Contribution of the Alkylquinolone Quorum-Sensing System to the Interaction of Pseudomonas aeruginosa With Bronchial Epithelial Cells

Iron starvation increases the production of the Pseudomonas aeruginosa RsmY and RsmZ sRNAs in static conditions

Pseudomonas aeruginosa is an opportunistic Gram-negative pathogen that induces virulence gene expression in response to host-mediated iron starvation. Recently, our laboratory showed that some virulence factors are responsive to iron limitation in static but not shaking growth conditions. One of these is the HSI-2-type six secretion system (T6SS), which is also induced during chronic infection. Iron regulation of T6SS was partially impacted by the iron-responsive PrrF sRNA and completely dependent upon the Pseudomonas quinolone signal (PQS) biosynthetic gene pqsA. Here, we analyzed the impact of iron on the expression of two small regulatory RNAs (sRNAs), RsmY and RsmZ, that activate the expression of T6SS by sequestering the RsmA translation inhibitor. Our results demonstrate that iron starvation induces the expression of RsmY and RsmZ in static but not shaking cultures. We further show that this induction occurs through the rsmY and rsmZ promoters and is dependent upon PqsA. Disruption of either the pqsR gene also eliminated iron-dependent regulation of rsmY and rsmZ promoter activity. Taken together, our results show novel targets of iron regulation that are specific to static growth, highlighting the importance of studying regulatory mechanisms in static communities that may be more representative of growth during chronic infection.IMPORTANCEIron is a central component of various bacterial metabolic pathways making it an important host-acquired nutrient for pathogens to establish infection. Previous iron regulatory studies primarily relied on shaking bacterial cultures; while these ensure cultural homogeneity, they do not reflect growth conditions during infection. We recently showed that static growth of Pseudomonas aeruginosa promotes iron-dependent regulation of a type six secretion system (T6SS), a virulence factor that is induced during chronic infections. In the current study, we found that static growth also promotes iron-dependent regulation of the RsmY and RsmZ sRNAs, which are global regulators that affect T6SS during chronic P. aeruginosa lung infection. Hence, our work demonstrates the Rsm sRNAs as potential effectors of iron regulation during static growth that may also be relevant in chronic infection.

Modulation of immune cell function, IDO expression and kynurenine production by the quorum sensor 2-heptyl-3-hydroxy-4-quinolone (PQS)

Many invasive micro-organisms produce 'quorum sensor' molecules which regulate colony expansion and may modulate host immune responses. We have examined the ability of Pseudomonas Quorum Sensor (PQS) to influence cytokine expression under conditions of inflammatory stress. The administration of PQS in vivo to mice with collagen-induced arthritis (CIA) increased the severity of disease. Blood and inflamed paws from treated mice had fewer regulatory T cells (Tregs) but normal numbers of Th17 cells. However, PQS (1μM) treatment of antigen-stimulated lymph node cells from collagen-immunised mice in vitro inhibited the differentiation of CD4+IFNγ+ cells, with less effect on CD4+IL-17+ cells and no change in CD4+FoxP3+Tregs. PQS also inhibited T cell activation by anti-CD3/anti-CD28 antibodies. PQS reduced murine macrophage polarisation and inhibited expression of IL1B and IL6 genes in murine macrophages and human THP-1 cells. In human monocyte-derived macrophages, IDO1 gene, protein and enzyme activity were all inhibited by exposure to PQS. TNF gene expression was inhibited in THP-1 cells but not murine macrophages, while LPS-induced TNF protein release was increased by high PQS concentrations. PQS is known to have iron scavenging activity and its suppression of cytokine release was abrogated by iron supplementation. Unexpectedly, PQS decreased the expression of indoleamine-2, 3-dioxygenase genes (IDO1 and IDO2), IDO1 protein expression and enzyme activity in mouse and human macrophages. This is consistent with evidence that IDO1 inhibition or deletion exacerbates arthritis, while kynurenine reduces its severity. It is suggested that the inhibition of IDO1 and cytokine expression may contribute to the quorum sensor and invasive actions of PQS.

Communication Breakdown: Into the Molecular Mechanism of Biofilm Inhibition by CeO2 Nanocrystal Enzyme Mimics and How It Can Be Exploited

Bacterial biofilm formation is a huge problem in industry and medicine. Therefore, the discovery of anti-biofilm agents may hold great promise. Biofilm formation is usually a consequence of bacterial cell-cell communication, a process called quorum sensing (QS). CeO₂ nanocrystals (NCs) have been established as haloperoxidase (HPO) mimics and ecologically beneficial biofilm inhibitors. They were suggested to interfere with QS, a mechanism termed quorum quenching (QQ), but their molecular mechanism remained elusive. We show that CeO₂ NCs are effective QQ agents, inactivating QS signals by bromination. Catalytic bromination of 3-oxo-C₁₂-AHL a QS signaling compound used by Pseudomonas aeruginosa, was detected in the presence of CeO₂ NCs, bromide ions, and hydrogen peroxide. Brominated acyl-homoserine lactones (AHLs) no longer act as QS signals but were not detected in the bacterial cultures. Externally added brominated AHLs also disappeared in P. aeruginosa cultures within minutes of their addition, indicating that they are rapidly degraded by the bacteria. Moreover, we detected the catalytic bromination of 2-heptyl-1-hydroxyquinolin-4(1H)-one (HQNO), a multifunctional non-AHL QS signal from P. aeruginosa with antibacterial and algicidal properties controlling the expression of many virulence genes. Brominated HQNO was not degraded by the bacteria in vivo. The repression of the Pseudomonas quinolone signal (PQS) production and biofilm formation in P. aeruginosa through the catalytic formation of Br-HQNO on surfaces with coatings containing CeO₂ enzyme mimics validates the non-toxic strategy for the development of anti-infectives.

Targeting the Holy Triangle of Quorum Sensing, Biofilm Formation, and Antibiotic Resistance in Pathogenic Bacteria

Chronic and recurrent bacterial infections are frequently associated with the formation of biofilms on biotic or abiotic materials that are composed of mono- or multi-species cultures of bacteria/fungi embedded in an extracellular matrix produced by the microorganisms. Biofilm formation is, among others, regulated by quorum sensing (QS) which is an interbacterial communication system usually composed of two-component systems (TCSs) of secreted autoinducer compounds that activate signal transduction pathways through interaction with their respective receptors. Embedded in the biofilms, the bacteria are protected from environmental stress stimuli, and they often show reduced responses to antibiotics, making it difficult to eradicate the bacterial infection. Besides reduced penetration of antibiotics through the intricate structure of the biofilms, the sessile biofilm-embedded bacteria show reduced metabolic activity making them intrinsically less sensitive to antibiotics. Moreover, they frequently express elevated levels of efflux pumps that extrude antibiotics, thereby reducing their intracellular levels. Some efflux pumps are involved in the secretion of QS compounds and biofilm-related materials, besides being important for removing toxic substances from the bacteria. Some efflux pump inhibitors (EPIs) have been shown to both prevent biofilm formation and sensitize the bacteria to antibiotics, suggesting a relationship between these processes. Additionally, QS inhibitors or quenchers may affect antibiotic susceptibility. Thus, targeting elements that regulate QS and biofilm formation might be a promising approach to combat antibiotic-resistant biofilm-related bacterial infections.

Biological and clinical significance of quorum sensing alkylquinolones: current analytical and bioanalytical methods for their quantification

Quorum sensing (QS) is a sophisticated bacterial communication system which plays a key role in the virulence and biofilm formation of many pathogens. The Pseudomonas aeruginosa QS network consists of four sets of connected systems (las, rlh, pqs and iqs) hierarchically organized. The pqs system involves characteristic autoinducers (AI), most of them sharing an alkylquinolone (AQ) structure, and is able to carry out several relevant biological functions besides its main signalling activity. Their role in bacterial physiology and pathogenicity has been widely studied. Indeed, the presence of these metabolites in several body fluids and infected tissues has pointed to their potential value as biomarkers of infection. In this review, we summarize the most recent findings about the biological implications and the clinical significance of the main P. aeruginosa AQs. These findings have encouraged the development of analytical and bioanalytical techniques addressed to assess the role of these metabolites in bacterial growth and survival, during pathogenesis or as biomarkers of infections. The availability of highly sensitive reliable analytical methods suitable for clinical analysis would allow getting knowledge about pathogenesis and disease prognosis or progression, supporting clinicians on the decision-making process for the management of these infections and guiding them on the application of more effective and appropriate treatments. The benefits from the implementation of the point-of-care (PoC)-type testing in infectious disease diagnostics, which are seen to improve patient outcomes by promoting earlier therapeutic interventions, are also discussed.

Role of epithelial-endothelial cell interaction in the pathogenesis of SARS-CoV-2 infection

Background

The coronavirus disease 2019 (COVID-19) pandemic caused by the novel severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) continues to threaten public health globally. Patients with severe COVID-19 disease progress to acute respiratory distress syndrome, with respiratory and multiple organ failure. It is believed that dysregulated production of proinflammatory cytokines and endothelial dysfunction contribute to the pathogenesis of severe diseases. However, the mechanisms of SARS-CoV-2 pathogenesis and the role of endothelial cells are poorly understood.

Methods

Well-differentiated human airway epithelial cells were used to explore cytokine and chemokine production after SARS-CoV-2 infection. We measured the susceptibility to infection, immune response, and expression of adhesion molecules in human pulmonary microvascular endothelial cells (HPMVECs) exposed to conditioned medium from infected epithelial cells. The effect of imatinib on HPMVECs exposed to conditioned medium was evaluated.

Results

We demonstrated the production of interleukin-6, interferon gamma-induced protein-10, and monocyte chemoattractant protein-1 from the infected human airway cells after infection with SARS-CoV-2. Although HPMVECs did not support productive replication of SARS-CoV-2, treatment of HPMVECs with conditioned medium collected from infected airway cells induced an upregulation of proinflammatory cytokines, chemokines, and vascular adhesion molecules. Imatinib inhibited the upregulation of these cytokines, chemokines, and adhesion molecules in HPMVECs treated with conditioned medium.

Conclusions

We evaluated the role of endothelial cells in the development of clinical disease caused by SARS-CoV-2 and the importance of endothelial cell–epithelial cell interaction in the pathogenesis of human COVID-19 diseases.

Novel quinazolinone inhibitors of the Pseudomonas aeruginosa quorum sensing transcriptional regulator PqsR

Rising numbers of cases of multidrug- and extensively drug-resistant Pseudomonas aeruginosa over recent years have created an urgent need for novel therapeutic approaches to cure potentially fatal infections. One such approach is virulence attenuation where anti-virulence compounds, designed to reduce pathogenicity without affording bactericidal effects, are employed to treat infections. P. aeruginosa uses the pqs quorum sensing (QS) system, to coordinate the expression of a large number of virulence determinants as well as bacterial-host interactions and hence represents an excellent anti-virulence target. We report the synthesis and identification of a new series of thiazole-containing quinazolinones capable of inhibiting PqsR, the transcriptional regulator of the pqs QS system. The compounds demonstrated high potency (IC50 < 300 nM) in a whole-cell assay, using a mCTX:PpqsA-lux-based bioreporter for the P. aeruginosa PAO1-L and PA14 strains. Structural evaluation defined the binding modes of four analogues in the ligand-binding domain of PqsR through X-ray crystallography. Further work showed the ability of 6-chloro-3((2-pentylthiazol-4-yl)methyl)quinazolin-4(3H)-one (18) and 6-chloro-3((2-hexylthiazol-4-yl)methyl)quinazolin-4(3H)-one (19) to attenuate production of the PqsR-regulated virulence factor pyocyanin. Compounds 18 and 19 showed a low cytotoxic profile in the A549 human epithelial lung cell line making them suitable candidates for further pre-clinical evaluation.

Plant Phenolics and Phenolic-Enriched Extracts as Antimicrobial Agents against Food-Contaminating Microorganisms

Phenolic compounds and extracts with bioactive properties can be obtained from many kinds of plant materials. These natural substances have gained attention in the food research as possible growth inhibitors of foodborne pathogenic and spoilage bacteria. Many phenolic-enriched plant extracts and individual phenolics have promising anti-quorum sensing potential as well and can suppress the biofilm formation and toxin production of food-related pathogens. Various studies have shown that plant phenolics can substitute or support the activity of synthetic food preservatives and disinfectants, which, by the way, can provoke serious concerns in consumers. In this review, we will provide a brief insight into the bioactive properties, i.e., the antimicrobial, anti-quorum sensing, anti-biofilm and anti-enterotoxin activities, of plant phenolic extracts and compounds, with special attention to pathogen microorganisms that have food relation. Carbohydrase aided applications to improve the antimicrobial properties of phenolic extracts are also discussed.

The third quorum-sensing system of Pseudomonas aeruginosa: Pseudomonas quinolone signal and the enigmatic PqsE protein

Pseudomonas aeruginosa is an opportunistic pathogen that produces several virulence factors such as lectin A, pyocyanin, elastase and rhamnolipids. These compounds are controlled transcriptionally by three quorum-sensing circuits, two based on the synthesis and detection of N-acyl-homoserine-lactone termed the Las and Rhl system and a third system named the Pseudomonas quinolone signal (PQS) system, which is responsible for generating 2-alkyl-4(1 h)-quinolones (AQs). The transcriptional regulator called PqsR binds to the promoter of pqsABCDE in the presence of PQS or HHQ creating a positive feedback-loop. PqsE, encoded in the operon for AQ synthesis, is a crucial protein for pyocyanin production, activating the Rhl system by a still not fully understood mechanism. In turn, the regulation of the PQS system is modulated by Las and Rhl systems, which act positively and negatively, respectively. This review focuses on the PQS system, from its discovery to its role in Pseudomonas pathogenesis, such as iron depletion and pyocyanin synthesis that involves the PqsE protein - an intriguing player of this system.