GDPmannose dehydrogenase and biosynthesis of alginate-like polysaccharide in a mucoid strain of Pseudomonas aeruginosa

The GDP-Mannose Dehydrogenase of Pseudomonas aeruginosa: An Old and New Target to Fight against Antibiotics Resistance of Mucoid Strains

Alginates play an important role in the resistance of mucoid strains of Pseudomonas aeruginosa to antibiotics, as well as their persistence by escaping the immune defense system. GDP-mannose dehydrogenase (GMD) is the key enzyme in alginate biosynthesis by catalyzing the irreversible double oxidation of GDP-mannose to GDP-mannuronate. GDP-mannose dehydrogenase purified from mucoid strains exhibits strong negative cooperativity for its substrate, the GDP-mannose, with a KM of 13 µM for the site of strong affinity and 3 mM for this weak of a binding. The presence of a nucleotide strongly associated with the enzyme was detected, confirming the fact that the substrate oxidation reaction takes place in two distinct steps, with the substrate blocked on the enzyme in a half-oxidation state in the form of a hemiacetal. As the GMD polypeptide has only one site for substrate binding, our results tend to confirm the fact that the enzyme functions in a dimer form. The GDP-mannose dehydrogenase inhibition strategy that we developed a few years ago, based on the synthesis of substrate analogs, has shown its effectiveness. The addition of an alkynyl radical on carbon 6 of the mannose grafted to an amino-sulfonyl-guanosine allows, at a concentration of 0.5 mM, to inhibit GMD by 90%. As we had previously shown the effectiveness of these analogs on the sensitivity of mucoid strains of Pseudomonas aeruginosa to aminoglycosides, this revives the interest in the synthesis of new inhibitors of GDP-mannose dehydrogenase.

Biosynthetic pathway of sugar nucleotides essential for welan gum production in Alcaligenes sp. CGMCC2428

Welan gum is a microbial polysaccharide produced by Alcaligenes sp. CGMCC2428 that has D-glucose, D-glucuronic acid, D-glucose, and L-rhamnose as the main structural unit. The biosynthetic pathway of sugar nucleotides essential for producing welan gum in this strain was established in the following ways: (1) the detection of the presence of several intermediates and key enzymes; (2) the analysis of the response upon addition of precursors to the culture medium; (3) the correlation of the activities between several key enzymes with the yields of welan gum. With addition of 200-microM glucose-6-phosphate and fructose-6-phosphate, the production of welan gum was improved by 18%. The activities of phosphoglucomutase, phosphomannose isomerase, UDP-glucose pyrophosphorylase, and dTDP-glucose pyrophosphorylase, correlated well with the yields of welan gum. According to these findings, the biosynthetic pathway was proposed to involve the metabolism of glucose via two discrete systems. The first involves conversion of glucose to glucose-6-phosphate, with further reactions producing glucose-1-phosphate and fructose-6-phosphate, which are metabolized to the nucleotide sugar precursors of welan gum. The second system involves metabolism of glucose to synthesize the basic structural skeleton of the cell via central metabolic pathways, including the Entner-Doudoroff pathway, the pentose phosphate pathway, and the tricarboxylic acid cycle.

Airway biofilms: implications for pathogenesis and therapy of respiratory tract infections

The differentiation of bacterial biofilms in the airway environment, the pathogenesis of airway biofilm, and possible therapeutic methods are discussed. Biofilm diseases that characteristically involve the respiratory system include cystic fibrosis (CF), diffuse panbronchiolitis (DPB), and bronchiectasia with Pseudomonas aeruginosa (P. aeruginosa) infection. There is evidence to suggest that almost all strains of P. aeruginosa have the genetic capacity to synthesize alginate, a main matrix of biofilms, when ecological conditions are unfavorable for their survival. The bacteria inside the mature biofilm show increased resistance to both antibacterials and phagocytic cells, express fewer virulence factors because of their stationary state of growth, and are less stimulatory to the mucosa because of the 'sandwich binding'. These factors facilitate both the colonization of bacteria and their extended survival even under unfavorable conditions. Since the biofilm limits colonization to a latent form, the clinical symptoms in this situation are unremarkable. However, the clinical progression of both CF and DPB proceeds in two characteristic directions. The first is an acute exacerbation caused by planktonic bacteria that have germinated from the biofilm. The second is a slow progression of disease that is induced by harmful immune reactions. The harmful reactions are mediated by alginate, which induces antigen antibody reactions around the airways, as well as formation of circulating immune complexes that are deposited on lung tissue. Furthermore, the highest titer of bacterial permeability increasing anti-neutrophil cytoplasmic autoantibodies (BPI-ANCA) is observed in association with highly impaired pulmonary function in patients with CF and DPB, as well as in patients with a lengthy period of colonization with P. aeruginosa. BPI-ANCA subsequently makes chronic airway infection even more intractable. The long-term use of 14- or 15-ring membered macrolides results in a favorable clinical outcome for patients with DPB and in some patients with CF. In the last 10 years, an increasing number of studies have reported secondary actions of macrolides that include effects on both airway and phagocytic cells, as well as an anti-biofilm activity. The 14- or 15-ring membered macrolides inhibit: (i) the alginate production from P. aeruginosa; (ii) the antibody reaction to alginate, which leads to a decrease in the immune complex formation; and (iii) the activation of the autoinducer 3-O-C12-homoserine lactone and subsequent expression of lasI and rhlI in quorum sensing systems in P. aeruginosa. These anti-biofilm actions of macrolides may represent their basic mechanisms of action on airway biofilm disease.

Influence of macrolides on mucoid alginate biosynthetic enzyme from Pseudomonas aeruginosa

OBJECTIVE: The long-term administration of erythromycin (EM), clarithromycin (CAM) or azithromycin (AZM) has generally resulted in a favorable outcome for patients with diffuse panbronchiolitis (DPB) infected with mucoid Pseudomonas aeruginosa. To elucidate the mechanism involved, the influence of macrolides on mucoid alginate production by P. aeruginosa was investigated in vitro. METHODS: The macrolides used in this study were EM with a 14-membered ring, AZM with a 15-membered ring, midecamycin (MDM) with a 16-membered ring, and CP-4305, which has had mycarose removed from MDM, The effects of macrolides on mucoid P. aeruginosa were investigated by quantitative assay of alginate production and inhibition of guanosine diphospho-D-mannose dehydrogenase activity. RESULTS: After incubation with EM, AZM and CP-4305, the structural material of P. aeruginosa biofilm was distorted, and the enzymatic activity of GDP-D-mannose dehydrogenase, the most important enzyme in mucoid alginate biosynthesis, was inhibited. However, these effects were not observed with the 16-membered macrolide MDM. CONCLUSIONS: The basic mechanism of clinical efficacy seen characteristically in 14- or 15-membered macrolides for patients with airway biofilm disease depends on the ability of such macrolides to inhibit alginate production by P. aeruginosa. Furthermore, this suggests that the inhibitory effect observed with 14-, 15- and 16-membered macrolides may depend on the sugar chain connected with the macrolide ring.

Microbial pathogenesis in cystic fibrosis: mucoid Pseudomonas aeruginosa and Burkholderia cepacia

Respiratory infections with Pseudomonas aeruginosa and Burkholderia cepacia play a major role in the pathogenesis of cystic fibrosis (CF). This review summarizes the latest advances in understanding host-pathogen interactions in CF with an emphasis on the role and control of conversion to mucoidy in P. aeruginosa, a phenomenon epitomizing the adaptation of this opportunistic pathogen to the chronic chourse of infection in CF, and on the innate resistance to antibiotics of B. cepacia, person-to-person spread, and sometimes rapidly fatal disease caused by this organism. While understanding the mechanism of conversion to mucoidy in P. aeruginosa has progressed to the point where this phenomenon has evolved into a model system for studying bacterial stress response in microbial pathogenesis, the more recent challenge with B. cepacia, which has emerged as a potent bona fide CF pathogen, is discussed in the context of clinical issues, taxonomy, transmission, and potential modes of pathogenicity.

A putative pathway for perosamine biosynthesis is the first function encoded within the rfb region of Vibrio cholerae O1

The first four genes (rfbA,B,D,E) of the rfb region of Vibrio cholerae O1 are predicted to encode the enzymes required for the biosynthesis of perosamine, which constitutes the backbone structure of the O-antigen of the lipopolysaccharide. Based on homology to known proteins/protein families, the following functions are predicted: RfbA, phosphomannose isomerase-guanosine diphosphomannose pyrophosphorylase; RfbB, phosphomanno-mutase; RfbD, oxido reductase and RfbE, perosamine synthetase (amino-transferase). Thus, perosamine is synthesized from fructose 6-phosphate via the intermediates mannose 6-phosphate by RfbA, to mannose 1-phosphate by RfbB, to GDP-mannose by RfbA, to GDP-4-keto-6-dideoxymannose by RfbD and to GDP-perosamine by RfbE. This final product would then serve as the substrate for the addition of the tetronate, which could then be polymerized into the O-antigen for transfer to the lipid A plus core oligosaccharide and export to the cell surface. The organization of these genes are such that one would expect them to be translationally coupled as part of the rfb operon. However, the absence of readily detectable promoter sequences suggests low levels of transcription, in line with other studies. The nucleotide sequence of these genes is absolutely conserved in the two isolates 569B (classical, Inaba) and O17 (El Tor, Ogawa) which were expected to show maximal sequence variation. This suggests very tight constraints on the micro-evolution within these sequences.

Alginate synthesis by Pseudomonas aeruginosa: a key pathogenic factor in chronic pulmonary infections of cystic fibrosis patients

Pulmonary infection by mucoid, alginate-producing Pseudomonas aeruginosa is the leading cause of mortality among patients suffering from cystic fibrosis. Alginate-producing P. aeruginosa is uniquely associated with the environment of the cystic fibrosis-affected lung, where alginate is believed to increase resistance to both the host immune system and antibiotic therapy. Recent evidence indicates that P. aeruginosa is most resistant to antibiotics when the infecting cells are present as a biofilm, as they appear to be in the lungs of cystic fibrosis patients. Inhibition of the protective alginate barrier with nontoxic compounds targeted against alginate biosynthetic and regulatory proteins may prove useful in eradicating P. aeruginosa from this environment. Our research has dealt with elucidating the biosynthetic pathway and regulatory mechanism(s) responsible for alginate synthesis by P. aeruginosa. This review summarizes reports on the role of alginate in cystic fibrosis-associated pulmonary infections caused by P. aeruginosa and provides details about the biosynthesis and regulation of this exopolysaccharide.

Microbiology of airway disease in patients with cystic fibrosis

Individuals with cystic fibrosis have abbreviated life spans primarily due to chronic airway infection. A limited number of types of organisms are responsible for these infections, with Staphylococcus aureus and Pseudomonas aeruginosa being of primary importance. In the pre-antibiotic era, greater than 90% of deaths due to infection were caused by S. aureus and death usually occurred in the first 2 years of life. With the advent of effective antistaphylococcal therapy, life spans increased and P. aeruginosa became the pathogen of primary importance. P. aeruginosa isolates recovered from patients with cystic fibrosis have a unique phenotypic characteristic referred to as "mucoid." The mucoid phenotype is due to the production of a mucoid exopolysaccharide. A mucoid exopolysaccharide is believed to play a central role in the establishment of chronic pseudomonal lung infection in these patients. A third organism, Pseudomonas cepacia, has recently been detected in the airways of older patients with cystic fibrosis and is associated with increased mortality. The virulence of P. cepacia is not understood, but the organism is extremely refractory to antimicrobial therapy. Other bacteria, including Haemophilus influenzae and members of the family Enterobacteriaceae, appear to play a secondary role in airway infection. Aspergillus fumigatus is the most important fungal agent causing allergic bronchopulmonary disease. The role of viruses has only recently been examined. At least in some patients with cystic fibrosis, respiratory syncytial virus may be important in predisposing to subsequent bacterial infections.

Polysaccharide antigens of Pseudomonas aeruginosa

The major polysaccharide antigens of P. aeruginosa are the cell-wall lipopolysaccharides many of which have an acidic polysaccharide chain (O-antigen) rich in unusual amino sugars. The D-rhamnose-rich polysaccharide antigen common to many serologically distinct strains is also associated with the lipopolysaccharide. The high-molecular-weight polysaccharides with O-specificity are present in extracellular slime produced by strains isolated from the environmental and from the immunocompromised hosts. The extracellular antigenic polysaccharide of another type (bacterial alginate) is expressed by mucoid strains isolated from patients with cystic fibrosis. Serotype-specific immune responses after infection are directed at the lipopolysaccharides and these heat-stable antigens serve as the basis for differentiation of P. aeruginosa strains. Both the cell-wall antigens including conjugates of the O-polysaccharides with different proteins and the extracellular antigens have been used to prepare specific antibodies tested for protection against infections due to P. aeruginosa.

Alginate biosynthetic enzymes in mucoid and nonmucoid Pseudomonas aeruginosa: overproduction of phosphomannose isomerase, phosphomannomutase, and GDP-mannose pyrophosphorylase by overexpression of the phosphomannose isomerase (pmi) gene

The specific activities of phosphomannose isomerase (PMI), phosphomannomutase (PMM), GDP-mannose pyrophosphorylase (GMP), and GDP-mannose dehydrogenase (GMD) were compared in a mucoid cystic fibrosis isolate of Pseudomonas aeruginosa and in two spontaneous nonmucoid revertants. In both revertants some or all of the alginate biosynthetic enzymes we examined appeared to be repressed, indicating that the loss of the mucoid phenotype may be a result of decreased formation of sugar-nucleotide precursors. The introduction and overexpression of the cloned P. aeruginosa phosphomannose isomerase (pmi) gene in both mucoid and nonmucoid strains led not only to the appearance of PMI levels in cell extracts several times higher than those present in the wild-type mucoid strain, but also in higher PMM and GMP specific activities. In extracts of both strains, however, the specific activity of GMD did not change as a result of pmi overexpression. In contrast, the introduction of the cloned Escherichia coli manA (pmi) gene in P. aeruginosa caused an increase in only PMI and PMM activities, having no effect on the level of GMP. This suggests that an increase in PMI activity alone does not induce high GMP activity in P. aeruginosa. The heterologous overexpression of the P. aeruginosa pmi gene in the E. coli manA mutant CD1 led to the appearance in cell extracts of not only PMI activity but also GMP activity, both of which are normally undetectable in extracts of CD1. We discuss the implications of these results and propose a mechanism by which overexpression of the P. aeruginosa pmi gene can cause an elevation in both PMM and GMP activities.

Construction and characterization of Pseudomonas aeruginosa algB mutants: role of algB in high-level production of alginate

The algB gene, which is involved in the production of alginate in Pseudomonas aeruginosa, was localized to approximately 2.2 kilobases of DNA from strain FRD by using transposon Tn501 insertion mutagenesis, subcloning, and complementation techniques. The previously reported alg-50(Ts) mutation, which confers the phenotype of temperature-sensitive alginate production, was here designated as an algB allele. A transduction-mediated gene replacement technique was used for site-directed mutagenesis to isolate and characterize algB::Tn501 mutants of P. aeruginosa FRD. Although algB::Tn501 mutants had a nonmucoid phenotype (indicating an alginate deficiency), they still produced about 1 to 5% of wild-type levels of alginate in most growth media and up to 16% in very rich media. The algB::Tn501 mutations had no apparent effect on growth rate or growth requirements. Using another gene replacement technique called excision marker rescue, we constructed a chromosomal algB deletion (delta algB) mutant of P. aeruginosa FRD. The delta algB mutant also produced low levels of alginate as did the algB::Tn501 mutants. The alginate produced by algB::Tn501 mutants resembled wild-type alginate by all criteria studied: molecular weight, acetylation, and proportion of mannuronic and guluronic acids. Thus, the algB gene product is apparently involved in the high-level production of alginate by P. aeruginosa and is not directly involved in the pathway leading to its biosynthesis. Chromosomal mapping of an algB::Tn501 insertion showed linkage to the trp-2 marker on the FRD chromosome as does the algB50(Ts) mutation. The excision marker rescue technique was also used to place the algB::Tn501 marker on the chromosome of characterized strains of P. aeruginosa PAO. The algB::Tn501 mutation mapped near 21 min on the PAO chromosome.

Gene algD coding for GDPmannose dehydrogenase is transcriptionally activated in mucoid Pseudomonas aeruginosa

Transcriptional regulation of alginate biosynthesis by Pseudomonas aeruginosa was studied. A DNA region complementing the alg-5 mutation within the alginate gene cluster was found by RNA-DNA dot blot and Northern hybridization to be transcriptionally activated in mucoid P. aeruginosa. This region was subcloned as a 3.2-kilobase BglII-ClaI DNA fragment on the broad-host-range controlled transcription vector pMMB24, and gene products were analyzed by expression from the tac promoter. A 48-kilodalton polypeptide was detected in extracts of P. aeruginosa and 35S-labeled Escherichia coli maxicells. By using the same expression system, GDPmannose dehydrogenase activity was detected in both P. aeruginosa and E. coli. Thus, gene algD coding for this enzyme was found to be present in the transcriptionally active DNA area. Insertion of the xylE gene within the BglII-ClaI fragment disrupted the induction of the 48-kilodalton polypeptide, GDPmannose dehydrogenase activity, and alg-5 complementing ability. With the algD-xylE transcription fusion, activation of algD gene expression was shown to occur in mucoid P. aeruginosa of different origins. In addition, regulation of the algD promoter activity was demonstrated to be mediated by a diffusible factor.

Overproduction and assay of Pseudomonas aeruginosa phosphomannose isomerase

Phosphomannose isomerase activity was undetectable in extracts of mucoid (alginate-producing) Pseudomonas aeruginosa. When a P. aeruginosa gene previously shown to complement an alginate-negative mutant was overexpressed under the control of the tac promoter in the broad-host-range controlled-expression vector pMMB22, phosphomannose isomerase activity could be measured in extracts of P. aeruginosa and in a manA (phosphomannose isomerase-negative) mutant of Escherichia coli. P. aeruginosa extracts containing induced levels of enzyme were shown to interconvert fructose 6-phosphate and mannose 6-phosphate. A 56,000-dalton polypeptide was visualized on sodium dodecyl sulfate-polyacrylamide gels after induction in both hosts. When RNA-DNA dot- blot hybridization analysis was used, transcription of algA, the gene coding for P. aeruginosa phosphomannose isomerase, was not measurable from the chromosomes of either mucoid or nonmucoid P. aeruginosa. However, a high level of algA transcription was detected after expression of algA under tac promoter control in pMMB22.

Cloning of Escherichia coli and Pseudomonas aeruginosa phosphomannose isomerase genes and their expression in alginate-negative mutants of Pseudomonas aeruginosa

The phosphomannose isomerase (pmi) gene of Escherichia coli was cloned on a broad-host-range cosmid vector and expressed in Pseudomonas aeruginosa at a low level. Plasmid pAD3, which harbors the E. coli pmi gene, contains a 6.2-kilobase-pair HindIII fragment derived from the chromosome of E. coli. Subcloning produced plasmids carrying the 1.5-kilobase-pair HindIII-HpaI subfragment of pAD3 that restored alginic acid production in a nonmucoid, alginate-negative mutant of P. aeruginosa. This fragment also complemented mannose-negative, phosphomannose isomerase-negative mutants of E. coli and showed no homology by DNA-DNA hybridization to P. aeruginosa chromosomal DNA. By using a BamHI constructed cosmid clone bank of the stable alginate producing strain 8830, we have been able to isolate a recombinant plasmid of P. aeruginosa origin that also restores alginate production in the alginate-negative mutant. This new recombinant plasmid, designated pAD4, contained a 9.9-kilobase-pair EcoRI-BamHI fragment with the ability to restore alginate synthesis in the alginate-negative P. aeruginosa. This fragment showed no homology to E. coli chromosomal DNA or to plasmid pAD3. Both mucoid and nonmucoid strains of P. aeruginosa had no detectable levels of phosphomannose isomerase activity as measured by mannose 6-phosphate-to-fructose 6-phosphate conversion. However, P. aeruginosa strains harboring the cloned pmi gene of E. coli contained measurable levels of phosphomannose isomerase activity as evidenced by examining the conversion of mannose 6-phosphate to fructose 6-phosphate.

Isolation and characterization of an alginase from mucoid strains of Pseudomonas aeruginosa

Strains of Pseudomonas aeruginosa which produce an alginate-like slime polysaccharide were shown to also synthesize an intracellular enzyme which can degrade these polysaccharides and the seaweed alginic acids. The enzyme acts as an eliminase introducing delta 4,5 unsaturation into the uronic acid moiety. It appears to be a polymannuronide lyase which degrades the polysaccharides, depending on their uronic acid composition, to a series of oligosaccharides, the smallest of which is a disaccharide. L-Guluronic acid linkages are not split. The Pseudomonas alginase resembles other bacterial alginases and enzymes from molluscs but differs in some important properties, such as extent of degradation and linkage preference. Nonmucoid forms of the organism produce detectable but much lower amounts of enzyme.

Cloning and expression in Pseudomonas aeruginosa of a gene involved in the production of alginate

Pseudomonas aeruginosa strains isolated from patients with cystic fibrosis commonly produce a capsule-like exopolysaccharide called alginate. The alginate-producing (Alg+) phenotype results in a mucoid colony morphology and is an unstable trait. A mutant of P. aeruginosa FRD (a cystic fibrosis isolate) was obtained which was temperature sensitive for alginate production ( Algts ). At elevated growth temperatures (41 degrees C), no alginate was detected in culture supernatants of the Algts mutant, but yields of alginate increased as the temperature of incubation was reduced. The mutation responsible for the Algts phenotype, alg-50(Ts), has been mapped to a region of the FRD chromosome closely linked to trp-2. The alg-50(Ts) marker did not map near the met-l-linked chromosomal mutations responsible for the instability of the Alg+ phenotype. A broad host range cosmid cloning system based upon derivatives of plasmid RK2 was used to construct a P. aeruginosa clone bank. After transfer of the clone bank to the Algts mutant, hybrid plasmids were obtained which complemented the Algts defect. Deletion mapping of the original 20.3 kilobases of P. aeruginosa DNA cloned showed that a 4.7-kilobase fragment would complement the alg-50(Ts) mutation.