The sensor kinase CbrA is a global regulator that modulates metabolism, virulence, and antibiotic resistance in Pseudomonas aeruginosa

Loss of LasR function leads to decreased repression of Pseudomonas aeruginosa PhoB activity at physiological phosphate concentrations

While the Pseudomonas aeruginosa LasR transcription factor plays a role in quorum sensing (QS) across phylogenetically-distinct lineages, isolates with loss-of-function mutations in lasR (LasR- strains) are commonly found in diverse settings including infections where they are associated with worse clinical outcomes. In LasR- strains, the transcription factor RhlR, which is controlled by LasR, can be alternately activated in low inorganic phosphate (Pi) concentrations via the two-component system PhoR-PhoB. Here, we demonstrate a new link between LasR and PhoB in which the absence of LasR increases PhoB activity at physiological Pi concentrations and raises the Pi concentration necessary for PhoB inhibition. PhoB activity was also less repressed by Pi in mutants lacking different QS regulators (RhlR and PqsR) and in mutants lacking genes required for the production of QS-regulated phenazines suggesting that decreased phenazine production was one reason for decreased PhoB repression by Pi in LasR- strains. In addition, the CbrA-CbrB two-component system, which is elevated in LasR- strains, was necessary for reduced PhoB repression by Pi and a Δcrc mutant, which lacks the CbrA-CbrB-controlled translational repressor, activated PhoB at higher Pi concentrations than the wild type. The ΔlasR mutant had a PhoB-dependent growth advantage in a medium with no added Pi and increased virulence-determinant gene expression in a medium with physiological Pi, in part through reactivation of QS. This work suggests PhoB activity may contribute to the virulence of LasR- P. aeruginosa and subsequent clinical outcomes.

Glycerol metabolism impacts biofilm phenotypes and virulence in Pseudomonas aeruginosa via the Entner-Doudoroff pathway

Pseudomonas aeruginosa is a ubiquitous bacterium and a notorious opportunistic pathogen that forms biofilm structures in response to many environmental cues. Biofilm formation includes attachment to surfaces and the production of the exopolysaccharide Pel, which is present in both the PAO1 and PA14 laboratory strains of P. aeruginosa. Biofilms help protect bacterial cells from host defenses and antibiotics and abet infection. The carbon source used by the cells also influences biofilm, but these effects have not been deeply studied. We show here that glycerol, which can be liberated from host surfactants during infection, encourages surface attachment and magnifies colony morphology differences. We find that glycerol kinase is important but not essential for glycerol utilization and relatively unimportant for biofilm behaviors. Among downstream enzymes predicted to take part in glycerol utilization, Edd stood out as being important for glycerol utilization and for enhanced biofilm phenotypes in the presence of glycerol. Thus, gluconeogenesis and catabolism of anabolically produced glucose appear to impact not only the utilization of glycerol but also glycerol-stimulated biofilm phenotypes. Finally, waxworm moth larvae and nematode infection models reveal that interruption of the Entner-Doudoroff pathway, but not abrogation of glycerol phosphorylation, unexpectedly increases P. aeruginosa lethality in both acute and chronic infections, even while stimulating a stronger immune response by Caenorhabditis elegans.IMPORTANCEPseudomonas aeruginosa, the ubiquitous environmental bacterium and human pathogen, forms multicellular communities known as biofilms in response to various stimuli. We find that glycerol, a common carbon source that bacteria can use for energy and biosynthesis, encourages biofilm behaviors such as surface attachment and colony wrinkling by P. aeruginosa. Glycerol can be derived from surfactants that are present in the human lungs, a common infection site. Glycerol-stimulated biofilm phenotypes do not depend on phosphorylation of glycerol but are surprisingly impacted by a glucose breakdown pathway, suggesting that it is glycerol utilization, and not its mere presence or cellular import, that stimulates biofilm phenotypes. Moreover, the same mutations that block glycerol-stimulated biofilm phenotypes also impact P. aeruginosa virulence in both acute and chronic animal models. Notably, a glucose-breakdown mutant (Δedd) counteracts biofilm phenotypes but shows enhanced virulence and stimulates a stronger immune response in Caenorhabditis elegans.

Mutations in genes lpxL1, bamA, and pmrB impair the susceptibility of cystic fibrosis strains of Pseudomonas aeruginosa to murepavadin

Murepavadin is a peptidomimetic exhibiting specific inhibitory activity against Pseudomonas species. In the present study, its in vitro activity was assessed on 230 cystic fibrosis (CF) strains of Pseudomonas aeruginosa isolated from 12 French hospitals, in comparison with 12 other antipseudomonal antibiotics. Although murepavadin is still in preclinical stage of development, 9.1% (n = 21) of strains had a minimum inhibitory concentration (MIC) >4 mg/L, a level at least 128-fold higher than the modal MIC value of the whole collection (≤0.06 mg/L). Whole-genome sequencing of these 21 strains along with more susceptible isogenic counterparts coexisting in the same patients revealed diverse mutations in genes involved in the synthesis (lpxL1 and lpxL2) or transport of lipopolysaccharides (bamA, lptD, and msbA), or encoding histidine kinases of two-component systems (pmrB and cbrA). Allelic replacement experiments with wild-type reference strain PAO1 confirmed that alteration of genes lpxL1, bamA, and/or pmrB can decrease the murepavadin susceptibility from 8- to 32-fold. Furthermore, we found that specific amino acid substitutions in histidine kinase PmrB (G188D, Q105P, and D45E) reduce the susceptibility of P. aeruginosa to murepavadin, colistin, and tobramycin, three antibiotics used or intended to be used (murepavadin) in aerosols to treat colonized CF patients. Whether colistin or tobramycin may select mutants resistant to murepavadin or the opposite needs to be addressed by clinical studies.

What are the signals that control catabolite repression in Pseudomonas?

Metabolically versatile bacteria exhibit a global regulatory response known as carbon catabolite repression (CCR), which prioritizes some carbon sources over others when all are present in sufficient amounts. This optimizes growth by distributing metabolite fluxes, but can restrict yields in biotechnological applications. The molecular mechanisms and preferred substrates for CCR vary between bacterial groups. Escherichia coli prioritizes glucose whereas Pseudomonas sp. prefer certain organic acids or amino acids. A significant issue in understanding (and potentially bypassing) CCR is the lack of information about the signals that trigger this regulatory response. In E. coli, several key compounds act as flux sensors, governing the flow of metabolites through catabolic pathways and preventing imbalances. These flux sensors can also modulate the CCR response. It has been suggested that the order of substrate preference is determined by carbon uptake flux rather than substrate identity. For Pseudomonas, much less information is available, as the signals that induce CCR are poorly understood. This article briefly discusses the available evidence on the signals that trigger CCR and the questions that remain to be answered in Pseudomonas.

Pseudomonas aeruginosa two-component system CprRS regulates HigBA expression and bacterial cytotoxicity in response to LL-37 stress

Pseudomonas aeruginosa is a highly pathogenic bacterium known for its ability to sense and coordinate the production of virulence factors in response to host immune responses. However, the regulatory mechanisms underlying this process have remained largely elusive. In this study, we investigate the two-component system CprRS in P. aeruginosa and unveil the crucial role of the sensor protein CprS in sensing the human host defense peptide LL-37, thereby modulating bacterial virulence. We demonstrate that CprS acts as a phosphatase in the presence of LL-37, leading to the phosphorylation and activation of the response regulator CprR. The results prove that CprR directly recognizes a specific sequence within the promoter region of the HigBA toxin-antitoxin system, resulting in enhanced expression of the toxin HigB. Importantly, LL-37-induced HigB expression promotes the production of type III secretion system effectors, leading to reduced expression of proinflammatory cytokines and increased cytotoxicity towards macrophages. Moreover, mutations in cprS or cprR significantly impair bacterial survival in both macrophage and insect infection models. This study uncovers the regulatory mechanism of the CprRS system, enabling P. aeruginosa to detect and respond to human innate immune responses while maintaining a balanced virulence gene expression profile. Additionally, this study provides new evidence and insights into the complex regulatory system of T3SS in P. aeruginosa within the host environment, contributing to a better understanding of host-microbe communication and the development of novel strategies to combat bacterial infections.

Natural and synthetic molecules with potential to enhance biofilm formation and virulence properties in Pseudomonas aeruginosa

Pseudomonas aeruginosa can efficiently adapt to changing environmental conditions due to its ubiquitous nature, intrinsic/acquired/adaptive resistance mechanisms, high metabolic versatility, and the production of numerous virulence factors. As a result, P. aeruginosa becomes an opportunistic pathogen, causing chronic infection in the lungs and several organs of patients suffering from cystic fibrosis. Biofilm established by P. aeruginosa in host tissues and medical device surfaces has been identified as a major obstruction to antimicrobial therapy. P. aeruginosa is very likely to be closely associated with the various microorganisms in the host tissues or organs in a pathogenic or nonpathogenic behavior. Aside from host-derived molecules, other beneficial and pathogenic microorganisms produce a diverse range of secondary metabolites that either directly or indirectly favor the persistence of P. aeruginosa. Thus, it is critical to understand how P. aeruginosa interacts with different molecules and ions in the host and abiotic environment to produce extracellular polymeric substances and virulence factors. Thus, the current review discusses how various natural and synthetic molecules in the environment induce biofilm formation and the production of multiple virulence factors.

Virulence and Metabolism Crosstalk: Impaired Activity of the Type Three Secretion System (T3SS) in a Pseudomonas aeruginosa Crc-Defective Mutant

Pseudomonas aeruginosa is a ubiquitous nosocomial opportunistic pathogen that harbors many virulence determinants. Part of P. aeruginosa success colonizing a variety of habitats resides in its metabolic robustness and plasticity, which are the basis of its capability of adaptation to different nutrient sources and ecological conditions, including the infected host. Given this situation, it is conceivable that P. aeruginosa virulence might be, at least in part, under metabolic control, in such a way that virulence determinants are produced just when needed. Indeed, it has been shown that the catabolite repression control protein Crc, which together with the RNA chaperon Hfq regulates the P. aeruginosa utilization of carbon sources at the post-transcriptional level, also regulates, directly or indirectly, virulence-related processes in P. aeruginosa. Among them, Crc regulates P. aeruginosa cytotoxicity, likely by modulating the activity of the Type III Secretion System (T3SS), which directly injects toxins into eukaryotic host cells. The present work shows that the lack of Crc produces a Type III Secretion-defective phenotype in P. aeruginosa. The observed impairment is a consequence of a reduced expression of the genes encoding the T3SS, together with an impaired secretion of the proteins involved. Our results support that the impaired T3SS activity of the crc defective mutant is, at least partly, a consequence of a defective protein export, probably due to a reduced proton motive force. This work provides new information about the complex regulation of the expression and the activity of the T3SS in P. aeruginosa. Our results highlight the need of a robust bacterial metabolism, which is defective in the ∆crc mutant, to elicit complex and energetically costly virulence strategies, as that provided by the T3SS.

Antimicrobial Resistance: Two-Component Regulatory Systems and Multidrug Efflux Pumps

The number of multidrug-resistant bacteria is rapidly spreading worldwide. Among the various mechanisms determining resistance to antimicrobial agents, multidrug efflux pumps play a noteworthy role because they export extraneous and noxious substrates from the inside to the outside environment of the bacterial cell contributing to multidrug resistance (MDR) and, consequently, to the failure of anti-infective therapies. The expression of multidrug efflux pumps can be under the control of transcriptional regulators and two-component systems (TCS). TCS are a major mechanism by which microorganisms sense and reply to external and/or intramembrane stimuli by coordinating the expression of genes involved not only in pathogenic pathways but also in antibiotic resistance. In this review, we describe the influence of TCS on multidrug efflux pump expression and activity in some Gram-negative and Gram-positive bacteria. Taking into account the strict correlation between TCS and multidrug efflux pumps, the development of drugs targeting TCS, alone or together with already discovered efflux pump inhibitors, may represent a beneficial strategy to contribute to the fight against growing antibiotic resistance.

Genetic and Environmental Investigation of a Novel Phenylamino Acetamide Inhibitor of the Pseudomonas aeruginosa Type III Secretion System

Traditional antibiotics target essential cellular components or metabolic pathways conserved in both pathogenic and nonpathogenic bacteria. Unfortunately, long-term antibiotic use often leads to antibiotic resistance and disruption of the overall microbiota. In this work, we identified a phenylamino acetamide compound, named 187R, that strongly inhibited the expression of the type III secretion system (T3SS) encoding genes and the secretion of the T3SS effector proteins in Pseudomonas aeruginosa. T3SS is an important virulence factor, as T3SS-deficient strains of P. aeruginosa are greatly attenuated in virulence. We further showed that 187R had no effect on bacterial growth, implying a reduced selective pressure for the development of resistance. 187R-mediated repression of T3SS was dependent on ExsA, the master regulator of T3SS in P. aeruginosa. The impact of 187R on the host-associated microbial community was also tested using the Arabidopsis thaliana phyllosphere as a model. Both culture-independent (Illumina sequencing) and culture-dependent (Biolog) methods showed that the application of 187R had little impact on the composition and function of microbial community compared to the antibiotic streptomycin. Together, these results suggested that compounds that target virulence factors could serve as an alternative strategy for disease management caused by bacterial pathogens. IMPORTANCE New antimicrobial therapies are urgently needed, since antibiotic resistance in human pathogens has become one of the world's most urgent public health problems. Antivirulence therapy has been considered a promising alternative for the management of infectious diseases, as antivirulence compounds target only the virulence factors instead of the growth of bacteria, and they are therefore unlikely to affect commensal microorganisms. However, the impacts of antivirulence compounds on the host microbiota are not well understood. We report a potent synthetic inhibitor of the P. aeruginosa T3SS, 187R, and its effect on the host microbiota of Arabidopsis. Both culture-independent (Illumina sequencing) and culture-dependent (Biolog) methods showed that the impacts of the antivirulence compound on the composition and function of host microbiota were limited. These results suggest that antivirulence compounds can be a potential alternative method to antibiotics.

In Vivo Role of Two-Component Regulatory Systems in Models of Urinary Tract Infections

Two-component signaling systems (TCSs) are finely regulated mechanisms by which bacteria adapt to environmental conditions by modifying the expression of target genes. In bacterial pathogenesis, TCSs play important roles in modulating adhesion to mucosal surfaces, resistance to antibiotics, and metabolic adaptation. In the context of urinary tract infections (UTI), one of the most common types infections causing significant health problems worldwide, uropathogens use TCSs for adaptation, survival, and establishment of pathogenicity. For example, uropathogens can exploit TCSs to survive inside bladder epithelial cells, sense osmolar variations in urine, promote their ascension along the urinary tract or even produce lytic enzymes resulting in exfoliation of the urothelium. Despite the usefulness of studying the function of TCSs in in vitro experimental models, it is of primary necessity to study bacterial gene regulation also in the context of host niches, each displaying its own biological, chemical, and physical features. In light of this, the aim of this review is to provide a concise description of several bacterial TCSs, whose activity has been described in mouse models of UTI.

CzcR Is Essential for Swimming Motility in Pseudomonas aeruginosa during Zinc Stress

Two-component system (TCS) plays a vital role in modulating target gene expression in response to the changing environments. Pseudomonas aeruginosa is a ubiquitous opportunistic pathogen that can survive under diverse stress conditions. The great adaptability of P. aeruginosa relies heavily on the abundant TCSs encoded by its genome. However, most TCSs in P. aeruginosa have not been well-characterized. CzcS/CzcR is a metal responsive TCS which displays multiple regulatory functions associated with metal hemostasis, quorum sensing activity and antibiotic resistance. In this study, we found that swimming motility of P. aeruginosa was completely abolished during zinc (Zn2+) stress when the czcR gene from the TCS CzcS/CzcR was deleted. Noticeably, CzcR was dispensable for swimming without the stress of Zn2+ excess. CzcR was shown to be activated by Zn2+ stress possibly through inducing its expression level and triggering its phosphorylation to positively regulate swimming which was abolished by Zn2+ stress in a CzcR-independent manner. Further TEM analyses and promoter activity examinations revealed that CzcR was required for the expression of genes involved in flagellar biosynthesis during Zn2+ stress. In vitro protein-DNA interaction assay showed that CzcR was capable of specifically recognizing and binding to the promoters of operons flgBCDE, flgFGHIJK, and PA1442/FliMNOPQR/flhB. Together, this study demonstrated a novel function of CzcR in regulating flagellar gene expression and motility in P. aeruginosa when the pathogen encounters Zn2+ stress conditions. IMPORTANCE The fitness of bacterial cells depends largely on their ability to sense and respond quickly to the changing environments. P. aeruginosa expresses a great number of signal sensing and transduction systems that enable the pathogen to grow and survive under diverse stress conditions and cause serious infections at different sites in many hosts. In addition to the previously characterized functions to regulate metal homeostasis, quorum sensing activity, and antibiotic resistance, here we report that CzcR is a novel regulator essential for flagellar gene expression and swimming motility in P. aeruginosa during Zn2+ stress. Since swimming motility is important for the virulence of P. aeruginosa, findings in this study might provide a new target for the treatment of P. aeruginosa infections with Zn2+-based antimicrobial agents in the future.

Dual GGDEF/EAL-Domain Protein RmcA Controls the Type III Secretion System of Pseudomonas aeruginosa by Interaction with CbrB

Cyclic diguanylate (c-di-GMP) is a major bacterial secondary signaling molecule that controls a multitude of cellular processes. More than 40 genes encoding diguanylate cyclases and phosphodiesterases have been identified in Pseudomonas aeruginosa, and many of them have been intensively investigated. However, the mechanism through which they achieve signaling specificity remains unclear. Here, we revealed that the absence of the dual GGDEF/EAL-domain protein RmcA significantly affected biofilm formation of P. aeruginosa PAO1 and led to upregulated expression of the type III secretion system (T3SS) genes; overexpression of RmcA strongly reduced the expression of T3SS. Further investigation showed that the regulatory function of RmcA was independent of the Gac/Rsm pathway. To identify the interaction partners of RmcA involved in this process, bacterial two-hybrid library screening was performed. We found that RmcA directly interacts with a two-component response regulator CbrB, which is involved in the regulation of biofilm formation and T3SS expression by RmcA. These findings reveal that the dual-domain GGDEF/EAL protein RmcA could achieve specificity of action through physical interaction with CbrB, which extends understanding the complex regulatory network of the c-di-GMP signaling.

Conquering the host: Bordetella spp. and Pseudomonas aeruginosa molecular regulators in lung infection

When bacteria sense cues from the host environment, stress responses are activated. Two component systems, sigma factors, small RNAs, ppGpp stringent response, and chaperones start coordinate the expression of virulence factors or immunomodulators to allow bacteria to respond. Although, some of these are well studied, such as the two-component systems, the contribution of other regulators, such as sigma factors or ppGpp, is increasingly gaining attention. Pseudomonas aeruginosa is the gold standard pathogen for studying the molecular mechanisms to sense and respond to environmental cues. Bordetella spp., on the other hand, is a microbial model for studying host-pathogen interactions at the molecular level. These two pathogens have the ability to colonize the lungs of patients with chronic diseases, suggesting that they have the potential to share a niche and interact. However, the molecular networks that facilitate adaptation of Bordetella spp. to cues are unclear. Here, we offer a side-by-side comparison of what is known about these diverse molecular mechanisms that bacteria utilize to counteract host immune responses, while highlighting the relatively unexplored interactions between them.

Balancing the Virulence and Antimicrobial Resistance in VISA DAP-R CA-MRSA Superbug

Background: Methicillin-resistant Staphylococcus aureus (MRSA) with intermediate resistance to Vancomycin (VISA) is reported worldwide. These strains frequently emerge among hospital-associated (HA)-MRSA and rarely within community-acquired (CA)-MRSA. Here, the genomic and transcriptomic adaptations distinguishing VISA daptomycin resistant (DAP-R) CA-MRSA, which emerged in a hospitalized patient under glycopeptide treatment, were explored. Methods: Whole-genome sequencing, RNA-Seq and bioinformatics were carried out. Results: Our CA-MRSA clustered in the USA400 lineage showing additional antimicrobial resistance (AMR) versus DAP and glycopeptides. Resistomics revealed adaptations related to glycopeptide, daptomycin and rifampin resistance (mprF nsSNPS and overexpression of glycopeptide and daptomycin-resistance related genes). Similar changes were detected in virulence traits (agrA HI-nsSNPs and toxin gene underexpression), in which a decrease was observed despite the abundance of virulence-related genes. Our results predicted a balance in adaptations, decreasing the virulence and biological costs to support the co-occurrence of extensive AMR in a hypervirulent genomic background. Conclusion: Our data show that VISA DAP-R CA-MRSA shifts the potential hypervirulent behavior of CA-MRSA towards the acquisition and maintenance of extensive AMR, by a decrease in virulence and biological costs mediated by a “compensatory modulatory mutation” silencing the Agr quorum-sensing cascade.

Pseudomonas aeruginosa: pathogenesis, virulence factors, antibiotic resistance, interaction with host, technology advances and emerging therapeutics

Pseudomonas aeruginosa (P. aeruginosa) is a Gram-negative opportunistic pathogen that infects patients with cystic fibrosis, burn wounds, immunodeficiency, chronic obstructive pulmonary disorder (COPD), cancer, and severe infection requiring ventilation, such as COVID-19. P. aeruginosa is also a widely-used model bacterium for all biological areas. In addition to continued, intense efforts in understanding bacterial pathogenesis of P. aeruginosa including virulence factors (LPS, quorum sensing, two-component systems, 6 type secretion systems, outer membrane vesicles (OMVs), CRISPR-Cas and their regulation), rapid progress has been made in further studying host-pathogen interaction, particularly host immune networks involving autophagy, inflammasome, non-coding RNAs, cGAS, etc. Furthermore, numerous technologic advances, such as bioinformatics, metabolomics, scRNA-seq, nanoparticles, drug screening, and phage therapy, have been used to improve our understanding of P. aeruginosa pathogenesis and host defense. Nevertheless, much remains to be uncovered about interactions between P. aeruginosa and host immune responses, including mechanisms of drug resistance by known or unannotated bacterial virulence factors as well as mammalian cell signaling pathways. The widespread use of antibiotics and the slow development of effective antimicrobials present daunting challenges and necessitate new theoretical and practical platforms to screen and develop mechanism-tested novel drugs to treat intractable infections, especially those caused by multi-drug resistance strains. Benefited from has advancing in research tools and technology, dissecting this pathogen's feature has entered into molecular and mechanistic details as well as dynamic and holistic views. Herein, we comprehensively review the progress and discuss the current status of P. aeruginosa biophysical traits, behaviors, virulence factors, invasive regulators, and host defense patterns against its infection, which point out new directions for future investigation and add to the design of novel and/or alternative therapeutics to combat this clinically significant pathogen.

Metabolic basis for the evolution of a common pathogenic Pseudomonas aeruginosa variant

Microbes frequently evolve in reproducible ways. Here, we show that differences in specific metabolic regulation rather than inter-strain interactions explain the frequent presence of lasR loss-of-function (LOF) mutations in the bacterial pathogen Pseudomonas aeruginosa. While LasR contributes to virulence through its role in quorum sensing, lasR mutants have been associated with more severe disease. A model based on the intrinsic growth kinetics for a wild type strain and its LasR- derivative, in combination with an experimental evolution based genetic screen and further genetics analyses, indicated that differences in metabolism were sufficient to explain the rise of these common mutant types. The evolution of LasR- lineages in laboratory and clinical isolates depended on activity of the two-component system CbrAB, which modulates substrate prioritization through the catabolite repression control pathway. LasR- lineages frequently arise in cystic fibrosis lung infections and their detection correlates with disease severity. Our analysis of bronchoalveolar lavage fluid metabolomes identified compounds that negatively correlate with lung function, and we show that these compounds support enhanced growth of LasR- cells in a CbrB-controlled manner. We propose that in vivo metabolomes contribute to pathogen evolution, which may influence the progression of disease and its treatment.

2-aminoethylphosphonate utilisation in Pseudomonas putida BIRD-1 is controlled by multiple master regulators

Bacteria possess various regulatory mechanisms to detect and coordinate a response to elemental nutrient limitation. In pseudomonads, the two‐component system regulators CbrAB, NtrBC and PhoBR, are responsible for regulating cellular response to carbon (C), nitrogen (N) and phosphorus (P) respectively. Phosphonates are reduced organophosphorus compounds produced by a broad range of biota and typified by a direct C‐P bond. Numerous pseudomonads can use the environmentally abundant phosphonate species 2‐aminoethylphosphonate (2AEP) as a source of C, N, or P, but only PhoBR has been shown to play a role in 2AEP utilization. On the other hand, utilization of 2AEP as a C and N source is considered substrate inducible. Here, using the plant‐growth‐promoting rhizobacterium Pseudomonas putida BIRD‐1 we present evidence that 2AEP utilization is under dual regulation and only occurs upon depletion of C, N, or P, controlled by CbrAB, NtrBC, or PhoBR respectively. However, the presence of 2AEP was necessary for full gene expression, i.e. expression was substrate inducible. Mutation of a LysR‐type regulator, termed AepR, upstream of the 2AEP transaminase‐phosphonatase system (PhnWX), confirmed this dual regulatory mechanism. To our knowledge, this is the first study identifying coordination between global stress response and substrate‐specific regulators in phosphonate metabolism.

The Regulatory Hierarchy Following Signal Integration by the CbrAB Two-Component System: Diversity of Responses and Functions

CbrAB is a two-component system, unique to bacteria of the family Pseudomonaceae, capable of integrating signals and involved in a multitude of physiological processes that allow bacterial adaptation to a wide variety of varying environmental conditions. This regulatory system provides a great metabolic versatility that results in excellent adaptability and metabolic optimization. The two-component system (TCS) CbrA-CbrB is on top of a hierarchical regulatory cascade and interacts with other regulatory systems at different levels, resulting in a robust output. Among the regulatory systems found at the same or lower levels of CbrAB are the NtrBC nitrogen availability adaptation system, the Crc/Hfq carbon catabolite repression cascade in Pseudomonas, or interactions with the GacSA TCS or alternative sigma ECF factor, such as SigX. The interplay between regulatory mechanisms controls a number of physiological processes that intervene in important aspects of bacterial adaptation and survival. These include the hierarchy in the use of carbon sources, virulence or resistance to antibiotics, stress response or definition of the bacterial lifestyle. The multiple actions of the CbrAB TCS result in an important competitive advantage.

Role of Two-Component System Networks in Pseudomonas aeruginosa Pathogenesis

Two-component systems (TCS) are the largest family of signaling systems in the bacterial kingdom. They enable bacteria to cope with a wide range of environmental conditions via the sensing of stimuli and the transduction of the signal into an appropriate cellular adaptation response. Pseudomonas aeruginosa possesses one of the richest arrays of TCSs in bacteria and they have been the subject of intense investigation for more than 20 years. Most of the P. aeruginosa TCSs characterized to date affect its pathogenesis, via the regulation of virulence factors expression, modulation of the synthesis of antibiotic/antimicrobial resistance mechanisms, and/or via linking virulence to energy metabolism. Here, we give an overview of the current knowledge on P. aeruginosa TCSs, citing key examples for each of the above-mentioned regulatory actions. We then conclude by mentioning few small molecule inhibitors of P. aeruginosa TCSs that have shown an antimicrobial action in vitro.

Biological Effects of Quinolones: A Family of Broad-Spectrum Antimicrobial Agents

Broad antibacterial spectrum, high oral bioavailability and excellent tissue penetration combined with safety and few, yet rare, unwanted effects, have made the quinolones class of antimicrobials one of the most used in inpatients and outpatients. Initially discovered during the search for improved chloroquine-derivative molecules with increased anti-malarial activity, today the quinolones, intended as antimicrobials, comprehend four generations that progressively have been extending antimicrobial spectrum and clinical use. The quinolone class of antimicrobials exerts its antimicrobial actions through inhibiting DNA gyrase and Topoisomerase IV that in turn inhibits synthesis of DNA and RNA. Good distribution through different tissues and organs to treat Gram-positive and Gram-negative bacteria have made quinolones a good choice to treat disease in both humans and animals. The extensive use of quinolones, in both human health and in the veterinary field, has induced a rise of resistance and menace with leaving the quinolones family ineffective to treat infections. This review revises the evolution of quinolones structures, biological activity, and the clinical importance of this evolving family. Next, updated information regarding the mechanism of antimicrobial activity is revised. The veterinary use of quinolones in animal productions is also considered for its environmental role in spreading resistance. Finally, considerations for the use of quinolones in human and veterinary medicine are discussed.

Roles of Two-Component Systems in Pseudomonas aeruginosa Virulence

Pseudomonas aeruginosa is an opportunistic pathogen that synthesizes and secretes a wide range of virulence factors. P. aeruginosa poses a potential threat to human health worldwide due to its omnipresent nature, robust host accumulation, high virulence, and significant resistance to multiple antibiotics. The pathogenicity of P. aeruginosa, which is associated with acute and chronic infections, is linked with multiple virulence factors and associated secretion systems, such as the ability to form and utilize a biofilm, pili, flagella, alginate, pyocyanin, proteases, and toxins. Two-component systems (TCSs) of P. aeruginosa perform an essential role in controlling virulence factors in response to internal and external stimuli. Therefore, understanding the mechanism of TCSs to perceive and respond to signals from the environment and control the production of virulence factors during infection is essential to understanding the diseases caused by P. aeruginosa infection and further develop new antibiotics to treat this pathogen. This review discusses the important virulence factors of P. aeruginosa and the understanding of their regulation through TCSs by focusing on biofilm, motility, pyocyanin, and cytotoxins.

Inactivation of CbrAB two-component system hampers root colonization in rhizospheric strain of Pseudomonas aeruginosa PGPR2

Two-component systems (TCS) are one of the signal transduction mechanisms, which sense physiological/biological restraints and respond to changing environmental conditions by regulating the gene expression. Previously, by employing a forward genetic screen (INSeq), we identified that cbrA gene is essential for the fitness of Pseudomonas aeruginosa PGPR2 during root colonization. Here, we report the functional characterization of cbrAB TCS in PGPR2 during root colonization. We constructed insertion mutants in cbrA and its cognate response regulator cbrB. Genetic characterization revealed drastic down-regultion of sRNA crcZ gene in both mutant strains which play a critical role in carbon catabolite repression (CCR). The mutant strains displayed 10-fold decreased root colonization efficiency when compared to the wild-type strain. On the other hand, mutant strains formed higher biofilm on the abiotic surface, and the expression of pelB and pslA genes involved in biofilm matrix formation was up-regulated. In contrast, the expression of algD, responsible for alginate production, and its associated sigma factor algU was significantly down-regulated in mutant strains. We further analyzed the transcript levels of rsmA, controlled by the algU sigma factor, and found that the expression of rsmA was hampered in both mutants. The ability of mutant strains to swim and swarm was significantly hindered. Also, the expression of genes associated with type III secretion system (T3SS) was dysregulated in mutant strains. Taken together, regulation of gene expression by CbrAB TCS is intricate, and we confirm its role beyond carbon and nitrogen assimilation.

Polymyxins, the last-resort antibiotics: Mode of action, resistance emergence, and potential solutions

Infections caused by multi-drug resistant (MDR) bacterial pathogens are a leading cause of mortality and morbidity across the world. Indiscriminate use of broad-spectrum antibiotics has seriously affected this situation. With the diminishing discovery of novel antibiotics, new treatment methods are urgently required to combat MDR pathogens. Polymyxins, the cationic lipopeptide antibiotics, discovered more than half a century ago, are considered to be the last-line of antibiotics available at the moment. This antibiotic shows a great bactericidal effect against Gram-negative bacteria. Polymyxins primarily target the bacterial membrane and disrupt them, causing lethality. Because of their membrane interacting mode of action, polymyxins cause nephrotoxicity and neurotoxicity in humans, limiting their usability. However, recent modifications in their chemical structure have been able to reduce the toxic effects. The development of better dosing regimens has also helped in getting better clinical outcomes in the infections caused by MDR pathogens. Since the mid-1990s the use of polymyxins has increased manifold in clinical settings, resulting in the emergence of polymyxin-resistant strains. The risk posed by the polymyxin-resistant nosocomial pathogens such as the Enterobacteriaceae group, Pseudomonas aeruginosa, and Acinetobacter baumannii, etc. is very serious considering these pathogens are resistant to almost all available antibacterial drugs. In this review article, the mode of action of the polymyxins and the genetic regulatory mechanism responsible for the emergence of resistance are discussed. Specifically, this review aims to update our current understanding in the field and suggest possible solutions that can be pursued for future antibiotic development. As polymyxins primarily target the bacterial membranes, resistance to polymyxins arises primarily by the modification of the lipopolysaccharides (LPS) in the outer membrane (OM). The LPS modification pathways are largely regulated by the bacterial two-component signal transduction (TCS) systems. Therefore, targeting or modulating the TCS signalling mechanisms can be pursued as an alternative to treat the infections caused by polymyxin-resistant MDR pathogens. In this review article, this aspect is also highlighted.

Tuning of Electrode Surface for Enhanced Bacterial Adhesion and Reactions: A Review on Recent Approaches

The study of bacterial adhesion and its consequences has great significance in different fields such as marine science, renewable energy sectors, soil and plant ecology, food industry, and the biomedical field. Generally, the adverse effects of microbial surface interactions have attained wide visibility. However, herein, we present distinct approaches to highlight the beneficial aspects of microbial surface interactions for various applications rather than deal with the conventional negative aspects or prevention strategies. The surface microbial reactions can be tuned for useful biochemical or bio-electrochemical applications, which are otherwise unattainable through conventional routes. In this context, the present review is a comprehensive approach to highlight the basic principles and signature parameters that are responsible for the useful microbial-electrode interactions. It also proposes various surface tuning strategies, which are useful for tuning the electrode characteristics particularly suitable for the enhanced bacterial adhesion and reactions. The tuning of surface characteristics of electrodes is discussed with a special reference to the Microbial Fuel Cell as an example.

Crc Regulates Succinate-Mediated Repression of Mineral Phosphate Solubilization in Acinetobacter sp. SK2 by Modulating Membrane Glucose Dehydrogenase

The plant growth-promoting Acinetobacter sp. SK2 isolated from Vigna radiata rhizosphere was characterized for mineral phosphate solubilization (MPS). To understand the contribution of the membrane glucose dehydrogenase (mGDH) and soluble glucose dehydrogenase (sGDH) in glucose oxidation and MPS, insertional inactivation of the corresponding genes was carried out. The disruption of mGDH encoding gene gdhA resulted in complete loss of mGDH activity, which confirmed its role in periplasmic glucose oxidation and gluconate-mediated MPS phenotype. The inactivation of sGDH encoding gene gdhB resulted in loss of sGDH activity, which did not alter the MPS or mGDH activity. Thus, it was also concluded that the sGDH was dispensable in gluconate-mediated MPS. Supplementation of succinate in glucose-containing medium suppressed the activity of mGDH (and sGDH) and therefore repressed the MPS phenotype. The catabolite repression control protein (Crc) of Pseudomonas was implicated in Acinetobacter sp. for a similar function in the presence of preferred and non-preferred carbon sources. To understand the regulatory linkage between Crc and genes for glucose oxidation, crc mutants were generated. The inactivation of crc resulted in increased activity of the mGDH in glucose + succinate-grown cells, indicating derepression. An increase in phosphate solubilization up to 44% in glucose + succinate-grown crc ^- compared with glucose-grown cells was recorded, which was significantly repressed in the wild-type strain under similar conditions. It is therefore proposed that in Acinetobacter sp. SK2, Crc is involved in the succinate-provoked repression of the MPS phenotype. The gene expression data indicated that Hfq may also have a regulating role in preferential utilization of carbon source by perhaps modulating Crc-Hfq functionality. V. radiata plants inoculated with the wild type improved both root and shoot length by 1.3 to 1.4-fold. However, crc ^- increased the root and shoot length by 1.6-fold, compared with the uninoculated controls. In mimicking the soil condition (in the presence of multiple carbon sources, e.g., succinate along with glucose), the crc ^- strain of Acinetobacter sp. SK2 performed better in supporting the growth of V. radiata in pot experiments.

Evolutionary Genomics of Niche-Specific Adaptation to the Cystic Fibrosis Lung in Pseudomonas aeruginosa

The comparative genomics of the transition of the opportunistic pathogen Pseudomonas aeruginosa from a free-living environmental strain to one that causes chronic infection in the airways of cystic fibrosis (CF) patients remain poorly studied. Chronic infections are thought to originate from colonization by a single strain sampled from a diverse, globally distributed population, followed by adaptive evolution to the novel, stressful conditions of the CF lung. However, we do not know whether certain clades are more likely to form chronic infections than others and we lack a comprehensive view of the suite of genes under positive selection in the CF lung. We analyzed whole-genome sequence data from 1,000 P. aeruginosa strains with diverse ecological provenances including the CF lung. CF isolates were distributed across the phylogeny, indicating little genetic predisposition for any one clade to cause chronic infection. Isolates from the CF niche experienced stronger positive selection on core genes than those derived from environmental or acute infection sources, consistent with recent adaptation to the lung environment. Genes with the greatest differential positive selection in the CF niche include those involved in core cellular processes such as metabolism, energy production, and stress response as well as those linked to patho-adaptive processes such as antibiotic resistance, cell wall and membrane modification, quorum sensing, biofilms, mucoidy, motility, and iron homeostasis. Many genes under CF-specific differential positive selection had regulatory functions, consistent with the idea that regulatory mutations play an important role in rapid adaptation to novel environments.

Two-Component Signaling Systems Regulate Diverse Virulence-Associated Traits in Pseudomonas aeruginosa

Pseudomonas aeruginosa is an opportunistic pathogen that can cause problematic infections at different sites throughout the human body. P. aeruginosa encodes a large suite of over 60 two-component signaling systems that enable cells to rapidly sense and respond to external signals. Previous work has shown that some of these sensory systems contribute to P. aeruginosa pathogenesis, but the virulence-associated processes and phenotypic traits that each of these systems controls are still largely unclear. To aid investigations of these sensory systems, we have generated deletion strains for each of 64 genes encoding histidine kinases and one histidine phosphotransferase in P. aeruginosa PA14. We carried out initial phenotypic characterizations of this collection by assaying these mutants for over a dozen virulence-associated traits, and we found that each of these phenotypes is regulated by multiple sensory systems. Our work highlights the usefulness of this collection for further studies of P. aeruginosa two-component signaling systems and provides insight into how these systems may contribute to P. aeruginosa infection.IMPORTANCE Pseudomonas aeruginosa can grow and survive under a wide range of conditions, including as a human pathogen. As such, P. aeruginosa must be able to sense and respond to diverse signals and cues in its environment. This sensory capability is endowed in part by the hundreds of two-component signaling proteins encoded in the P. aeruginosa genome, but the precise roles of each remain poorly defined. To facilitate systematic study of the signaling repertoire of P. aeruginosa PA14, we generated a library of deletion strains, each lacking one of the 64 histidine kinases. By subjecting these strains to a battery of phenotypic assays, we confirmed the functions of many and unveiled roles for dozens of previously uncharacterized histidine kinases in controlling various traits, many of which are associated with P. aeruginosa virulence. Thus, this work provides new insight into the functions of two-component signaling proteins and provides a resource for future investigations.

Peptide 1018 inhibits swarming and influences Anr-regulated gene expression downstream of the stringent stress response in Pseudomonas aeruginosa

Pseudomonas aeruginosa is a ubiquitous opportunistic pathogen that causes considerable human morbidity and mortality, particularly in nosocomial infections and individuals with cystic fibrosis. P. aeruginosa can adapt to surface growth by undergoing swarming motility, a rapid multicellular movement that occurs on viscous soft surfaces with amino acids as a nitrogen source. Here we tested the small synthetic host defense peptide, innate defense regulator 1018, and found that it inhibited swarming motility at concentrations as low as 0.75 μg/ml, well below the MIC for strain PA14 planktonic cells (64 μg/ml). A screen of the PA14 transposon insertion mutant library revealed 29 mutants that were more tolerant to peptide 1018 during swarming, five of which demonstrated significantly greater swarming than the WT in the presence of peptide. Transcriptional analysis (RNA-Seq) of cells that were inoculated on swarming plates containing 1.0 μg/ml peptide revealed differential expression of 1,190 genes compared to cells swarming on plates without peptide. Furthermore, 1018 treatment distinctly altered the gene expression profile of cells when compared to that untreated cells in the centre of the swarm colonies. Peptide-treated cells exhibited changes in the expression of genes implicated in the stringent stress response including those regulated by anr, which is involved in anaerobic adaptation, indicative of a mechanism by which 1018 might inhibit swarming motility. Overall, this study illustrates potential mechanisms by which peptide 1018 inhibits swarming surface motility, an important bacterial adaptation associated with antibiotic resistance, virulence, and dissemination of P. aeruginosa.

An atlas of the binding specificities of transcription factors in Pseudomonas aeruginosa directs prediction of novel regulators in virulence

A high-throughput systematic evolution of ligands by exponential enrichment assay was applied to 371 putative TFs in Pseudomonas aeruginosa, which resulted in the robust enrichment of 199 unique sequence motifs describing the binding specificities of 182 TFs. By scanning the genome, we predicted in total 33,709 significant interactions between TFs and their target loci, which were more than 11-fold enriched in the intergenic regions but depleted in the gene body regions. To further explore and delineate the physiological and pathogenic roles of TFs in P. aeruginosa, we constructed regulatory networks for nine major virulence-associated pathways and found that 51 TFs were potentially significantly associated with these virulence pathways, 32 of which had not been characterized before, and some were even involved in multiple pathways. These results will significantly facilitate future studies on transcriptional regulation in P. aeruginosa and other relevant pathogens, and accelerate to discover effective treatment and prevention strategies for the associated infectious diseases.

Integrated regulatory network in Pseudomonas syringae reveals dynamics of virulence

Pseudomonas syringae, a Gram-negative plant pathogen, expresses multitudinous transcriptional regulators to control the type III secretion system (T3SS) and response to diverse environmental challenges. Although the mechanisms of virulence-associated regulators of P. syringae have been studied for decades, the overall crosstalk underlying these regulators is still elusive. Here, we identify five T3SS regulators (EnvZ-OmpR, CbrAB2, PhoPQ, PilRS, and MgrA), and find that the two-component systems EnvZ-OmpR and CbrAB2 negatively regulate the T3SS. To elucidate crosstalk between 16 virulence-associated regulators in P. syringae, we map an online intricate network called "PSRnet" (Pseudomonas syringae regulatory network) by combining the differentially expressed genes (DEGs) of these 16 regulators by RNA sequencing (RNA-seq) and their binding loci by chromatin immunoprecipitation sequencing (ChIP-seq). Consequently, we identify 238 and 153 functional genes involved in the T3SS and other virulence-related pathways in KB and MM media, respectively. Our results provide insights into the mechanism of plant infections caused by P. syringae.

Prokaryotic Solute/Sodium Symporters: Versatile Functions and Mechanisms of a Transporter Family

The solute/sodium symporter family (SSS family; TC 2.A.21; SLC5) consists of integral membrane proteins that use an existing sodium gradient to drive the uphill transport of various solutes, such as sugars, amino acids, vitamins, or ions across the membrane. This large family has representatives in all three kingdoms of life. The human sodium/iodide symporter (NIS) and the sodium/glucose transporter (SGLT1) are involved in diseases such as iodide transport defect or glucose-galactose malabsorption. Moreover, the bacterial sodium/proline symporter PutP and the sodium/sialic acid symporter SiaT play important roles in bacteria–host interactions. This review focuses on the physiological significance and structural and functional features of prokaryotic members of the SSS family. Special emphasis will be given to the roles and properties of proteins containing an SSS family domain fused to domains typically found in bacterial sensor kinases.

Metabotypes of Pseudomonas aeruginosa Correlate with Antibiotic Resistance, Virulence and Clinical Outcome in Cystic Fibrosis Chronic Infections

Pseudomonas aeruginosa (P.a) is one of the most critical antibiotic resistant bacteria in the world and is the most prevalent pathogen in cystic fibrosis (CF), causing chronic lung infections that are considered one of the major causes of mortality in CF patients. Although several studies have contributed to understanding P.a within-host adaptive evolution at a genomic level, it is still difficult to establish direct relationships between the observed mutations, expression of clinically relevant phenotypes, and clinical outcomes. Here, we performed a comparative untargeted LC/HRMS-based metabolomics analysis of sequential isolates from chronically infected CF patients to obtain a functional view of P.a adaptation. Metabolic profiles were integrated with expression of bacterial phenotypes and clinical measurements following multiscale analysis methods. Our results highlighted significant associations between P.a “metabotypes”, expression of antibiotic resistance and virulence phenotypes, and frequency of clinical exacerbations, thus identifying promising biomarkers and therapeutic targets for difficult-to-treat P.a infections

Polymyxins, the last-resort antibiotics: Mode of action, resistance emergence, and potential solutions

Infections caused by multi-drug resistant (MDR) bacterial pathogens are a leading cause of mortality and morbidity across the world. Indiscriminate use of broad-spectrum antibiotics has seriously affected this situation. With the diminishing discovery of novel antibiotics, new treatment methods are urgently required to combat MDR pathogens. Polymyxins, the cationic lipopeptide antibiotics, discovered more than half a century ago, are considered to be the last-line of antibiotics available at the moment. This antibiotic shows a great bactericidal effect against Gram-negative bacteria. Polymyxins primarily target the bacterial membrane and disrupt them, causing lethality. Because of their membrane interacting mode of action, polymyxins cause nephrotoxicity and neurotoxicity in humans, limiting their usability. However, recent modifications in their chemical structure have been able to reduce the toxic effects. The development of better dosing regimens has also helped in getting better clinical outcomes in the infections caused by MDR pathogens. Since the mid1990s the use of polymyxins has increased manifold in clinical settings, resulting in the emergence of polymyxin-resistant strains. The risk posed by the polymyxin-resistant nosocomial pathogens such as the Enterobacteriaceae group, Pseudomonas aeruginosa, and Acinetobacter baumannii, etc. is very serious considering these pathogens are resistant to almost all available antibacterial drugs. In this review article, the mode of action of the polymyxins and the genetic regulatory mechanism responsible for the emergence of resistance are discussed. Specifically, this review aims to update our current understanding in the field and suggest possible solutions that can be pursued for future antibiotic development. As polymyxins primarily target the bacterial membranes, resistance to polymyxins arises primarily by the modification of the lipopolysaccharides (LPS) in the outer membrane (OM). The LPS modification pathways are largely regulated by the bacterial two-component signal transduction (TCS) systems. Therefore, targeting or modulating the TCS signalling mechanisms can be pursued as an alternative to treat the infections caused by polymyxin-resistant MDR pathogens. In this review article, this aspect is also highlighted.

Murepavadin antimicrobial activity against and resistance development in cystic fibrosis Pseudomonas aeruginosa isolates

Background

Murepavadin, a novel peptidomimetic antibiotic, is being developed as an inhalation therapy for treatment of Pseudomonas aeruginosa respiratory infection in people with cystic fibrosis (CF). It blocks the activity of the LptD protein in P. aeruginosa causing outer membrane alterations.

Objectives

To determine the in vitro activity of murepavadin against CF P. aeruginosa isolates and to investigate potential mechanisms of resistance.

Methods

MIC values were determined by both broth microdilution and agar dilution and results compared. The effect of artificial sputum and lung surfactant on in vitro activity was also measured. Spontaneous mutation frequency was estimated. Bactericidal activity was investigated using time–kill assays. Resistant mutants were studied by WGS.

Results

The murepavadin MIC50 was 0.125 versus 4 mg/L and the MIC90 was 2 versus 32 mg/L by broth microdilution and agar dilution, respectively. Essential agreement was >90% when determining in vitro activity with artificial sputum or lung surfactant. It was bactericidal at a concentration of 32 mg/L against 95.4% of the strains within 1–5 h. Murepavadin MICs were 2–9 two-fold dilutions higher for the mutant derivatives (0.5 to >16 mg/L) than for the parental strains. Second-step mutants were obtained for the PAO mutS reference strain with an 8×MIC increase. WGS showed mutations in genes involved in LPS biosynthesis (lpxL1, lpxL2, bamA2, lptD, lpxT and msbA).

Conclusions

Murepavadin characteristics, such as its specific activity against P. aeruginosa, its unique mechanism of action and its strong antimicrobial activity, encourage the further clinical evaluation of this drug.

An integrated model system to gain mechanistic insights into biofilm-associated antimicrobial resistance in Pseudomonas aeruginosa MPAO1

Pseudomonas aeruginosa MPAO1 is the parental strain of the widely utilized transposon mutant collection for this important clinical pathogen. Here, we validate a model system to identify genes involved in biofilm growth and biofilm-associated antibiotic resistance. Our model employs a genomics-driven workflow to assemble the complete MPAO1 genome, identify unique and conserved genes by comparative genomics with the PAO1 reference strain and genes missed within existing assemblies by proteogenomics. Among over 200 unique MPAO1 genes, we identified six general essential genes that were overlooked when mapping public Tn-seq data sets against PAO1, including an antitoxin. Genomic data were integrated with phenotypic data from an experimental workflow using a user-friendly, soft lithography-based microfluidic flow chamber for biofilm growth and a screen with the Tn-mutant library in microtiter plates. The screen identified hitherto unknown genes involved in biofilm growth and antibiotic resistance. Experiments conducted with the flow chamber across three laboratories delivered reproducible data on P. aeruginosa biofilms and validated the function of both known genes and genes identified in the Tn-mutant screens. Differential protein abundance data from planktonic cells versus biofilm confirmed the upregulation of candidates known to affect biofilm formation, of structural and secreted proteins of type VI secretion systems, and provided proteogenomic evidence for some missed MPAO1 genes. This integrated, broadly applicable model promises to improve the mechanistic understanding of biofilm formation, antimicrobial tolerance, and resistance evolution in biofilms.

Novel therapeutic strategies for treating Pseudomonas aeruginosa infection

INTRODUCTION

Persistent infections caused by the superbug Pseudomonas aeruginosa and its resistance to multiple antimicrobial agents are huge threats to patients with cystic fibrosis as well as those with compromised immune systems. Multidrug-resistant P. aeruginosa has posed a major challenge to conventional antibiotics and therapeutic approaches, which show limited efficacy and cause serious side effects. The public demand for new antibiotics is enormous; yet, drug development pipelines have started to run dry with limited targets available for inventing new antibacterial drugs. Consequently, it is important to uncover potential therapeutic targets.

AREAS COVERED

The authors review the current state of drug development strategies that are promising in terms of the development of novel and potent drugs to treat P. aeruginosa infection.

EXPERT OPINION

The prevention of P. aeruginosa infection is increasingly challenging. Furthermore, targeting key virulence regulators has great potential for developing novel anti-P. aeruginosa drugs. Additional promising strategies include bacteriophage therapy, immunotherapies, and antimicrobial peptides. Additionally, the authors believe that in the coming years, the overall network of molecular regulatory mechanism of P. aeruginosa virulence will be fully elucidated, which will provide more novel and promising drug targets for treating P. aeruginosa infections.

Pseudomonas aeruginosa core metabolism exerts a widespread growth-independent control on virulence

To assess the role of core metabolism genes in bacterial virulence - independently of their effect on growth - we correlated the genome, the transcriptome and the pathogenicity in flies and mice of 30 fully sequenced Pseudomonas strains. Gene presence correlates robustly with pathogenicity differences among all Pseudomonas species, but not among the P. aeruginosa strains. However, gene expression differences are evident between highly and lowly pathogenic P. aeruginosa strains in multiple virulence factors and a few metabolism genes. Moreover, 16.5%, a noticeable fraction of the core metabolism genes of P. aeruginosa strain PA14 (compared to 8.5% of the non-metabolic genes tested), appear necessary for full virulence when mutated. Most of these virulence-defective core metabolism mutants are compromised in at least one key virulence mechanism independently of auxotrophy. A pathway level analysis of PA14 core metabolism, uncovers beta-oxidation and the biosynthesis of amino-acids, succinate, citramalate, and chorismate to be important for full virulence. Strikingly, the relative expression among P. aeruginosa strains of genes belonging in these metabolic pathways is indicative of their pathogenicity. Thus, P. aeruginosa strain-to-strain virulence variation, remains largely obscure at the genome level, but can be dissected at the pathway level via functional transcriptomics of core metabolism.

Pseudomonas aeruginosa biofilm formation on endotracheal tubes requires multiple two-component systems

Introduction. Indwelling medical devices such as endotracheal tubes (ETTs), urinary catheters, vascular access devices, tracheostomies and feeding tubes are often associated with hospital-acquired infections. Bacterial biofilm formed on the ETTs in intubated patients is a significant risk factor associated with ventilator-associated pneumonia. Pseudomonas aeruginosa is one of the four frequently encountered bacteria responsible for causing pneumonia, and the biofilm formation on ETTs. However, understanding of biofilm formation on ETT and interventions to prevent biofilm remains lagging. The ability to sense and adapt to external cues contributes to their success. Thus, the biofilm formation is likely to be influenced by the two-component systems (TCSs) that are composed of a membrane-associated sensor kinase and an intracellular response regulator.Aim. This study aims to establish an in vitro method to analyse the P. aeruginosa biofilm formation on ETTs, and identify the TCSs that contribute to this process.Methodology. In total, 112 P. aeruginosa PA14 TCS mutants were tested for their ability to form biofilm on ETTs, their effect on quorum sensing (QS) and motility.Results. Out of 112 TCS mutants studied, 56 had altered biofilm biomass on ETTs. Although the biofilm formation on ETTs is QS-dependent, none of the 56 loci controlled quorum signal. Of these, 18 novel TCSs specific to ETT biofilm were identified, namely, AauS, AgtS, ColR, CopS, CprR, NasT, KdpD, ParS, PmrB, PprA, PvrS, RcsC, PA14_11120, PA14_32580, PA14_45880, PA14_49420, PA14_52240, PA14_70790. The set of 56 included the GacS network, TCS proteins involved in fimbriae synthesis, TCS proteins involved in antimicrobial peptide resistance, and surface-sensing. Additionally, several of the TCS-encoding genes involved in biofilm formation on ETTs were found to be linked to flagellum-dependent swimming motility.Conclusions. Our study established an in vitro method for studying P. aeruginosa biofilm formation on the ETT surfaces. We also identified novel ETT-specific TCSs that could serve as targets to prevent biofilm formation on indwelling devices frequently used in clinical settings.

Pseudomonad reverse carbon catabolite repression, interspecies metabolite exchange, and consortial division of labor

Microorganisms acquire energy and nutrients from dynamic environments, where substrates vary in both type and abundance. The regulatory system responsible for prioritizing preferred substrates is known as carbon catabolite repression (CCR). Two broad classes of CCR have been documented in the literature. The best described CCR strategy, referred to here as classic CCR (cCCR), has been experimentally and theoretically studied using model organisms such as Escherichia coli. cCCR phenotypes are often used to generalize universal strategies for fitness, sometimes incorrectly. For instance, extremely competitive microorganisms, such as Pseudomonads, which arguably have broader global distributions than E. coli, have achieved their success using metabolic strategies that are nearly opposite of cCCR. These organisms utilize a CCR strategy termed 'reverse CCR' (rCCR), because the order of preferred substrates is nearly reverse that of cCCR. rCCR phenotypes prefer organic acids over glucose, may or may not select preferred substrates to optimize growth rates, and do not allocate intracellular resources in a manner that produces an overflow metabolism. cCCR and rCCR have traditionally been interpreted from the perspective of monocultures, even though most microorganisms live in consortia. Here, we review the basic tenets of the two CCR strategies and consider these phenotypes from the perspective of resource acquisition in consortia, a scenario that surely influenced the evolution of cCCR and rCCR. For instance, cCCR and rCCR metabolism are near mirror images of each other; when considered from a consortium basis, the complementary properties of the two strategies can mitigate direct competition for energy and nutrients and instead establish cooperative division of labor.

TpiA is a Key Metabolic Enzyme That Affects Virulence and Resistance to Aminoglycoside Antibiotics through CrcZ in Pseudomonas aeruginosa

The increase in bacterial resistance against antibiotics imposes a severe threat to public health. It is urgent to identify new drug targets and develop novel antimicrobials. Metabolic homeostasis of bacteria plays an essential role in their virulence and resistance to antibiotics. Recent studies demonstrated that antibiotic efficacies can be improved by modulating the bacterial metabolism. Pseudomonas aeruginosa is an important opportunistic human pathogen that causes various infections. The bacterium is intrinsically resistant to antibiotics. In this study, we provide clear evidence that TpiA (triosephosphate isomerase) plays an essential role in the metabolism of P. aeruginosa and influences bacterial virulence and antibiotic resistance. The significance of this work is in identifying a key enzyme in the metabolic network, which will provide clues as to the development of novel treatment strategies against infections caused by P. aeruginosa.

Carbon metabolism plays an essential role in bacterial pathogenesis and susceptibility to antibiotics. In Pseudomonas aeruginosa, Crc, Hfq, and a small RNA, CrcZ, are central regulators of carbon metabolism. By screening mutants of genes involved in carbon metabolism, we found that mutation of the tpiA gene reduces the expression of the type III secretion system (T3SS) and bacterial resistance to aminoglycoside antibiotics. TpiA is a triosephosphate isomerase that reversibly converts glyceraldehyde 3-phosphate to dihydroxyacetone phosphate, a key step connecting glucose metabolism with glycerol and phospholipid metabolisms. We found that mutation of the tpiA gene enhances the bacterial carbon metabolism, respiration, and oxidative phosphorylation, which increases the membrane potential and promotes the uptake of aminoglycoside antibiotics. Further studies revealed that the level of CrcZ is increased in the tpiA mutant due to enhanced stability. Mutation of the crcZ gene in the tpiA mutant background restored the expression of the T3SS genes and the bacterial resistance to aminoglycoside antibiotics. Overall, this study reveals an essential role of TpiA in the metabolism, virulence, and antibiotic resistance in P. aeruginosa.

Down-Regulation of Flagellar, Fimbriae, and Pili Proteins in Carbapenem-Resistant Klebsiella pneumoniae (NDM-4) Clinical Isolates: A Novel Linkage to Drug Resistance

The emergence and spread of carbapenem-resistant Klebsiella pneumoniae infections have worsened the current situation worldwide, in which totally drug-resistant strains (bad bugs) are becoming increasingly prominent. Bacterial biofilms enable bacteria to tolerate higher doses of antibiotics and other stresses, which may lead to the drug resistance. In the present study, we performed proteomics on the carbapenem-resistant NDM-4-producing K. pneumoniae clinical isolate under meropenem stress. Liquid chromatography coupled with mass spectrometry (LC–MS/MS) analysis revealed that 69 proteins were down-regulated (≤0.42-fold change) under meropenem exposure. Within the identified down-regulated proteome (69 proteins), we found a group of 13 proteins involved in flagellar, fimbriae, and pili formation and their related functions. Further, systems biology approaches were employed to reveal their networking pathways. We suggest that these down-regulated proteins and their interactive partners cumulatively contribute to the emergence of a biofilm-like state and the survival of bacteria under drug pressure, which could reveal novel mechanisms or pathways involved in drug resistance. These down-regulated proteins and their pathways might be used as targets for the development of novel therapeutics against antimicrobial-resistant (AMR) infections.

Systems-level analysis of NalD mutation, a recurrent driver of rapid drug resistance in acute Pseudomonas aeruginosa infection

Pseudomonas aeruginosa, a main cause of human infection, can gain resistance to the antibiotic aztreonam through a mutation in NalD, a transcriptional repressor of cellular efflux. Here we combine computational analysis of clinical isolates, transcriptomics, metabolic modeling and experimental validation to find a strong association between NalD mutations and resistance to aztreonam-as well as resistance to other antibiotics-across P. aeruginosa isolated from different patients. A detailed analysis of one patient's timeline shows how this mutation can emerge in vivo and drive rapid evolution of resistance while the patient received cancer treatment, a bone marrow transplantation, and antibiotics up to the point of causing the patient's death. Transcriptomics analysis confirmed the primary mechanism of NalD action-a loss-of-function mutation that caused constitutive overexpression of the MexAB-OprM efflux system-which lead to aztreonam resistance but, surprisingly, had no fitness cost in the absence of the antibiotic. We constrained a genome-scale metabolic model using the transcriptomics data to investigate changes beyond the primary mechanism of resistance, including adaptations in major metabolic pathways and membrane transport concurrent with aztreonam resistance, which may explain the lack of a fitness cost. We propose that metabolic adaptations may allow resistance mutations to endure in the absence of antibiotics and could be targeted by future therapies against antibiotic resistant pathogens.

Phenotypic and Genetic Predictors of Pathogenicity and Virulence in Flavobacterium psychrophilum

Flavobacterium psychrophilum causes bacterial cold-water disease (BCWD) in farmed rainbow trout (Oncorhynchus mykiss), with the multilocus sequence typing (MLST) clonal complex (CC) CC-ST10 accounting for the majority of outbreaks globally. The development of alternative strategies to antibiotic treatment of BCWD using bacteriophage-based control of F. psychrophilum, or virulence factors as targets for therapy, requires knowledge of the phage-sensitivity of outbreak strains and of universal traits contributing to their pathogenicity. To examine the association between virulence and both genetic (MLST sequence type (ST) and PCR-serotype) and phenotypic characteristics (adherence, antibiotic resistance, colony spreading motility, hemolytic and proteolytic activity), the median lethal dose (LD₅₀) of 26 geographically disparate F. psychrophilum isolates was determined in rainbow trout. Furthermore, the in vitro sensitivity of the isolates against five bacteriophages was determined by the efficiency of plating (EOP). The tested F. psychrophilum isolates were mainly represented by CC-ST10 genotypes (22 out of 26) and showed up to 3-log differences in LD₅₀ (8.9 × 10³ to 3.1 × 10⁶ CFU). No association between MLST ST and virulence was found because of a high variation in LD₅₀ within STs. All identified serotypes (0, 1, and 2) were pathogenic, but ten most virulent isolates belonged to serotype 1 or 2. Isolates of high (LD₅₀ < 10⁵ CFU), moderate (LD₅₀ = 10⁵–10⁶ CFU), and weak (LD₅₀ > 10⁶ CFU) virulence were similar in phenotypic characteristics in vitro. However, the only non-virulent CC-ST10 isolate was deficient in spreading motility and proteolytic activity, indicating that the characteristics are required for pathogenicity in F. psychrophilum. Univariate correlation studies found only non-significant associations between LD₅₀ and the measured phenotypic characteristics, and the multivariable analysis did neither reveal any significant predictors of virulence. The majority of isolates (16 out of 26) were sensitive to at least four bacteriophages, with up to a 6-log variation in the EOP. Most CC-ST10 isolates (16 out of 22) were sensitive to the examined phages, including 5 out of the 7 most virulent isolates represented by prevalent and antibiotic-resistant STs. Our findings suggest that control of BCWD using lytic phages or interventions targeting shared characteristics of pathogenic F. psychrophilum strains should be further explored.

Interplay between the catabolite repression control protein Crc, Hfq and RNA in Hfq-dependent translational regulation in Pseudomonas aeruginosa

In Pseudomonas aeruginosa the RNA chaperone Hfq and the catabolite repression control protein (Crc) act as post-transcriptional regulators during carbon catabolite repression (CCR). In this regard Crc is required for full-fledged Hfq-mediated translational repression of catabolic genes. RNA_seq based transcriptome analyses revealed a significant overlap between the Crc and Hfq regulons, which in conjunction with genetic data supported a concerted action of both proteins. Biochemical and biophysical approaches further suggest that Crc and Hfq form an assembly in the presence of RNAs containing A-rich motifs, and that Crc interacts with both, Hfq and RNA. Through these interactions, Crc enhances the stability of Hfq/Crc/RNA complexes, which can explain its facilitating role in Hfq-mediated translational repression. Hence, these studies revealed for the first time insights into how an interacting protein can modulate Hfq function. Moreover, Crc is shown to interfere with binding of a regulatory RNA to Hfq, which bears implications for riboregulation. These results are discussed in terms of a working model, wherein Crc prioritizes the function of Hfq toward utilization of favored carbon sources.

Unraveling the role of the CbrA histidine kinase in the signal transduction of the CbrAB two-component system in Pseudomonas putida

The histidine kinase CbrA of the CbrAB two-component system of Pseudomonas putida is a key element to recognise the activating signal and mediate auto- and trans-phosphorylation of the response element CbrB. CbrA is encoded by the gene cbrA which is located downstream of a putative open reading frame we have named cbrX. We describe the role of the CbrX product in the expression of CbrA and show there is translational coupling of the genes. We also explore the role of the transmembrane (TM) and PAS domains of CbrA in the signal recognition. A ΔcbrXA mutant lacking its TM domains is uncoupled in its growth in histidine and citrate as carbon sources, but its overexpression restores the ability to grow in such carbon sources. In these conditions ΔTM-CbrA is able to respond to carbon availability, thus suggesting an intracellular nature for the signal sensed.

Pseudomonas donghuensis HYS virulence towards Caenorhabditis elegans is regulated by the Cbr/Crc system

Pseudomonas donghuensis HYS is the type strain of a recently identified species, P. donghuensis, which has pathogenic potential with an unclear virulence mechanism. In this study, we used Caenorhabditis elegans as a host to explore the virulence mechanism of P. donghuensis HYS. Based on a correlation between P. donghuensis HYS virulence and its repellence property, we identified 68 potential virulence-related genes, among them the Cbr/Crc system, which regulates the virulence of prokaryotic microorganisms. Slow-killing assays indicated that cbrA, cbrB, or specific sRNA-encoding genes all affected P. donghuensis virulence positively, whereas crc affected it negatively. Transcriptome analyses demonstrated that the Cbr/Crc system played an important role in the pathogenesis of P. donghuensis. In addition, experiments using the worm mutant KU25 pmk-1(km25) showed a correlation between P. donghuensis HYS virulence and the PMK-1/p38 MAPK pathway in C. elegans. In conclusion, our data show that Crc plays a novel role in the Cbr/Crc system, and the P. donghuensis virulence phenotype therefore differs from that of P. aeruginosa. This process also involves C. elegans innate immunity. These findings significantly increase the available information about Cbr/Crc-based virulence mechanisms in the genus Pseudomonas.