DeltacgR_1595 through cgR_1604) from the WT strain, showed cell elongation, and several lines appeared on the cell surface (bamboo shape). The morphological changes were suppressed by overexpression of cgR_1596. Single disruption of cgR_1596 in WT C. glutamicum R resulted in morphological changes similar to those observed in the RD41 strain. CgR_1596 has a predicted secretion signal peptide in the amino-terminal region and a predicted NlpC/P60 domain, which is conserved in cell wall hydrolases, in the carboxyl-terminal region. In C. glutamicum R, CgR_0802, CgR_1596, CgR_2069, and CgR_2070 have the NlpC/P60 domain; however, only simultaneous disruption of cgR_1596 and cgR_2070, and not cgR_2070 single disruption, resulted in cell growth delay and more severe morphological changes than disruption of cgR_1596. Transmission electron microscopy revealed multiple septa within individual cells of cgR_1596 single and cgR_1596-cgR_2070 double disruptants. Taken together, these results suggest that cgR_1596 and cgR_2070 are involved in cell separation and cell growth in C. glutamicum.

Cell envelope fluidity modification for an effective glutamate excretion in Corynebacterium glutamicum 2262

1-(4-trimethylammoniumphenyl)-6-phenyl-1,3,5-hexatriene (TMA-DPH) was used to assess the cell envelope fluidity of Corynebacterium glutamicum 2262 during a temperature-triggered glutamate producing process. Because the fluorescence lifetime of TMA-DPH was shown to be constant all over the process, fluorescence anisotropy can be considered as a good index of cell envelope fluidity. When the temperature of the fed-batch culture was increased from 33 to 39 degrees C to induce glutamate excretion, the fluorescence anisotropy values decreased from 0.212 +/- 0.002 to 0.186 +/- 0.002 (corresponding to an increase in the cell fluidity), while the specific glutamate production rate reached its maximal value. The increase in fluidity of the C. glutamicum cell envelope was not due to a physical effect related to the temperature elevation, but rather to an alteration of the composition of the cell envelope. Using a mutant devoid of corynomycolates, significant differences in fluorescence anisotropy values were obtained compared to the wild-type strain, suggesting that TMA-DPH is mainly anchored into the corynomycomembrane. Differences in fluorescence anisotropy were also observed when the bacteria were cultivated at 33, 36, 38, and 39 degrees C in batch cultures, and a linear relationship was obtained between the maximum specific glutamate production rate and the measured fluidity. When using the glutamate non-producing variant of C. glutamicum 2262, the fluorescence anisotropy remained constant at 0.207 +/- 0.003 whatever the applied temperature shift. This suggests that the fluidity of the Corynebacteria mycomembrane plays an important role in glutamate excretion during the temperature-triggered process.