Involvement of MexS and MexEF-OprN in Resistance to Toxic Ion Chelators in Pseudomonas putida KT2440

A transcription factor-based bacterial biosensor system and its application for on-site detection of explosives

Detecting unexploded landmines is critical due to the environmental pollution and potential humanitarian risks caused by buried landmines. Therefore, this study focused on developing a biosensor system capable of detecting explosives safely and efficiently. A novel transcription factor-based Escherichia coli biosensor was designed to detect 1,3-dinitrobenzene (1,3-DNB). The MexT transcription factor from Pseudomonas putida (P. putida) was identified as the fundamental sensing element in this biosensor. The study found that MexT positively regulated the transcription of PP_2827 by binding to the bidirectional promoter region between them, and significantly enhanced the expression of downstream genes under the condition of 1,3-DNB. The MexT-based biosensor for 1,3-DNB was developed by adopting different combinations of the mexT gene and promoters. The optimized biosensor demonstrated adequate sensitivity for detecting 0.1 μg/mL of 1,3-DNB in a liquid solution with satisfactory specificity and long-term stability. Subsequently, the MexT-based biosensor was integrated into a detection device to simulate the in-field exploration of explosives. The system exhibited a detection sensitivity of 0.5 mg/kg for 1,3-DNB in the sand, and realized the detection of on-site and large-scale area and the location of buried 1,3-DNB under the soil. The study provided a novel transcription factor-based bacterial biosensor and a complete system (China Earth Eye, CEE) for sensitive detection of 1,3-DNB. The good performance of this biosensor system can facilitate the development of accurate, on-site, and high-efficient exploration of explosives in real extensive minefields. Moreover, this 1,3-DNB biosensor can be complementary to the 2,4-DNT biosensor reported before, demonstrating its potential applications in military situations.

The RND efflux system ParXY affects siderophore secretion in Pseudomonas putida KT2440

IMPORTANCE

Gram-negative bacteria from the Pseudomonas group are survivors in various environmental niches. For example, the bacteria secrete siderophores to capture ferric ions under deficiency conditions. Tripartite efflux systems are involved in the secretion of siderophores, which are also important for antibiotic resistance. For one of these efflux systems, the resistance-nodulation-cell division transporter ParXY from the model organism Pseudomonas putida KT2440, we show that it influences the secretion of the siderophore pyoverdine in addition to its already known involvement in antibiotic resistance. Phenotypically, its role in pyoverdine secretion is only apparent when other pyoverdine secretion systems are inactive. The results confirm that the different tripartite efflux systems have overlapping substrate specificities and can at least partially functionally substitute for each other, especially in important physiological activities such as supplying the cell with iron ions. This fact must be taken into account when developing specific inhibitors for tripartite efflux systems.

The Role of MexCD-OprJ and MexEF-OprN Efflux Systems in the Multiple Antibiotic Resistance of Pseudomonas aeruginosa Isolated from Clinical Samples

Increasing antimicrobial resistance and the development of multi-drug resistant (MDR) Pseudomonas aeruginosa is dependent on the expression of efflux pumps. This study aimed to investigate the role of overexpression of MexCD-OprJ and MexEF-OprN efflux pumps in reduced susceptibility to antimicrobial agents among P. aeruginosa strains. Totally, 100 clinical isolates of P. aeruginosa were collected from patients and the strains were identified by standard diagnostic tests. The MDR isolates were detected using the disk agar diffusion method. The expression levels of MexCD-OprJ and MexEF-OprN efflux pumps were evaluated by the real-time PCR. Forty-one isolates showed MDR phenotype, while piperacillin-tazobactam and levofloxacin were the most- and least-effective antibiotics, respectively. Also, all 41 MDR isolates showed a more than tenfold increase in the expression of mexD and mexF genes. In this study, a significant relationship was observed between the rate of antibiotic resistance, the emergence of MDR strains, and increasing the expression levels of MexEF-OprN and MexCD-OprJ efflux pumps (P < 0.05). Efflux systems mediated resistance was a noteworthy mechanism causative to multidrug resistance in P. aeruginosa clinical isolates. The study results demonstrated mexE and mexF overexpression as the primary mechanism conferring in the emergence of MDR phenotypes among P. aeruginosa strains. In addition, we also show that piperacillin/tazobactam exhibited a stronger ability in the management of infections caused by MDR P. aeruginosa in this area.

Towards robust Pseudomonas cell factories to harbour novel biosynthetic pathways

Biotechnological production in bacteria enables access to numerous valuable chemical compounds. Nowadays, advanced molecular genetic toolsets, enzyme engineering as well as the combinatorial use of biocatalysts, pathways, and circuits even bring new-to-nature compounds within reach. However, the associated substrates and biosynthetic products often cause severe chemical stress to the bacterial hosts. Species of the Pseudomonas clade thus represent especially valuable chassis as they are endowed with multiple stress response mechanisms, which allow them to cope with a variety of harmful chemicals. A built-in cell envelope stress response enables fast adaptations that sustain membrane integrity under adverse conditions. Further, effective export machineries can prevent intracellular accumulation of diverse harmful compounds. Finally, toxic chemicals such as reactive aldehydes can be eliminated by oxidation and stress-induced damage can be recovered. Exploiting and engineering these features will be essential to support an effective production of natural compounds and new chemicals. In this article, we therefore discuss major resistance strategies of Pseudomonads along with approaches pursued for their targeted exploitation and engineering in a biotechnological context. We further highlight strategies for the identification of yet unknown tolerance-associated genes and their utilisation for engineering next-generation chassis and finally discuss effective measures for pathway fine-tuning to establish stable cell factories for the effective production of natural compounds and novel biochemicals.

Involvement of the MxtR/ErdR (CrbS/CrbR) Two-Component System in Acetate Metabolism in Pseudomonas putida KT2440

MxtR/ErdR (also called CrbS/CrbR) is a two-component system previously identified as important for the utilization of acetate in Vibrio cholerae and some Pseudomonas species. In addition, evidence has been found in Pseudomonas aeruginosa for a role in regulating the synthesis and expression, respectively, of virulence factors such as siderophores and RND transporters. In this context, we investigated the physiological role of the MxtR/ErdR system in the soil bacterium Pseudomonas putida KT2440. To that end, mxtR and erdR were individually deleted and the ability of the resulting mutants to metabolize different carbon sources was analyzed in comparison to wild type. We also assessed the impact of the deletions on siderophore production, expression of mexEF-oprN (RND transporter), and the biocontrol properties of the strain. Furthermore, the MxtR/ErdR-dependent expression of putative target genes and binding of ErdR to respective promoter regions were analyzed. Our results indicated that the MxtR/ErdR system is active and essential for acetate utilization in P. putida KT2440. Expression of scpC, pp_0354, and acsA-I was stimulated by acetate, while direct interactions of ErdR with the promoter regions of the genes scpC, pp_0354, and actP-I were demonstrated by an electromobility shift assay. Finally, our results suggested that MxtR/ErdR is neither involved in regulating siderophore production nor the expression of mexEF-oprN in P. putida KT2440 under the conditions tested.