Analysis of two quorum sensing-deficient isolates of Pseudomonas aeruginosa

Anti-Biofilm Strategies: A Focused Review on Innovative Approaches

Biofilm (BF) can give rise to systemic infections, prolonged hospitalization times, and, in the worst case, death. This review aims to provide an overview of recent strategies for the prevention and destruction of pathogenic BFs. First, the main phases of the life cycle of BF and maturation will be described to identify potential targets for anti-BF approaches. Then, an approach acting on bacterial adhesion, quorum sensing (QS), and the extracellular polymeric substance (EPS) matrix will be introduced and discussed. Finally, bacteriophage-mediated strategies will be presented as innovative approaches against BF inhibition/destruction.

Unveiling therapeutic efficacy of extract and multi-targeting phytocompounds from Christella dentata (Forssk.) Brownsey & Jermy against multidrug-resistant Pseudomonas aeruginosa

Christella dentata (Forssk.) Brownsey & Jermy has been commonly used in traditional medicinal practices but its effects on multi-drug-resistant (MDR) bacteria have remained unexplored. We aimed to assess the in vitro antibacterial potential of the ethanol extract of Christella dentata (EECD) against MDR Pseudomonas aeruginosa and to identify potential multi-targeting antibacterial phytocompounds through computer-aided drug design focusing on the LasR and LpxC proteins. PPS, FT-IR and GC-MS were used for profiling of the phytocompounds in EECD. The antimicrobial activity of EECD was assessed using in vitro agar well diffusion, disc diffusion, MIC and MBC. Computer-aided drug design was used to identify multi-targeting leads from GC-MS-annotated phytocompounds. EECD exhibited dose-dependent antibacterial activity and revealed the presence of 51 phytocompounds in GC-MS analysis. Among these, three phytocompounds; (2E,4E)-N-isobutylhexadeca-2,4-dienamide (CID 6442402), bicyclo[4.3.0]nonane, 2,2,6,7-tetramethyl-7-hydroxy- (CID 536446) and 1,4-diethylbenzene (CID 7734) were identified as promising antibacterial phytocompounds as they strongly bonded with LasR and LpxC. Of them, CID 536446 and CID 7734 exhibited multiple targeting abilities with LasR and LpxC. On further screening, both CID 536446 and CID 7734 exhibited favorable drug-able, pharmacokinetics and toxicity properties. Finally, molecular dynamics (MD) simulation proved the binding stability of bicyclo[4.3.0]nonane, 2,2,6,7-tetramethyl-7-hydroxy- and 1,4-diethylbenzene to active pockets of LasR and LpxC. The results of this study offer scientific validation for the traditional use of Christella dentata in bacterial infection-related diseases. It also suggests that bicyclo[4.3.0]nonane, 2,2,6,7-tetramethyl-7-hydroxy- and 1,4-diethylbenzene from Christella dentata might be responsible for the antibacterial activity and could act as phytopharmacological leads for the development of LasR and LpxC inhibitors against MDR P. aeruginosa.

Quorum quenching by a type IVA secretion system effector

Proteobacteria primarily utilize acyl-homoserine lactones (AHLs) as quorum-sensing signals for intra-/interspecies communication to control pathogen infections. Enzymatic degradation of AHL represents the major quorum-quenching mechanism that has been developed as a promising approach to prevent bacterial infections. Here we identified a novel quorum-quenching mechanism revealed by an effector of the type IVA secretion system (T4ASS) in bacterial interspecies competition. We found that the soil antifungal bacterium Lysobacter enzymogenes OH11 (OH11) could use T4ASS to deliver the effector protein Le1288 into the cytoplasm of another soil microbiome bacterium Pseudomonas fluorescens 2P24 (2P24). Le1288 did not degrade AHL, whereas its delivery to strain 2P24 significantly impaired AHL production through binding to the AHL synthase PcoI. Therefore, we defined Le1288 as LqqE1 (Lysobacter quorum-quenching effector 1). Formation of the LqqE1-PcoI complex enabled LqqE1 to block the ability of PcoI to recognize/bind S-adenosy-L-methionine, a substrate required for AHL synthesis. This LqqE1-triggered interspecies quorum-quenching in bacteria seemed to be of key ecological significance, as it conferred strain OH11 a better competitive advantage in killing strain 2P24 via cell-to-cell contact. This novel quorum-quenching also appeared to be adopted by other T4ASS-production bacteria. Our findings suggest a novel quorum-quenching that occurred naturally in bacterial interspecies interactions within the soil microbiome by effector translocation. Finally, we presented two case studies showing the application potential of LqqE1 to block AHL signaling in the human pathogen Pseudomonas aeruginosa and the plant pathogen Ralstonia solanacearum.

Design, Synthesis, and Biological Evaluation of N-Acyl-Homoserine Lactone Analogs of Quorum Sensing in Pseudomonas aeruginosa

Quorum sensing plays a necessary role in the production of virulence factors and the formation of biofilm on Pseudomonas aeruginosa. Thus, the development of inhibition of quorum sensing is one of the most promising methods to control bacterial infection and antibiotic resistance. In this work, nine novel AHL analogs were designed, synthesized, and evaluated as potential quorum sensing inhibitors. The results depicted that structural modifications have significant effects on quorum sensing inhibition activity of AHL molecules. Without inhibiting the growth of P. aeruginosa, 2-(4-bromophenyl)-N-(2-oxotetrapyridinefuran-3-yl) butanamide (compound no.10) showed the excellent performance in inhibiting biofilm formation and virulence factor production among all the compounds through robustly suppressing the expression of QS related genes. In a molecular docking study, compound no.10 exhibited a higher affinity toward LasR than other AHL analogs. In addition, compound no.10 also exhibits the best inhibition effect on virulence production in the Caenorhabditis elegans infection model.

Occurrence of Pseudomonas spp. in Raw Vegetables: Molecular and Phenotypical Analysis of Their Antimicrobial Resistance and Virulence-Related Traits

Pseudomonas is characterized by its great capacity to colonize different ecological niches, but also by its antimicrobial resistance and pathogenicity, causing human, animal, or plant diseases. Raw and undercooked food is a potential carrier of foodborne disease. The aim of this study was to determine the occurrence of Pseudomonas spp. among raw vegetables, analysing their antimicrobial resistance, virulence, and molecular typing. A total of 163 Pseudomonas spp. isolates (12 different species) were recovered from 77 of the 145 analysed samples (53.1%) and were classified into 139 different pulsed-field gel electrophoresis patterns. Low antimicrobial resistance levels, but one multidrug-resistant isolate, were found. Among the 37 recovered P. aeruginosa strains, 28 sequence-types and nine serotypes were detected. Eleven OprD patterns and an insertion sequence (ISPa1635) truncating the oprD gene of one imipenem-resistant strain were found. Ten virulotypes were observed, including four exoU-positive and thirty-one exoS-positive strains. The lasR gene was absent in three ST155 strains and was truncated by different insertion sequences (ISPre2, IS1411, and ISPst7) in other three strains. High biofilm, motility, pigment, elastase, and rhamnolipid production were detected. Our study demonstrated a low occurrence of P. aeruginosa (18%) and low antimicrobial resistance, but a high number of virulence-related traits in these P. aeruginosa strains, highlighting their pathological importance.

A Novel Use of Allopurinol as A Quorum-Sensing Inhibitor in Pseudomonas aeruginosa

Pseudomonas aeruginosa can cause a variety of healthcare-associated infections by its arsenal of virulence factors. Virulence factor production is largely controlled by the cell-to-cell communication system termed quorum sensing (QS). Targeting QS may be a good approach to inhibit the production of virulence factors and attenuate pathogenicity without exerting selective stress on bacterial growth. This will greatly reduce the emergence of resistant mutants. In this work, we investigated the anti-virulence and anti-QS activities of the FDA-approved drug allopurinol against the P. aeruginosa PAO1 strain. Allopurinol at 200 µg/mL (1/10 MIC) significantly decreased the production of the QS-controlled Chromobacterium violaceum CV026 violet pigment violacein and other P. aeruginosa QS-controlled virulence factors phenotypically. Furthermore, allopurinol reduced the infiltration of P. aeruginosa and leucocytes and diminished the congestion in the liver and kidney tissues of infected mice. In silico study showed that allopurinol could compete with the autoinducers on binding to the receptors LasR and RhlR by hydrogen bonding. On the molecular level, qRT-PCR proved that allopurinol showed a significant downregulating effect on all tested QS-encoding genes that regulate virulence factor production. In summary, allopurinol is a promising QS inhibitor that may be useful in the future treatment of P. aeruginosa infection.

Heterogeneity in biofilm formation and identification of biomarkers of strong biofilm formation among field isolates of Pseudomonas spp

The biofilm formation ability of a collection of thirty-three Pseudomonas spp. isolates from food processing facilities was investigated in order to find biomarkers of strong biofilm production, a characteristic that can determine persistence in food processing environments. The strains were classified according to the colony pigmentation on solid media as green, brown or not pigmented. The biofilm production on stainless steel and polystyrene was assessed by spectrometric determination of the fixed crystal violet, and the biofilm formed on glass, through confocal laser scanning microscopy. Besides, pyoverdine production, catalase activity, RpoS status and cellular hydrophobicity were also monitored. A significantly higher biofilm production level on stainless steel and polystyrene was observed for green-pigmented strains as compared to brown or not pigmented strains. The influence of iron availability on biofilm formation on stainless steel was studied through the addition of the iron scavenger 2,2-bipyridine resulting in a decrease of 40 % in biofilm formation for the not pigmented strains. For most of the potential biomarkers studied (i.e., pyoverdine production, catalase activity, cellular hydrophobicity), the phenotypic heterogeneity observed among strains was mainly dependent on the Pseudomonas species and no strong associations with the biofilm formation capacity were detected. However, the green colony pigmentation on solid media showed good potential as a biomarker of strong biofilm formation on stainless steel and polystyrene both in P. aeruginosa and Pseudomonas spp.

Strategies for Interfering With Bacterial Early Stage Biofilms

Biofilm-related bacteria show high resistance to antimicrobial treatments, posing a remarkable challenge to human health. Given bacterial dormancy and high expression of efflux pumps, persistent infections caused by mature biofilms are not easy to treat, thereby driving researchers toward the discovery of many anti-biofilm molecules that can intervene in early stage biofilms formation to inhibit further development and maturity. Compared with mature biofilms, early stage biofilms have fragile structures, vigorous metabolisms, and early attached bacteria are higher susceptibility to antimicrobials. Thus, removing biofilms at the early stage has evident advantages. Many reviews on anti-biofilm compounds that prevent biofilms formation have already been done, but most of them are based on compound classifications to introduce anti-biofilm effects. This review discusses the inhibitory effects of anti-biofilm compounds on early stage biofilms formation from the perspective of the mechanisms of action, including hindering reversible adhesion, reducing extracellular polymeric substances production, interfering in the quorum sensing, and modifying cyclic di-GMP. This information can be exploited further to help researchers in designing new molecules with anti-biofilm activity.

Transcriptomic Analysis of Pseudomonas aeruginosa Response to Pine Honey via RNA Sequencing Indicates Multiple Mechanisms of Antibacterial Activity

Pine honey is a unique type of honeydew honey produced exclusively in Eastern Mediterranean countries like Greece and Turkey. Although the antioxidant and anti-inflammatory properties of pine honey are well documented, few studies have investigated so far its antibacterial activity. This study investigates the antibacterial effects of pine honey against P. aeruginosa PA14 at the molecular level using a global transcriptome approach via RNA-sequencing. Pine honey treatment was applied at sub-inhibitory concentration and short exposure time (0.5× of minimum inhibitory concentration –MIC- for 45 min). Pine honey induced the differential expression (>two-fold change and p ≤ 0.05) of 463 genes, with 274 of them being down-regulated and 189 being up-regulated. Gene ontology (GO) analysis revealed that pine honey affected a wide range of biological processes (BP). The most affected down-regulated BP GO terms were oxidation-reduction process, transmembrane transport, proteolysis, signal transduction, biosynthetic process, phenazine biosynthetic process, bacterial chemotaxis, and antibiotic biosynthetic process. The up-regulated BP terms, affected by pine honey treatment, were those related to the regulation of DNA-templated transcription, siderophore transport, and phosphorylation. Pathway analysis revealed that pine honey treatment significantly affected two-component regulatory systems, ABC transporter systems, quorum sensing, bacterial chemotaxis, biofilm formation and SOS response. These data collectively indicate that multiple mechanisms of action are implicated in antibacterial activity exerted by pine honey against P. aeruginosa.

Caries-Associated Biosynthetic Gene Clusters in Streptococcus mutans

Early childhood caries (ECC) is a chronic disease affecting the oral health of children globally. This disease is multifactorial, but a primary factor is cariogenic microorganisms such as Streptococcus mutans. Biosynthetic gene clusters (BGCs) encode small molecules with diverse biological activities that influence the development of many microbial diseases, including caries. The purpose of this study was to identify BGCs in S. mutans from a high-caries risk study population using whole-genome sequencing and assess their association with ECC. Forty representative S. mutans isolates were selected for genome sequencing from a large-scale epidemiological study of oral microbiology and dental caries in children from a localized Alabama population. A total of 252 BGCs were identified using the antiSMASH BGC-mining tool. Three types of BGCs identified herein-butyrolactone-like, ladderane-like, and butyrolactone-ladderane-like hybrid (BL-BGC)-have not been reported in S. mutans. These 3 BGCs were cross-referenced against public transcriptomics data, and were found to be highly expressed in caries subjects. Furthermore, based on a polymerase chain reaction screening for core BL genes, 93% of children with BL-BGC had ECC. The role of BL-BGC was further investigated by examining cariogenic traits and strain fitness in a deletion mutant using in vitro biofilm models. Deletion of the BL-BGC significantly increased biofilm pH as compared to the parent strain, while other virulence and fitness properties remained unchanged. Intriguingly, BL-BGC containing strains produced more acid, a key cariogenic feature, and less biofilm than the model cariogenic strain S. mutans UA159, suggesting the importance of this BL-BGC in S. mutans-mediated cariogenesity. The structure of any BL-BGC derived metabolites, their functions, and mechanistic connection with acid production remain to be elucidated. Nevertheless, this study is the first to report the clinical significance of a BL-BGC in S. mutans. This study also highlights pangenomic diversity, which is likely to affect phenotype and virulence.

Anti-bacterial activity of baicalin against APEC through inhibition of quorum sensing and inflammatory responses

Avian pathogenic Escherichia coli (APEC), collectively known as causative agent of extraintestinal infections, is an important cause of morbidity and mortality in poultry. Currently, quorum sensing (QS), biofilm formation and virulence factors are considered as novel prospective targets for antimicrobial therapy to control APEC invasion. In addition, inflammatory responses are also served as the major pathological features of APEC invasion. This study was aimed to explore the effect of baicalin on APEC and APEC-induced inflammatory responses. After treatment with baicalin, we mainly examined the AI-2 secretion, biofilm formation, expression of virulence genes of APEC, and the levels of inflammatory cytokines, as well as the expression of NF-κB pathway. Our results showed that baicalin significantly inhibited the QS via decreasing the AI-2 secretion, biofilm formation, and the expression of virulence genes of APEC such as LsrB, LsrK, LuxS, pfs, H-NS, fimA, fimB, fyuA, csgA, csgB, and rpoS. Moreover, baicalin significantly attenuated the release of lactate dehydrogenase (LDH), and the adhesion of APEC to chicken type II pneumocytes to reduce cell damage. Furthermore, baicalin also inhibited the expression of pro-inflammatory cytokines and NF-κB activation. Thus, our data revealed that baicalin could interfere with the quorum sensing, biofilm formation and virulence genes expression to relieve the APEC pathogenicity. Additionally, baicalin decreased the inflammatory responses of chicken type II pneumocytes induced by APEC. Taken together, these data provide a novel potential pharmaco-therapeutic approach to chicken colibacillosis.

Diallyl Disulfide From Garlic Oil Inhibits Pseudomonas aeruginosa Quorum Sensing Systems and Corresponding Virulence Factors

Previously, we determined that diallyl disulfide (DADS) from garlic oil can inhibit Pseudomonas aeruginosa PAO1 pathogenic factors by inactivating the transcription of key genes from three quorum sensing (QS) systems (las, rhl, and pqs) based on the effects of DADS on growth, virulence factor production (elastase, pyocyanin, biofilm, and swarming motility), and RNA transcription (real-time q-PCR). To further investigate the mechanisms underlying the inhibition of the three P. aeruginosa QS systems by DADS, high-throughput RNA and proteome sequencing techniques were used to study differences in the transcriptional and proteome expression of P. aeruginosa PAO1 following treatment with DADS. The RNA-seq and proteomic data are available via NCBI Gene Expression Omnibus database with accession number GSE118801 and ProteomeXchange with identifier PXD011144, respectively. The experimental results indicated that all key genes of the three QS systems (las, rhl, and pqs) of P. aeruginosa PAO1 as well as the virulence factors (including exoprotease LasA, elastase LasB, lectin LecA and LecB, pyocyanin biosynthesis, and biofilm formation) regulated by these three QS systems were inhibited. This is consistent with our previous studies on the physiology, biochemistry, and RNA expression of P. aeruginosa treated with DADS. Additionally, our results also indicated that bacterial motility, chemotaxis, and two-component systems were inhibited by DADS treatment. All these changes abolish the sensitivity of P. aeruginosa PAO1 to environmental stimuli and cause the cells to be in a state of passivation. Further research is needed to determine how QS systems regulate these functions. Our findings could potentially contribute to the treatment and control of P. aeruginosa infection, virulence, and pathogenicity.