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.

Dual-channel glycolysis balances cofactor supply for l-homoserine biosynthesis in Corynebacterium glutamicum

l-Homoserine is an important platform compound that is widely used to produce many valuable bio-based products, but production of l-homoserine in Corynebacterium glutamicum remains low. In this study, an efficient l-homoserine-producing strain was constructed. Native pentose phosphate pathway (PPP) was enhanced and heterologous Entner-Doudoroff (ED) pathway was carefully introduced into l-homoserine-producing strain, which increased the l-homoserine titer. Coexpression of NADH-dependent aspartate-4-semialdehyde dehydrogenase and aspartate dehydrogenase could increase the titer from 11.3 to 13.3 g/L. Next, NADP⁺-dependent glyceraldehyde-3-phosphate dehydrogenase (NADP-GPD) was coexpressed with that of NAD⁺-dependent (NAD-GPD) to construct dual-channel glycolysis for balance of intracellular cofactors, which increased the l-homoserine titer by 48.6 % to 16.8 g/L. Finally, engineered strain Cg18-1 accumulated 63.5 g/L l-homoserine after 96 h in a 5 L bioreactor, the highest titer reported to date for C. glutamicum. This dual-channel glycolysis strategy provides a reference for automatic cofactor regulation to promote efficient biosynthesis of other target products.

Glucose Oxidation to Pyruvate Is Not Essential for Brucella suis Biovar 5 Virulence in the Mouse Model

Brucella species cause brucellosis, a worldwide extended zoonosis. The brucellae are related to free-living and plant-associated α2-Proteobacteria and, since they multiply within host cells, their metabolism probably reflects this adaptation. To investigate this, we used the rodent-associated Brucella suis biovar 5, which in contrast to the ruminant-associated Brucella abortus and Brucella melitensis and other B. suis biovars, is fast-growing and conserves the ancestral Entner-Doudoroff pathway (EDP) present in the plant-associated relatives. We constructed mutants in Edd (glucose-6-phosphate dehydratase; first EDP step), PpdK (pyruvate phosphate dikinase; phosphoenolpyruvate ⇌ pyruvate), and Pyk (pyruvate kinase; phosphoenolpyruvate → pyruvate). In a chemically defined medium with glucose as the only C source, the Edd mutant showed reduced growth rates and the triple Edd-PpdK-Pyk mutant did not grow. Moreover, the triple mutant was also unable to grow on ribose or xylose. Therefore, B. suis biovar 5 sugar catabolism proceeds through both the Pentose Phosphate shunt and EDP, and EDP absence and exclusive use of the shunt could explain at least in part the comparatively reduced growth rates of B. melitensis and B. abortus. The triple Edd-PpdK-Pyk mutant was not attenuated in mice. Thus, although an anabolic use is likely, this suggests that hexose/pentose catabolism to pyruvate is not essential for B. suis biovar 5 multiplication within host cells, a hypothesis consistent with the lack of classical glycolysis in all Brucella species and of EDP in B. melitensis and B. abortus. These results and those of previous works suggest that within cells, the brucellae use mostly 3 and 4 C substrates fed into anaplerotic pathways and only a limited supply of 5 and 6 C sugars, thus favoring the EDP loss observed in some species.

Characterization of Pectobacterium carotovorum proteins differentially expressed during infection of Zantedeschia elliotiana in vivo and in vitro which are essential for virulence

The identification of phytopathogen proteins that are differentially expressed during the course of the establishment of an infection is important to better understand the infection process. In vitro approaches, using plant extracts added to culture medium, have been used to identify such proteins, but the biological relevance of these findings for in planta infection are often uncertain until confirmed by in vivo studies. Here, we compared the proteins of Pectobacterium carotovorum ssp. carotovorum strain PccS1 differentially expressed in Luria-Bertani medium supplemented with extracts of the ornamental plant Zantedeschia elliotiana cultivar 'Black Magic' (in vitro) and in plant tissues (in vivo) by two-dimensional electrophoresis coupled with mass spectrometry. A total of 53 differentially expressed proteins (>1.5-fold) were identified (up-regulated or down-regulated in vitro, in vivo or both). Proteins that exhibited increased expression in vivo but not in vitro, or in both conditions, were identified, and deletions were made in a number of genes encoding these proteins, four of which (clpP, mreB, flgK and eda) led to a loss of virulence on Z. elliotiana, although clpP and mreB were later also shown to be reduced in growth in rich and minimal media. Although clpP, flgK and mreB have previously been reported as playing a role in virulence in plants, this is the first report of such a role for eda, which encodes 2-keto-3-deoxy-6-phosphogluconate (KDPG) aldolase, a key enzyme in Entner-Doudoroff metabolism. The results highlight the value of undertaking in vivo as well as in vitro approaches for the identification of new bacterial virulence factors.

Metabolic flux analyses of Pseudomonas aeruginosa cystic fibrosis isolates

Pseudomonas aeruginosa is a metabolically versatile wide-ranging opportunistic pathogen. In humans P. aeruginosa causes infections of the skin, urinary tract, blood, and the lungs of Cystic Fibrosis patients. In addition, P. aeruginosa's broad environmental distribution, relatedness to biotechnologically useful species, and ability to form biofilms have made it the focus of considerable interest. We used ¹³C metabolic flux analysis (MFA) and flux balance analysis to understand energy and redox production and consumption and to explore the metabolic phenotypes of one reference strain and five strains isolated from the lungs of cystic fibrosis patients. Our results highlight the importance of the oxidative pentose phosphate and Entner-Doudoroff pathways in P. aeruginosa growth. Among clinical strains we report two divergent metabolic strategies and identify changes between genetically related strains that have emerged during a chronic infection of the same patient. MFA revealed that the magnitude of fluxes through the glyoxylate cycle correlates with growth rates.

Pseudomonas putida KT2440 Strain Metabolizes Glucose through a Cycle Formed by Enzymes of the Entner-Doudoroff, Embden-Meyerhof-Parnas, and Pentose Phosphate Pathways

The soil bacterium Pseudomonas putida KT2440 lacks a functional Embden-Meyerhof-Parnas (EMP) pathway, and glycolysis is known to proceed almost exclusively through the Entner-Doudoroff (ED) route. To investigate the raison d'être of this metabolic arrangement, the distribution of periplasmic and cytoplasmic carbon fluxes was studied in glucose cultures of this bacterium by using (13)C-labeled substrates, combined with quantitative physiology experiments, metabolite quantification, and in vitro enzymatic assays under both saturating and non-saturating, quasi in vivo conditions. Metabolic flux analysis demonstrated that 90% of the consumed sugar was converted into gluconate, entering central carbon metabolism as 6-phosphogluconate and further channeled into the ED pathway. Remarkably, about 10% of the triose phosphates were found to be recycled back to form hexose phosphates. This set of reactions merges activities belonging to the ED, the EMP (operating in a gluconeogenic fashion), and the pentose phosphate pathways to form an unforeseen metabolic architecture (EDEMP cycle). Determination of the NADPH balance revealed that the default metabolic state of P. putida KT2440 is characterized by a slight catabolic overproduction of reducing power. Cells growing on glucose thus run a biochemical cycle that favors NADPH formation. Because NADPH is required not only for anabolic functions but also for counteracting different types of environmental stress, such a cyclic operation may contribute to the physiological heftiness of this bacterium in its natural habitats.

Role of central metabolism in the osmoadaptation of the halophilic bacterium Chromohalobacter salexigens

Bacterial osmoadaptation involves the cytoplasmic accumulation of compatible solutes to counteract extracellular osmolarity. The halophilic and highly halotolerant bacterium Chromohalobacter salexigens is able to grow up to 3 m NaCl in a minimal medium due to the de novo synthesis of ectoines. This is an osmoregulated pathway that burdens central metabolic routes by quantitatively drawing off TCA cycle intermediaries. Consequently, metabolism in C. salexigens has adapted to support this biosynthetic route. Metabolism of C. salexigens is more efficient at high salinity than at low salinity, as reflected by lower glucose consumption, lower metabolite overflow, and higher biomass yield. At low salinity, by-products (mainly gluconate, pyruvate, and acetate) accumulate extracellularly. Using [1-(13)C]-, [2-(13)C]-, [6-(13)C]-, and [U-(13)C6]glucose as carbon sources, we were able to determine the main central metabolic pathways involved in ectoines biosynthesis from glucose. C. salexigens uses the Entner-Doudoroff pathway rather than the standard glycolytic pathway for glucose catabolism, and anaplerotic activity is high to replenish the TCA cycle with the intermediaries withdrawn for ectoines biosynthesis. Metabolic flux ratios at low and high salinity were similar, revealing a certain metabolic rigidity, probably due to its specialization to support high biosynthetic fluxes and partially explaining why metabolic yields are so highly affected by salinity. This work represents an important contribution to the elucidation of specific metabolic adaptations in compatible solute-accumulating halophilic bacteria.