The role of individual exopolysaccharides in antibiotic tolerance of Pseudomonas aeruginosa aggregates

Burkholderia cenocepacia and Pseudomonas aeruginosa in dual-species models: Insights into population distribution, antibiotic susceptibility, and virulence

Multispecies biofilms are communities composed of different microorganisms embedded in an auto-synthesized polymeric matrix. Pseudomonas aeruginosa and Burkholderia cenocepacia are two multidrug-resistant and biofilm-forming opportunistic pathogens often found in the lungs of people living with cystic fibrosis. In this context, planktonic, static, and dynamic biofilms and in vivo models of both species were optimized in this work to understand their population dynamics, disposition, virulence, and antibiotic susceptibility. From the coculture models optimized in this work, we determined that B. cenocepacia grows in a clustered, aggregative manner at the bottom layers of biofilms, in close contact with P. aeruginosa, that tends to occupy the top layers. Their coexistence increases virulence-related gene expression in both species at early stages of coinfection and in in vivo models, while there was a general downregulation of virulence-related genes after longer coexistence periods as they eventually reach a non-competitive stage during chronic infections. When evaluating antimicrobial susceptibility, a decrease of antimicrobial tolerance was observed in both species when co-cultured. These findings shed light on the differential behavior of P. aeruginosa and B. cenocepacia in dual-species systems, stressing the relevance of multispecies studies in the clinical context.

Emerging strategies to target virulence in Pseudomonas aeruginosa respiratory infections

Pseudomonas aeruginosa is an opportunistic pathogen that is responsible for infections in people living with chronic respiratory conditions, such as cystic fibrosis (CF) and non-CF bronchiectasis (NCFB). Traditionally, in people with chronic respiratory disorders, P. aeruginosa infection has been managed with a combination of inhaled and intravenous antibiotic therapies. However, due in part to the prolonged use of antibiotics in these people, the emergence of multi-drug resistant P. aeruginosa strains is a growing concern. The development of anti-virulence therapeutics may provide a new means of treating P. aeruginosa lung infections whilst also combatting the AMR crisis, as these agents are presumed to exert reduced pressure for the emergence of drug resistance as compared to antibiotics. However, the pipeline for developing anti-virulence therapeutics is poorly defined, and it is currently unclear as to whether in vivo and in vitro models effectively replicate the complex pulmonary environment sufficiently to enable development and testing of such therapies for future clinical use. Here, we discuss potential targets for P. aeruginosa anti-virulence therapeutics and the effectiveness of the current models used to study them. Focus is given to the difficulty of replicating the virulence gene expression patterns of P. aeruginosa in the CF and NCFB lung under laboratory conditions and to the challenges this poses for anti-virulence therapeutic development.

The role of exopolysaccharides Psl and Pel in resistance of Pseudomonas aeruginosa to the oxidative stressors sodium hypochlorite and hydrogen peroxide

Pseudomonas aeruginosa is well-known for its antimicrobial resistance and the ability to survive in harsh environmental conditions due to an abundance of resistance mechanisms, including the formation of biofilms and the production of exopolysaccharides. Exopolysaccharides are among the major components of the extracellular matrix in biofilms and aggregates of P. aeruginosa. Although their contribution to antibiotic resistance has been previously shown, their roles in resistance to oxidative stressors remain largely elusive. Here, we studied the function of the exopolysaccharides Psl and Pel in the resistance of P. aeruginosa to the commonly used disinfectants and strong oxidizing agents NaOCl and H2O2. We observed that the simultaneous inactivation of Psl and Pel in P. aeruginosa PAO1 mutant strain ∆pslA pelF resulted in a significant increase in susceptibility to both NaOCl and H2O2. Further analyses revealed that Pel is more important for oxidative stress resistance in P. aeruginosa and that the form of Pel (i.e., cell-associated or cell-free) did not affect NaOCl susceptibility. Additionally, we show that Psl/Pel-negative strains are protected against oxidative stress in co-culture biofilms with P. aeruginosa PAO1 WT. Taken together, our results demonstrate that the EPS matrix and, more specifically, Pel exhibit protective functions against oxidative stressors such as NaOCl and H2O2 in P. aeruginosa.

IMPORTANCE

Biofilms are microbial communities of cells embedded in a self-produced polymeric matrix composed of polysaccharides, proteins, lipids, and extracellular DNA. Biofilm bacteria have been shown to possess unique characteristics, including increased stress resistance and higher antimicrobial tolerance, leading to failures in bacterial eradication during chronic infections or in technical settings, including drinking and wastewater industries. Previous studies have shown that in addition to conferring structure and stability to biofilms, the polysaccharides Psl and Pel are also involved in antibiotic resistance. This work provides evidence that these biofilm matrix components also contribute to the resistance of Pseudomonas aeruginosa to oxidative stressors including the widely used disinfectant NaOCl. Understanding the mechanisms by which bacteria escape antimicrobial agents, including strong oxidants, is urgently needed in the fight against antimicrobial resistance and will help in developing new strategies to eliminate resistant strains in any environmental, industrial, and clinical setting.

A Simple Allelic Exchange Method for Efficient Seamless Knockout of Up to 34-kbp-Long Gene Cassettes in Pseudomonas

Gene knockout is a widely used technique for engineering bacterial genomes, investigating the roles of genes in metabolism, and conferring biological characteristics. Herein, we developed a rapid, efficient, and simple method for the knockout of long gene cassettes in Pseudomonas spp., based on a traditional allelic exchange strategy. The upstream and downstream sequences of the target gene cluster to be deleted were amplified using primers with 5'-end sequences identical to the multiple cloning sites of a suicide plasmid (mutant allele insert vector). The sequences were then fused with the linearized suicide plasmid in one step via seamless cloning. The resulting allelic exchange vector (recombinant plasmid) was introduced from the donor strain (Escherichia coli SM 10) into recipient cells (Pseudomonas putida, P. composti, and P. khazarica) via conjugation. Single-crossover merodiploids (integrates the vector into host chromosome by homologous recombination) were screened based on antibiotic resistance conferred by the plasmid, and double-crossover haploids (deleting the target gene clusters and inserted alien plasmid backbone) were selected using sucrose-mediated counterselection. Unlike other approaches, the method described herein introduces no selective marker genes into the genomes of the knockout mutants. Using our method, we successfully deleted polysaccharide-encoding gene clusters in P. putida, P. composti, and P. khazarica and generated four mutants with single-gene cassette deletions up to 18 kbp and one mutant with double-gene cassette deletion of approximately 34 kbp. Collectively, our results indicate that this method is ideal for the deletion of long genetic sequences, yielding seamless mutants of various Pseudomonas spp.

Novel biostimulant bacterial exopolysaccharides production via solid-state fermentation as a valorisation strategy for agri-food waste

Bacterial exopolysaccharides (EPS) are extracellular polymer-based substances recently defined as potential plant biostimulants, as they can increase nutrient uptake, water retention, and resistance to abiotic stress. As sugar-based substances, the bacteria producing them need to grow in a sugar-rich substrate. Hence, some agri-food by-products could be used as suitable carbon sources for EPS production as a cost-effective and more sustainable alternative to conventional substrates. Thus, this study aimed to produce EPS from specific bacterial strains through solid-state fermentation (SSF) using agri-food waste as a low-cost substrate. Six residues and five bacterial strains were tested in a lab-scale SSF system. From the assessed substrate-strain combinations, Burkholderia cepacia with ginger juice waste (GJW) resulted in the most promising considering several process parameters (EPS production, cumulative oxygen consumption, biomass growth, reducing sugars consumption). Also, dynamic monitoring of the system allowed for establishing 5 days as a suitable fermentation time. Then, using response surface methodology (Box-Behnken design), the process was optimised based on airflow rate (AF), inoculum size (IS), and micronutrient concentration (MN). In this stage, the best conditions found were at 0.049 (± 0.014) L h-1 per gram of dry matter (DM) for AF, 8.4 (± 0.9) E + 09 CFU g-1 DM for IS, and 0.07 (± 0.01) mL g-1 DM for MN, reaching up to 71.1 (± 3.2) mg crude EPS g-1 DM. Results show the potential of this approach to provide a new perspective on the value chain for the agri-food industry by introducing it to a circular economy framework.

Evaluation of Minimum Inhibitory Concentration of Heavy Metals Contained in Packaging Material Digest on Prominent Gut Microbiota

Several scientific investigations have revealed that the leaching of metals from packaging material into the packed food is an unavoidable process. Hence, this study is aimed at investigating the effect of leached heavy metals from food packing materials on normal human gut flora. We analysed the effect of vanadium, arsenic, cadmium, and mercury present in digested packaging materials (DPM) on standard strains of Escherichia coli ATCC 25923, Pseudomonas aeruginosa ATCC 27853, Klebsiella pneumoniae ATCC 70063, and Enterococcus faecalis ATCC 29212. The minimum inhibitory concentration (MIC) of laboratory-grade heavy metal salts and heavy metals present in DPM was determined by the agar dilution method. For all four bacteria, the MIC of cadmium and arsenic in the DPM was 7 μg/ml and 1.6 μg/ml, respectively. The MIC of mercury in DPM was 1.6 μg/ml for E. coli, K. pneumoniae, and E. faecalis and 1.4 μg/ml for P. aeruginosa. MIC of vanadium for E. coli, P. aeruginosa, and E. faecalis was 2.2 μg/ml, and for K. pneumoniae was 2.0 μg/ml. The difference in MICs of heavy metals in DPMs and heavy metal salts was not statistically significant. MICs were within CODEX-specified permissible levels. Though heavy metals in packaging material have not shown a deleterious effect on representative human gut flora, there is scope to study their effect on the gut microbiome. Thus, understanding the risk of heavy metal ingestion through unknown sources and avoiding any possible ingestion is crucial to preventing chronic diseases.

Implication of Mn-cofactored superoxide dismutase in the tolerance of swarmer Pseudomonas aeruginosa to polymixin, ciprofloxacin and meropenem antibiotics

The protective role of superoxide dismutase (Sod) against oxidative stress, resulting from the common antibiotic pathway of action, has been studied in the wild type and mutant strains of swarmer Pseudomonas aeruginosa, lacking Cytosolic Mn-Sod (sodM), Fe-Sod (sodB) or both Sods (sodMB).Our results showed that inactivation of sodB genes leads to significant motility defects and tolerance to meropenem. This resistance is correlated with a greater membrane unsaturation as well as an effective intervention of Mn-Sod isoform, in antibiotic tolerance.Moreover, loss of Mn-Sod in sodM mutant, leads to polymixin intolerance and is correlated with membrane unsaturation. Effectivelty, sodM mutant showed an enhanced swarming motility and a conserved rhamnolipid production. Whereas, in the double mutant sodMB, ciprofloxacin tolerance would be linked to an increase in the percentage of saturated fatty acids in the membrane, even in the absence of superoxide dismutase activity.The overall results showed that Mn-Sod has a protective role in the tolerance to antibiotics, in swarmer P.aeruginosa strain. It has been further shown that Sod intervention in antibiotic tolerance is through change in membrane fatty acid composition.

The non-attached biofilm aggregate

Biofilms have conventionally been perceived as dense bacterial masses on surfaces, following the five-step model of development. Initial biofilm research focused on surface-attached formations, but detached aggregates have received increasing attention in the past decade due to their pivotal role in chronic infections. Understanding their nature sparked fervent discussions in biofilm conferences and scientific literature. This review consolidates current insights on non-attached aggregates, offering examples of their occurrence in nature and diseases. We discuss their formation and dispersion mechanisms, resilience to antibiotics and immune-responses, drawing parallels to surface-attached biofilms. Moreover, we outline available in vitro models for studying non-attached aggregates.