Molecular Microbiology (original) (raw)
May 11, 2026: 125(6):546-551
For many decades the existence of strict aerobic bacteria was part of every textbook. However, considering habitats like soils or surfaces, many of these microorganisms are exposed to drastic changes in oxygen tension. A simple rain shower can change oxygen diffusion rates by a factor of 10.000. Thus, for many of the so-called strict aerobic bacteria, anaerobic growth and survival strategies were discovered, mainly relying on the use of alternative electron acceptors to oxygen, redox-active metabolites, or fermentation processes generating ATP at the substrate level. Survival without growth was recognized as an important lifestyle of bacteria. With the increasing availability of genome data, many highly diverse growth and survival strategies have become apparent in bacteria. But the overall picture is far from complete. Only recently, a novel puzzle piece of the anaerobic survival strategy of the opportunistic pathogen and model bacterium Pseudomonas aeruginosa in the absence of alternative electron acceptors was elucidated. It relies on the re-wiring of carbon flux away from the Entner-Doudoroff pathway towards the pentose–phosphate pathway and use of a phosphoketolase to allow for metabolic flux while preventing nonproductive NADH formation under these fermentation conditions and for ATP generation via acetate kinase.
April 16, 2026: 125(6):506-545
The bacterial cell wall is an essential protective barrier, the frontline of cellular interactions with the environment and also a target for numerous antimicrobial agents. Accordingly, its integrity and homeostasis are closely monitored, and rapid adaptive responses by transcriptional reprogramming induce appropriate counter-measures against perturbations. Here, we report a comprehensive and comparative transcriptional profiling of the cell envelope stress response (CESR) in the Gram-positive model bacterium Bacillus subtilis, by integrating RNAseq and high-resolution tiling array data. We exposed growing cells to a range of antimicrobial compounds that interfere with cytoplasmic, membrane-coupled or extracellular steps of peptidoglycan (PG) biosynthesis. Transcriptomic studies revealed the complexity of the resulting cell wall stress responses of B. subtilis and unraveled the contribution of extracytoplasmic function (ECF) sigma factors and two-component signal transduction systems (TCSs). While membrane-anchored steps are tightly controlled, early cytoplasmic and late extracellular steps of PG biosynthesis are hardly monitored at all. The ECF σ factors _σ_W and _σ_M both respond to several cell wall synthesis inhibitors and protect the cytoplasmic membrane and cell wall, respectively, while _σ_V is almost exclusively induced by lysozyme, against which it provides specific resistance. Remarkably, _σ_X was slightly repressed by most antibiotics, pointing towards a role in envelope homeostasis rather than cell wall stress. It shares this role with the essential TCS WalRK, which balances cell wall growth with controlled autolysis. In contrast, all remaining TCSs are envelope stress-inducible systems. LiaRS is induced by a wide range of PG synthesis inhibitors, while the three paralogous systems BceAB, PsdRS, and ApeRS are more compound-specific detoxification modules. Induction of the CssRS TCS by all antibiotics interfering with membrane-anchored steps of PG biosynthesis points toward a physiological link between CESR and secretion stress. Based on the expression signatures, a series of CESR-specific B. subtilis whole-cell biosensors were developed and carefully evaluated. Overall, this comprehensive transcriptomic study establishes a reference framework for future studies of Gram-positive stress responses triggered by cell wall-targeting antibiotics.
April 13, 2026: 125(6):493-505
The MsrPQ system is essential for the repair of oxidized methionine residues in periplasmic proteins, ensuring bacterial resistance to oxidative stress and envelope integrity. While ubiquinones were initially proposed as the primary electron source for MsrPQ, in vitro studies suggested that the flavin reductase Fre and the FMN cofactor of MsrQ are essential for electron transfer from cytoplasmic NADH to MsrP. However, their physiological relevance in vivo remained unclear. Here, we investigated the role of Fre and FMN in the MsrPQ system using Escherichia coli as a model organism. We demonstrate that deletion of the fre gene does not impair MsrPQ activity, as Δ_fre_ mutants exhibit wild-type growth on methionine sulfoxide and maintain normal colony morphology under anaerobic chlorate stress. Additionally, the redox state of periplasmic proteins remains unaffected in Δ_fre_ strains, unlike in Δ_msrPQ_ mutants where these proteins accumulate in an oxidized form. Furthermore, we show that the MsrQH151A and MsrQR77A/R78A variants, which lack the FMN cofactor and exhibit altered quinone-binding capacity, respectively, retain full functionality in vivo, albeit with a delayed growth phenotype on methionine sulfoxide. However, we show that Fre becomes necessary for efficient methionine sulfoxide utilization by the MsrPQ system when MsrQ is plasmid-expressed, and that this dependence is further increased when MsrQ is impaired in ubiquinone binding. These observations indicate that the contribution of Fre is conditionally determined by MsrQ abundance. Collectively, our findings reveal that the MsrPQ system operates through a redundant network of electron transfer pathways, with ubiquinone as a central player. This redundancy likely represents an evolutionary adaptation to ensure robust proteostasis even when specific components of the electron transfer chain are compromised.
April 13, 2026: 125(6):475-492
The typhoid toxin is a secreted virulence factor of typhoidal serovars of the bacterial pathogen Salmonella enterica implicated in typhoid fever and chronic infections. The toxin causes a DNA damage response in human cells, characterised by cell-cycle arrest and cellular distension, resulting in cellular senescence and increased bacterial burden. To better understand host responses to typhoid toxin, we performed a transcriptomic analysis of intoxicated host cells and found that the toxin induced expression of genes relating to the type-I interferon response, including the ubiquitin-like protein ISG15. ISG15 was upregulated in a STING-dependent manner, reduced bacterial burden, and was found to be critical to host cell survival in response to the typhoid toxin and interferon. This highlights ISG15 as an important component of the host cell defence to the typhoid toxin.
April 10, 2026: 125(6):461-474
Methylglyoxal is a toxic aldehyde produced during cellular metabolism across all domains of life. To cope with methylglyoxal stress, Escherichia coli employs the glyoxalase detoxification pathway coupled with Kef-mediated potassium/proton antiport. However, the Kef system is protective only when extracellular potassium is well below concentrations typically found in mammalian hosts. The regulatory phosphotransferase system (PTS) that is historically known as the nitrogen-related PTS (PTSNtr) has previously been shown to modulate potassium homeostasis and other processes in E. coli. Here, we identified this regulatory PTS as a mediator of methylglyoxal resistance for potassium concentrations that are comparable to those encountered in the context of host colonization or infection. We found that loss of unphosphorylated PtsN increases survival in methylglyoxal, and that this depended on the potassium/proton antiporter YcgO, whose activity decreases intracellular potassium and pH. While cytoplasmic acidification has been hypothesized to underlie protection from methylglyoxal via potassium/proton antiport, our results suggest the effects of acidification and intracellular potassium cannot be easily separated. Loss of potassium import through Trk increased survival in methylglyoxal and decreased intracellular potassium with only a relatively small decrease in pH. Moreover, the addition of acetate, which acidifies the cytoplasm and protects cells from methylglyoxal, also decreased intracellular potassium. Our results demonstrate that for extracellular potassium levels relevant for host infection and colonization, PtsN modulates methylglyoxal resistance by regulating potassium transport, and that low intracellular potassium, in addition to acidification, could play a direct role in protecting against methylglyoxal stress.
Molecular Microbiology. June 01, 2026: 125(6)
Molecular Microbiology. May 11, 2026: 125(6):546-551
For many decades the existence of strict aerobic bacteria was part of every textbook. However, considering habitats like soils or surfaces, many of these microorganisms are exposed to drastic changes in oxygen tension. A simple rain shower can change oxygen diffusion rates by a factor of 10.000. Thus, for many of the so-called strict aerobic bacteria, anaerobic growth and survival strategies were discovered, mainly relying on the use of alternative electron acceptors to oxygen, redox-active metabolites, or fermentation processes generating ATP at the substrate level. Survival without growth was recognized as an important lifestyle of bacteria. With the increasing availability of genome data, many highly diverse growth and survival strategies have become apparent in bacteria. But the overall picture is far from complete. Only recently, a novel puzzle piece of the anaerobic survival strategy of the opportunistic pathogen and model bacterium Pseudomonas aeruginosa in the absence of alternative electron acceptors was elucidated. It relies on the re-wiring of carbon flux away from the Entner-Doudoroff pathway towards the pentose–phosphate pathway and use of a phosphoketolase to allow for metabolic flux while preventing nonproductive NADH formation under these fermentation conditions and for ATP generation via acetate kinase.
Molecular Microbiology. May 03, 2026: 125(5)