Why Should Nodule Cysteine-Rich (NCR) Peptides Be Absent From Nodules of Some Groups of Legumes but Essential for Symbiotic N-Fixation in Others?

In nitrogen-fixing nodules of legumes such as pea (Pisum) and Medicago spp. the plant induces terminal differentiation in the rhizobial endosymbionts by targeting nodule-specific cysteine-rich defensin-like peptides into the bacteria. However, in nodules of other legumes such as soybean and Lotus spp. terminal bacterial differentiation does not occur; these legumes lack genes encoding equivalent peptides controlling rhizobial development. Here, we review the effects of some of these peptides on rhizobia and address the question as to how and why such peptides may have evolved to enslave rhizobia and become essential for nitrogen fixation in some clades of legumes but not in others.

The alternative sigma factor σX mediates competence shut-off at the cell pole in Streptococcus pneumoniae

Competence is a widespread bacterial differentiation program driving antibiotic resistance and virulence in many pathogens. Here, we studied the spatiotemporal localization dynamics of the key regulators that master the two intertwined and transient transcription waves defining competence in Streptococcus pneumoniae. The first wave relies on the stress-inducible phosphorelay between ComD and ComE proteins, and the second on the alternative sigma factor σX, which directs the expression of the DprA protein that turns off competence through interaction with phosphorylated ComE. We found that ComD, σX and DprA stably co-localize at one pole in competent cells, with σX physically conveying DprA next to ComD. Through this polar DprA targeting function, σX mediates the timely shut-off of the pneumococcal competence cycle, preserving cell fitness. Altogether, this study unveils an unprecedented role for a transcription σ factor in spatially coordinating the negative feedback loop of its own genetic circuit.

Bacterial Differentiation in Oral Multi-Species Biofilm Models Using Confocal Raman Microscopy

<p><strong>Objectives:</strong> Confocal Raman microscopy can give inside knowledge on the composition and structure of biofilms and give some understanding on the strain’s role in the development and then maturation of biofilm structures. This information can then be helpful in understanding the impact of structure on both disease formation and the impact of therapies on biofilms. While different techniques like CLSM real-time PCR, SEM and others have been used for biofilm analysis successfully, confocal Raman microscopy has the advantage to be non-destructive and can thus consider the construction of biofilms over time.</p> <p><strong>Methods:</strong> S.oralis (DSM20066), V.dispar (DSM20735) and A.denticolens (DSM20671) were cultivated in brain-heart infusion broth under anaerobic conditions (37°C, 80% N<sub>2</sub>, 15% CO<sub>2</sub>, 5% H<sub>2</sub>) for 96h. Biofilms were formed on hydroxyapatite discs under the same conditions by using the static ‘Zürich’ model. A Renishaw inVia Qontor instrument was used for confocal Raman analysis of planktonic cells and biofilms (532nm Laser, 50mW laser power, 1s exposure time, 15 scans, spectral detection range from 268-2017cm<sup>-1</sup>). After spectral processing a bacterial database was created using planktonic bacteria. Predictions of species in a biofilm and mapping was performed using multivariate statistical analysis.</p> <p><strong>Results:</strong> The Raman spectra show a specific fingerprint region (600-1800 cm<sup>-1</sup>) where differentiation between different strains are apparent. Using multivariate statistical methods it is possible to differentiate the strains of interest. In combination with structural spectral analysis, it is possible to predict species in artificially grown biofilms. Two-dimensional mapping allowed the visualization of the distribution based on their location of the acquired spectra in a specified random window.</p> <p><strong>Conclusion:</strong> Confocal Raman microscopy was able to differentiate clusters of oral bacteria in a subgingival biofilm model. This preliminary analysis is showing the potential of this technology in biofilm assessment and bacterial differentiation and may be used as an alternative to confocal laser scanning microscopy in the future. In addition, the developed methodology has the potential to be applied to different multi-species biofilms beyond the scope of oral biofilms.</p>

Involvement of Glutaredoxin and Thioredoxin Systems in the Nitrogen-Fixing Symbiosis between Legumes and Rhizobia

Leguminous plants can form a symbiotic relationship with Rhizobium bacteria, during which plants provide bacteria with carbohydrates and an environment appropriate to their metabolism, in return for fixed atmospheric nitrogen. The symbiotic interaction leads to the formation of a new organ, the root nodule, where a coordinated differentiation of plant cells and bacteria occurs. The establishment and functioning of nitrogen-fixing symbiosis involves a redox control important for both the plant-bacteria crosstalk and the regulation of nodule metabolism. In this review, we discuss the involvement of thioredoxin and glutaredoxin systems in the two symbiotic partners during symbiosis. The crucial role of glutathione in redox balance and S-metabolism is presented. We also highlight the specific role of some thioredoxin and glutaredoxin systems in bacterial differentiation. Transcriptomics data concerning genes encoding components and targets of thioredoxin and glutaredoxin systems in connection with the developmental step of the nodule are also considered in the model system Medicago truncatula–Sinorhizobium meliloti.

Impaired competence in flagellar mutants ofBacillus subtilisis connected to the regulatory network governed by DegU

SummaryThe competent state is a developmentally distinct phase, in which bacteria are able to take up and integrate exogenous DNA into their genome.Bacillus subtilisis one of the naturally competent bacterial species and the domesticated laboratory strain 168 is easily transformable. In this study, we report a reduced transformation frequency ofB. subtilismutants lacking functional and structural flagellar components. This includeshag, the gene encoding the flagellin protein forming the filament of the flagellum. We confirm that the observed decrease of the transformation frequency is due to reduced expression of competence genes, particularly of the main competence regulatorcomK. The impaired competence is due to an increase in the phosphorylated form of the response regulator DegU, which is involved in regulation of both flagellar motility and competence. Altogether, our study identified a close link between motility and natural competence inB. subtilissuggesting that hindrance in motility has great impact on differentiation of this bacterium not restricted only to the transition towards sessile growth stage.Originality-Significance statementUnderstanding how versatile bacterial phenotypes influence each other is important for our basic understanding of microbial ecology. Our research highlights the novel intertwinement of bacterial differentiation and reveal how lack of single cell motility adjusts DNA exchange among bacterial strains.