Cloning of genes controlling alginate biosynthesis from a mucoid cystic fibrosis isolate of Pseudomonas aeruginosa.

Mucoid strains of Pseudomonas aeruginosa isolated from the sputum of cystic fibrosis patients produce copious quantities of an exopolysaccharide known as alginic acid. Since clinical isolates of the mucoid variants are unstable with respect to alginate synthesis and revert spontaneously to the more typical nonmucoid phenotype, it has been difficult to isolate individual structural gene mutants defective in alginate synthesis. The cloning of the genes controlling alginate synthesis has been facilitated by the isolation of a stable alginate-producing strain, 8830. The stable mucoid strain was mutagenized with ethyl methanesulfonate to obtain various mutants defective in alginate biosynthesis. Several nonmucoid (Alg-) mutants were isolated. A mucoid P. aeruginosa gene library was then constructed, using a cosmid cloning vector. DNA isolated from the stable mucoid strain 8830 was partially digested with the restriction endonuclease HindIII and ligated to the HindIII site of the broad host range cosmid vector, pCP13. After packaging in lambda particles, the recombinant DNA was introduced via transfection into Escherichia coli AC80. The clone bank was mated (en masse) from E. coli into various P. aeruginosa 8830 nonmucoid mutants with the help of pRK2013, which provided donor functions in trans, and tetracycline-resistant exconjugants were screened for the ability to form mucoid colonies. Three recombinant plasmids, pAD1, pAD2, and pAD3, containing DNA inserts of 20, 9.5, and 6.2 kilobases, respectively, were isolated based on their ability to restore alginate synthesis in various strain 8830 nonmucoid (Alg-) mutants. Mutants have been assigned to at least four complementation groups, based on complementation by pAD1, pAD2, or pAD3 or by none of them. Introduction of pAD1 into the spontaneous nonmucoid strain 8822, as well as into other nonmucoid laboratory strains of P. aeruginosa such as PAO and SB1, was found to slowly induce alginate synthesis. This alginate-inducing ability was found to reside on a 7.5-kilobase EcoRI fragment that complemented the alg-22 mutation of strain 8852. The pAD1 chromosomal insert which complements the alg-22 mutation was subsequently mapped at ca. 19 min of the P. aeruginosa PAO chromosome.     

Genetic rearrangement associated with in vivo mucoid conversion of Pseudomonas aeruginosa PAO is due to insertion elements.

The conversion of Pseudomonas aeruginosa PAO to the mucoid phenotype has been reported for a chronic pulmonary infection model in rats (D. E. Woods, P. A. Sokol, L. E. Bryan, D. G. Storey, S. J. Mattingly, H. J. Vogel, and H. Ceri, J. Infect. Dis. 163:143-149, 1991). This conversion was associated with a genetic rearrangement upstream of the exotoxin A gene. To characterize the genetic rearrangement, the region upstream of the toxA gene was cloned from PAO, PAO-muc (a mucoid strain), and PAO-rev (a nonmucoid revertant strain). The nucleotide sequence of a 4.8-kb fragment from PAO-muc was determined. A+T-rich regions of approximately 2 kb (IS-PA-4) and 0.4 kb (IS-PA-5) were identified in this fragment. DNA probes constructed internal to these regions hybridized to PAO-muc but not to PAO or PAO-rev, suggesting that PAO-muc contains an insertion element. Sequence analysis of the nonmucoid clones indicated that a 2,561-bp fragment corresponding to IS-PA-4 and a 992-bp fragment corresponding to IS-PA-5 were not present in PAO or PAO-rev. Both nonmucoid clones, however, contained in the same location as IS-PA-4, a 1,313-bp region which was not present in PAO-muc. DNA probes complementary to this sequence, designated IS-PA-6, did not hybridize with PAO-muc, indicating that this sequence had been replaced upon conversion to the mucoid phenotype. Between IS-PA-4 and IS-PA-5 there was a 500-bp sequence which was 94% identical to the 500-bp sequence downstream of IS-PA-6. These insertion elements had some DNA sequence similarity to plasmid and transposon sequences, suggesting that they may be of plasmid origin. IS-PA-4 and IS-PA-5 were shown also to be present in two mucoid isolates from cystic fibrosis patients. The insertions occurred in the same location upstream of the toxA gene, suggesting that this type of genetic recombination may also be associated with mucoid conversion in some P. aeruginosa clinical isolates.     

Siderophore synthesis by mucoid Pseudomonas aeruginosa strains isolated from cystic fibrosis patients.

Nonmucoid Pseudomonas aeruginosa responds to iron deprivation by synthesizing the siderophores pyochelin and pyoverdine. When grown in iron-deficient medium, six mucoid P. aeruginosa strains isolated from cystic fibrosis patients synthesized copious amounts of the exopolysaccharide alginate. A procedure that eliminated the interference of alginate was developed so that siderophores could be extracted from the growth medium. All six isolates were then noted to produce both pyoverdine and pyochelin. This report thus confirms that mucoid P. aeruginosa, like its nonmucoid counterparts, elicits the siderophores commonly cited as those of the microbe.     

Partial purification and characterization of a polymannuronic acid depolymerase produced by a mucoid strain of Pseudomonas aeruginosa isolated from a patient with cystic fibrosis

An exopolysaccharide depolymerase was isolated from a mucoid strain of Pseudomonas aeruginosa of cystic fibrosis origin. Purified preparations of the depolymerase showed maximum activity against the unacetylated polymannuronic acid exopolysaccharide from the same strain and little activity against commercially prepared alginic acid. The evidence suggests that the enzyme is either periplasmic in location or associated with the outer cell membrane and is released extracellularly, in the absence of cell lysis, after a reduction of the culture magnesium (Mg2+) concentration below 3.0 mM. The depolymerase is also released after the addition of sublethal concentrations of EDTA to cultures containing 3.0 mM Mg2+. A survey of additional mucoid P. aeruginosa isolates recovered from patients with cystic fibrosis showed that nearly 60% demonstrated similar depolymerase activity while none of the nonmucoid revertants of the parent strains produced detectable depolymerase activity.     

Pseudomonas aeruginosa flagellin and alginate elicit very distinct gene expression patterns in airway epithelial cells: implications for cystic fibrosis disease

Infection with the opportunistic pathogen Pseudomonas aeruginosa remains a major health concern. Two P. aeruginosa phenotypes relevant in human disease include motility and mucoidy. Motility is characterized by the presence of flagella and is essential in the establishment of acute infections, while mucoidy, defined by the production of the exopolysaccharide alginate, is critical in the development of chronic infections, such as the infections seen in cystic fibrosis patients. Indeed, chronic infection of the lung by mucoid P. aeruginosa is a major cause of morbidity and mortality in cystic fibrosis patients. We have used Calu-3 human airway epithelial cells to investigate global responses to infection with motile and mucoid P. aeruginosa. The response of airway epithelial cells to exposure to P. aeruginosa motile strains is characterized by a specific increase in gene expression in pathways controlling inflammation and host defense. By contrast, the response of airway epithelia to the stimuli presented by mucoid P. aeruginosa is not proinflammatory and, hence, may not be conducive to the effective elimination of the pathogen. The pattern of gene expression directed by flagellin, but not alginate, includes innate host defense genes, proinflammatory cytokines, and chemokines. By contrast, infection with alginate-producing P. aeruginosa results in an overall attenuation of host responses and an antiapoptotic effect.     

Partial purification and characterization of a polymannuronic acid depolymerase produced by a mucoid strain of Pseudomonas aeruginosa isolated from a patient with cystic fibrosis.

An exopolysaccharide depolymerase was isolated from a mucoid strain of Pseudomonas aeruginosa of cystic fibrosis origin. Purified preparations of the depolymerase showed maximum activity against the unacetylated polymannuronic acid exopolysaccharide from the same strain and little activity against commercially prepared alginic acid. The evidence suggests that the enzyme is either periplasmic in location or associated with the outer cell membrane and is released extracellularly, in the absence of cell lysis, after a reduction of the culture magnesium (Mg2+) concentration below 3.0 mM. The depolymerase is also released after the addition of sublethal concentrations of EDTA to cultures containing 3.0 mM Mg2+. A survey of additional mucoid P. aeruginosa isolates recovered from patients with cystic fibrosis showed that nearly 60% demonstrated similar depolymerase activity while none of the nonmucoid revertants of the parent strains produced detectable depolymerase activity.     

Gene amplification induces mucoid phenotype in rec-2 Pseudomonas aeruginosa exposed to kanamycin.

Gene amplification in the chromosome of rec-2 Pseudomonas aeruginosa PAO2003 upon growth on kanamycin-supplemented media led to a stable mucoid phenotype. The chromosomal region controlling alginate biosynthesis was shown to be amplified four to six times as a direct tandem repeat of at least 16.8 kilobase pairs. This amplification was deduced from Southern DNA-DNA hybridization patterns of the chromosomal DNA digested with restriction endonucleases BglII and EcoRI and probed with a cloned DNA segment complementing the alg-22 mutation. The part of the amplified unit carrying the novel DNA joint was cloned. The EcoRI junction fragment was further subcloned and used to probe chromosomes of parental strain PAO2003 and mucoid variant VD2003M. As predicted, the EcoRI junction fragment hybridized to the two chromosomal fragments required to produce the novel junction. Though the mucoid phenotype caused by gene amplification was stable, nonmucoid revertants were obtained at a low frequency on tetracycline-containing media. Southern hybridization of chromosomal DNA from a nonmucoid revertant revealed a reduction in the copy number of amplified DNA. These results suggest a direct relationship between amplification of this chromosomal segment and the induction of mucoidy.     

Analysis of the Pseudomonas aeruginosa regulon controlled by the sensor kinase KinB and sigma factor RpoN

Alginate overproduction by Pseudomonas aeruginosa, also known as mucoidy, is associated with chronic endobronchial infections in cystic fibrosis. Alginate biosynthesis is initiated by the extracytoplasmic function sigma factor (σ(22); AlgU/AlgT). In the wild-type (wt) nonmucoid strains, such as PAO1, AlgU is sequestered to the cytoplasmic membrane by the anti-sigma factor MucA that inhibits alginate production. One mechanism underlying the conversion to mucoidy is mutation of mucA. However, the mucoid conversion can occur in wt mucA strains via the degradation of MucA by activated intramembrane proteases AlgW and/or MucP. Previously, we reported that the deletion of the sensor kinase KinB in PAO1 induces an AlgW-dependent proteolysis of MucA, resulting in alginate overproduction. This type of mucoid induction requires the alternate sigma factor RpoN (σ(54)). To determine the RpoN-dependent KinB regulon, microarray and proteomic analyses were performed on a mucoid kinB mutant and an isogenic nonmucoid kinB rpoN double mutant. In the kinB mutant of PAO1, RpoN controlled the expression of approximately 20% of the genome. In addition to alginate biosynthetic and regulatory genes, KinB and RpoN also control a large number of genes including those involved in carbohydrate metabolism, quorum sensing, iron regulation, rhamnolipid production, and motility. In an acute pneumonia murine infection model, BALB/c mice exhibited increased survival when challenged with the kinB mutant relative to survival with PAO1 challenge. Together, these data strongly suggest that KinB regulates virulence factors important for the development of acute pneumonia and conversion to mucoidy.     

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.     

Adherence of clinical isolates ofPseudomonas aeruginosa to hamster trachael epithelium in vitro

The adherence to hamster tracheal epithelium, of mucoid and nonmucoid clinical isolates ofPseudomonas aeruginosa from cystic fibrosis patients, was studied using tracheal organ cultures. Tracheal cultures were infected with 10⁷ colony-forming units per ml of either mucoid or nonmucoid clinical isolates ofP aeruginosa. The tracheal explants were rinsed at various time intervals to remove nonadherent bacteria, fixed, and prepared for transmission-and scanning-electron microscopy. Mucoid isolates were seen adhering to the ciliated epithelium as early as 4 h after initiation of infection, whereas nonmucoid isolates were only observed adhering at 6 to 8 h after infection. Mucoid organisms were found as clusters of bacteria embedded in an extensive extracellular matrix. The nonmucoid isolates were generally found as single organisms with no evidence of an extracellular matrix. These results suggest that the prevalence of mucoid isolates ofP. aeruginosa in cystic fibrosis may be due to adherent properties of the mucoid organism.     

Alginate overproduction affects Pseudomonas aeruginosa biofilm structure and function

During the course of chronic cystic fibrosis (CF) infections, Pseudomonas aeruginosa undergoes a conversion to a mucoid phenotype, which is characterized by overproduction of the exopolysaccharide alginate. Chronic P. aeruginosa infections involve surface-attached, highly antibiotic-resistant communities of microorganisms organized in biofilms. Although biofilm formation and the conversion to mucoidy are both important aspects of CF pathogenesis, the relationship between them is at the present unclear. In this study, we report that the overproduction of alginate affects biofilm development on an abiotic surface. Biofilms formed by an alginate-overproducing strain exhibit a highly structured architecture and are significantly more resistant to the antibiotic tobramycin than a biofilm formed by an isogenic nonmucoid strain. These results suggest that an important consequence of the conversion to mucoidy is an altered biofilm architecture that shows increasing resistance to antimicrobial treatments.