Mycoredoxins Are Required for Redox Homeostasis and Intracellular Survival in the Actinobacterial Pathogen Rhodococcus equi

Redox-sensitive fluorescent biosensors detect Sinorhizobium meliloti intracellular redox changes under free-living and symbiotic lifestyles

Reactive oxygen species such as hydrogen peroxide (H₂O₂) are key signaling molecules that control the setup and functioning of Rhizobium-legume symbiosis. This interaction results in the formation of a new organ, the root nodule, in which bacteria enter the host cells and differentiate into nitrogen (N₂)-fixing bacteroids. The interaction between Sinorhizobium meliloti and Medicago truncatula is a genetic model to study N₂-fixing symbiosis. In previous work, S. meliloti mutants impaired in the antioxidant defense, showed altered symbiotic properties, emphasizing the importance of redox-based regulation in the bacterial partner. However, direct measurements of S. meliloti intracellular redox state have never been performed. Here, we measured dynamic changes of intracellular H₂O₂ and glutathione redox potential by expressing roGFP2-Orp1 and Grx1-roGFP2 biosensors in S. meliloti. Kinetic analyses of redox changes under free-living conditions showed that these biosensors are suitable to monitor the bacterial redox state in real-time, after H₂O₂ challenge and in different genetic backgrounds. In planta, flow cytometry and confocal imaging experiments allowed the determination of sensor oxidation state in nodule bacteria. These cellular studies establish the existence of an oxidative shift in the redox status of S. meliloti during bacteroid differentiation. Our findings open up new possibilities for in vivo studies of redox dynamics during N₂-fixing symbiosis.

Responses to Ecopollutants and Pathogenization Risks of Saprotrophic Rhodococcus Species

Under conditions of increasing environmental pollution, true saprophytes are capable of changing their survival strategies and demonstrating certain pathogenicity factors. Actinobacteria of the genus Rhodococcus, typical soil and aquatic biotope inhabitants, are characterized by high ecological plasticity and a wide range of oxidized organic substrates, including hydrocarbons and their derivatives. Their cell adaptations, such as the ability of adhering and colonizing surfaces, a complex life cycle, formation of resting cells and capsule-like structures, diauxotrophy, and a rigid cell wall, developed against the negative effects of anthropogenic pollutants are discussed and the risks of possible pathogenization of free-living saprotrophic Rhodococcus species are proposed. Due to universal adaptation features, Rhodococcus species are among the candidates, if further anthropogenic pressure increases, to move into the group of potentially pathogenic organisms with "unprofessional" parasitism, and to join an expanding list of infectious agents as facultative or occasional parasites.

Identification of genes required for the fitness of Rhodococcus equi during the infection of mice via signature-tagged transposon mutagenesis

Rhodococcus equi is a Gram-positive facultative intracellular bacterium that causes pyogranulomatous pneumonia in foals and immunocompromised people. In the present study, signature-tagged transposon mutagenesis was applied for the negative selection of R. equi mutants that cannot survive in vivo. Twenty-five distinguishable plasmid-transposon (plasposon) vectors by polymerase chain reaction (PCR), each containing a unique oligonucleotide tag, were constructed and used to select the transposon mutants that have in vivo fitness defects using a mouse systemic infection model. Of the 4,560 transposon mutants, 102 mutants were isolated via a real-time PCR-based screening as the mutants were unable to survive in the mouse model. Finally, 50 single transposon insertion sites were determined via the self-cloning strategy. The insertion of the transposon was seen on the virulence plasmid in 15 of the 50 mutants, whereas the remaining 35 mutants had the insertion of transposon on the chromosome. The chromosomal mutants contained transposon insertions in genes involved in cellular metabolism, DNA repair and recombination, gene regulation, non-ribosomal peptide synthesis, and unknown functions. Additionally, seven of the chromosomal mutants showed a reduced ability to multiply in the macrophages in vitro. In this study, we have identified several biosynthetic pathways as fitness factors associated with the growth within macrophages and survival in mice.

Stress response in Rhodococcus strains

Rhodococci are bacteria which can survive under various extreme conditions, in the presence of toxic compounds, and in other hostile habitats. Their tolerance of unfavorable conditions is associated with the structure of their cell wall and their large array of enzymes, which degrade or detoxify harmful compounds. Their physiological and biotechnological properties, together with tools for their genetic manipulation, enable us to apply them in biotransformations, biodegradation and bioremediation. Many such biotechnological applications cause stresses that positively or negatively affect their efficiency. Whereas numerous reviews on rhodococci described their enzyme activities, the optimization of degradation or production processes, and corresponding technological solutions, only a few reviews discussed some specific effects of stresses on the physiology of rhodococci and biotechnological processes. This review aims to comprehensively describe individual stress responses in Rhodococcus strains, the interconnection of different types of stresses and their consequences for cell physiology. We examine here the responses to (1) environmental stresses (desiccation, heat, cold, osmotic and pH stress), (2) the presence of stress-inducing compounds (metals, organic compounds and antibiotics) in the environment (3) starvation and (4) stresses encountered during biotechnological applications. Adaptations of the cell envelope, the formation of multicellular structures and stresses induced by the interactions of hosts with pathogenic rhodococci are also included. The roles of sigma factors of RNA polymerase in the global regulation of stress responses in rhodococci are described as well. Although the review covers a large number of stressful conditions, our intention was to provide an overview of the selected stress responses and their possible connection to biotechnological processes, not an exhaustive survey of the scientific literature. The findings on stress responses summarized in this review and the demonstration of gaps in current knowledge may motivate researchers working to fill these gaps.

Novel Methods to Identify Oxidative Stress-Producing Antibiotics

Antibiotherapy is the main therapeutic strategy in the fight against bacterial pathogens. However, the misuse of antimicrobials has led to the appearance of antimicrobial-resistant strains. The rate at which we isolate multidrug-resistant bacteria is now much faster than the discovery rate of new antimicrobials. Therefore, the repurposing of approved drugs against multidrug-resistant bacteria is a very promising strategy to find new therapies against these pathogens. Some antibiotics generate oxidative stress as part of their mechanism of action. We have recently applied different methods to find new oxidative stress-producing antimicrobials with synergistic action against intracellular pathogens. Here, we detail several procedures that could be used to identify oxidative stress-producing antimicrobials with a synergistic mechanism of action.

Oxidative Stress-Generating Antimicrobials, a Novel Strategy to Overcome Antibacterial Resistance

Antimicrobial resistance is becoming one of the most important human health issues. Accordingly, the research focused on finding new antibiotherapeutic strategies is again becoming a priority for governments and major funding bodies. The development of treatments based on the generation of oxidative stress with the aim to disrupt the redox defenses of bacterial pathogens is an important strategy that has gained interest in recent years. This approach is allowing the identification of antimicrobials with repurposing potential that could be part of combinatorial chemotherapies designed to treat infections caused by recalcitrant bacterial pathogens. In addition, there have been important advances in the identification of novel plant and bacterial secondary metabolites that may generate oxidative stress as part of their antibacterial mechanism of action. Here, we revised the current status of this emerging field, focusing in particular on novel oxidative stress-generating compounds with the potential to treat infections caused by intracellular bacterial pathogens.

The extracellular thioredoxin Etrx3 is required for macrophage infection in Rhodococcus equi

Rhodococcus equi is an intracellular veterinary pathogen that is becoming resistant to current antibiotherapy. Genes involved in preserving redox homeostasis could be promising targets for the development of novel anti-infectives. Here, we studied the role of an extracellular thioredoxin (Etrx3/REQ_13520) in the resistance to phagocytosis. An etrx3-null mutant strain was unable to survive within macrophages, whereas the complementation with the etrx3 gene restored its intracellular survival rate. In addition, the deletion of etrx3 conferred to R. equi a high susceptibility to sodium hypochlorite. Our results suggest that Etrx3 is essential for the resistance of R. equi to specific oxidative agents.

A Novel Screening Strategy Reveals ROS-Generating Antimicrobials That Act Synergistically against the Intracellular Veterinary Pathogen Rhodococcus equi

Rhodococcus equi is a facultative intracellular pathogen that causes infections in foals and many other animals such as pigs, cattle, sheep, and goats. Antibiotic resistance is rapidly rising in horse farms, which makes ineffective current antibiotic treatments based on a combination of macrolides and rifampicin. Therefore, new therapeutic strategies are urgently needed to treat R. equi infections caused by antimicrobial resistant strains. Here, we employed a R. equi mycoredoxin-null mutant strain highly susceptible to oxidative stress to screen for novel ROS-generating antibiotics. Then, we used the well-characterized Mrx1-roGFP2 biosensor to confirm the redox stress generated by the most promising antimicrobial agents identified in our screening. Our results suggest that different combinations of antibacterial compounds that elicit oxidative stress are promising anti-infective strategies against R. equi. In particular, the combination of macrolides with ROS-generating antimicrobial compounds such as norfloxacin act synergistically to produce a potent antibacterial effect against R. equi. Therefore, our screening approach could be applied to identify novel ROS-inspired therapeutic strategies against intracellular pathogens.