The inner-core lipopolysaccharide biosynthetic waaE gene: function and genetic distribution among some Enterobacteriaceaeb

Luis Izquierdo2, Nihal Abitiu1, Núria Coderch1, Beatriz Hita1, Susana Merino2, Rosalina Gavin2, Juan M. Tomás2 and Miguel Regué1

Departamento de Microbiología y Parasitología Sanitarias, División de Ciencias de la Salud, Facultad de Farmacia, Universidad de Barcelona, Av. Joan XXIII s/n, Barcelona 08028, Spain1
Departamento de Microbiología, Facultad de Biología, Universidad de Barcelona, Diagonal 645, 08071 Barcelona, Spain2

Author for correspondence: Miguel Regué. Tel: +34 3 4024496. Fax: +34 3 4024498. e-mail: regue{at}farmacia.far.ub.es


   ABSTRACT
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
 
To determine the function of the waaE gene in the biosynthesis of the inner-core LPS of Klebsiella pneumoniae, a waaE non-polar mutant has been constructed. Data obtained from the comparative chemical analysis of LPS samples obtained from the wild-type, the mutant strain and the complemented mutant demonstrated that the waaE gene is involved in substitution of {alpha}-L-glycero-D-manno-heptopyranose I (L,D-HeppI) at the O-4 position by a ß-D-glucopyranose (ß-D-Glcp) residue. In addition, DNA amplification and nucleotide sequence determination studies revealed that waaE homologues located between the waaA and coaD genes are present in clinical isolates of Enterobacteriaceae containing the structure ß-D-Glcp-(1->4)-{alpha}-L,D-HeppI (K. pneumoniae, Proteus mirabilis and Yersinia enterocolitica), as well as in strains of Serratia marcescens and Enterobacter aerogenes of unknown LPS-core structures. Complementation studies using non-polar waaE mutants prove that all the waaE homologues perform the same function. Furthermore, K. pneumoniae, Ser. marcescens and P. mirabilis non-polar waaE mutants showed reduced adhesion and pathogenicity. In addition, the Ser. marcescens and P. murabilis waaE mutants showed reduced swarming motility and ability to form biofilms in vitro. All these characteristics were rescued by reintroduction of the waaE gene independently of its origin. An easy DNA amplification method to detect this gene was established, which also helps in finding the potential presence of this structural feature [ß-D-Glcp-(1->4)-{alpha}-L,D-HeppI] in the inner-core LPS of Enterobacteriaceae members with unknown LPS-core structures.

Keywords: LPS, waaE homologues, non-polar mutant characterization, cross-complementation

Abbreviations: {alpha}-D-Gal(p)A, {alpha}-D-galacturonic acid; GlcN(p), glucosamine; ß-D-Glc(p), ß-D-glucopyranose; L,D-Hep(p), {alpha}-L-glycero-D-manno-heptopyranose; Kdo(p), 3-deoxy-D-manno-octulopyranosonic acid; BATH, bacterial adherence to hydrocarbons; HIC, hydrophobic interaction chromatography; NHS, non-immune human serum

b The GenBank accession number for the waaE gene sequences of P. mirabilis CECT170, Y. enterocolitica R102 and Ent. aerogenes CECT684 reported in this paper are AY075039, AY075041 and AY075040, respectively.


   INTRODUCTION
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ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
 
In Gram-negative bacteria LPS is one of the major structural and immunodominant molecules of the outer membrane. It consists of three domains: lipid A, core oligosaccharide and O-specific antigen or O side chain. The genetics of LPS core biosynthesis have been studied in some genera of the Enterobacteriaceae and in other Gram-negative bacteria (Allen & Maskell, 1996 ; Kalher et al., 1996 ; Filiatrault et al., 2000 ; Regué et al., 2001 ). The genes involved in LPS core biosynthesis in Escherichia coli and Salmonella enterica are usually found in the waa (rfa) gene cluster (Schnaitman & Klena, 1993 ; Heinrichs et al., 1998 ). [The nomenclature proposed by Reeves et al. (1996) for proteins and genes involved in core LPS biosynthesis is used in this work.]

Comparison of the known structures of core LPS from different Enterobacteriaceae reveals that the inner core structure contains two residues of 3-deoxy-D-manno-octulopyranosonic acid (Kdop) and three residues of L-glycero-D-manno-heptopyranose (L,D-HeppI, L,D-HeppII and L,D-HeppIII) (Radziejewska-Lebrecht et al., 1994 ; Heinrichs et al., 1998 ; Vinogradov et al., 2000 ; Vinogradov & Perry, 2001 ). In addition, some Enterobacteriaceae, like E. coli, contain a phosphoryl group modification at the L,D-HeppI residue (Heinrichs et al., 1998 ). In other species, like Klebsiella pneumoniae, a substitution of L,D-HeppI at the O-4 position by a ß-D-glucopyranose residue (ß-D-Glcp) is found (Vinogradov & Perry, 2001 ). Furthermore, in K. pneumoniae, Proteus mirabilis and Yersinia enterocolitica a substitution of L,D-HeppII at the O-3 position by an {alpha}-D-galacturonic acid residue ({alpha}-D-GalpA) residue is found (Radziejewska-Lebrecht et al., 1994 ; Vinogradov et al., 2000 ; Vinogradov & Perry, 2001 ), while a substitution of L,D-HeppII at the O-3 position by a Glcp residue is found in E. coli (Heinrichs et al., 1998 ).

The waaE gene is present in K. pneumoniae and Serratia marcescens waa gene clusters, but not in waa gene clusters from E. coli and Sal. enterica (Guasch et al., 1996 ; Regué et al., 2001 ). In this study we present results showing that the K. pneumoniae waaE gene is responsible for the attachment of the branched Glcp residue to L,D-HeppI. In addition, the presence of waaE homologues is determined for representative species of Enterobacteriaceae, and we show by complementation studies that all the waaE homologues perform the same function in inner core biosynthesis. We also investigated waaE gene function in some pathogenic features, as well as its role in motility and biofilm formation.


   METHODS
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ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
 
Bacterial strains, plasmids and growth conditions.
Serratia marcescens N28b (Guasch et al., 1996 ) and Klebsiella pneumoniae 889, (serovar O:8) (Hansen et al., 1999 ), as well as Escherichia coli, Salmonella enterica, Shigella dysenteriae, Shigella flexneri, Shigella sonnei, Proteus mirabilis and Yersinia enterocolitica strains isolated from clinical samples were used in this study. As controls the following strains from the Spanish Culture Collection were used: Y. enterocolitica R102 (O:8), P. mirabilis CECT170 (O:3) and Enterobacter aerogenes CECT684. Bacterial strains were grown in LB Miller broth and LB Miller agar (Miller, 1972 ). Plasmids used in this study and their characteristics are shown in Table 1. LB media was supplemented with ampicillin (100 µg ml-1 or 3 mg ml-1) and chloramphenicol (25 µg ml-1) when required.


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Table 1. Plasmids used in this study

 
General DNA methods.
General DNA manipulations were done as described by Sambrook et al. (1989) . DNA restriction endonucleases, T4 DNA ligase, E. coli DNA polymerase (Klenow fragment) and alkaline phosphatase were used as recommended by the suppliers.

LPS isolation and analysis.
Cultures for analysis of LPS were grown in TSB at 37 °C. LPS was purified by the method of Westphal & Jann (1965) . For screening purposes LPS was obtained after proteinase K digestion of whole cells (Hitchcock & Brown, 1983 ). LPS samples were separated by SDS-PAGE or SDS-Tricine-PAGE and visualized by silver staining as described previously (Tsai & Frasch, 1982 ; Pradel & Schnaitman, 1991 ). For chemical analyses, purified LPS was deacylated by treatment with acetic acid (Süsskind et al., 1998 ) and the core region was purified by column chromatography on Sephadex G50. The core fractions were hydrolysed with 1 M trifluoroacetic acid for 4 h at 100 °C. Monosaccharides were analysed as their alditol acetate derivatives by GLC on a Rtx-2330 column (Restek). Alditol acetate monosaccharides were obtained by a procedure described by Vinogradov et al. (1992) . Colorimetric analysis of Kdo was performed by the method of Karkhanis et al. (1978) . Methylation of carboxyl-reduced core oligosaccharides was performed according to the method of Ciucanu & Kerek (1984) . Briefly, the methylated samples were hydrolysed in 2 M trifluoroacetic acid at 100 °C for 2 h, reduced, acetylated and analysed using GLC-MS.

DNA sequencing and computer analysis of sequence data.
Double-stranded DNA sequencing was performed by using the dideoxy-chain termination method (Sanger et al., 1977 ) with the ABI Prism dye terminator cycle sequencing kit (Perkin Elmer). The relevant PCR-amplified genomic DNAs were sequenced by using oligonucleotides used in the amplification reactions. Other sequence-derived oligonucleotides were used to extend the nucleotide sequence. Primers used for genomic DNA amplification experiments and for DNA sequencing were purchased from Pharmacia LKB Biotechnology. The DNA sequence was translated in all six frames and all ORFs were inspected. Deduced amino acid sequences were compared with those of DNA translated in all six frames from non-redundant GenBank and EMBL databases by using the BLAST (Altschul et al., 1990 , 1997 ) and FASTA (Pearson & Lipman, 1988 ) network service at the National Center for Biotechnology Information and the European Biotechnology Information, respectively. CLUSTAL W (Thompson et al., 1994 ) was used for multiple sequence alignments.

DNA amplification.
Genomic DNA from different strains of representative Enterobacteriaceae was isolated and used as template in PCR experiments using primers designed to amplify DNA fragments containing the 3' end of the waaA gene and the 3' end of the fpg gene, respectively, at their ends. Oligonucleotide pairs Pr2 (5'-GATGGCGGGGAAC-3') and Fpg2 (5'-CATTGCCAACATGGTGAA-3'), kdtb1 (5'-GTCATGGGATCGAACGTC-3') and Yer3 (5'-CTGTTGACCGACGAAGACTA-3'), and Nuk9 (5'-TGCTTTATACCACCCTACT-3') and Nuk10 (5'-GATAAACGACCACTCTTTG-3') were used to amplify and to subclone the waaE homologues from P. mirabilis, Y. enterocolitica and Ent. aerogenes, respectively, in vector pGEMT.

Complementation studies.
Complementation analysis of relevant waaE mutants was performed by transformation of waaE genes from different Enterobacteriaceae cloned in pGEMT. Transformants were selected on LB agar containing ampicillin (100 µg ml-1) and LPS was isolated and analysed by Tricine-SDS-PAGE.

Serum killing.
The survival of exponential-phase bacteria in non-immune human serum was measured as described previously (Merino et al., 1992 ).

Cell surface hydrophobicity.
Cell surface hydrophobicity was determined by two different methods. The first method used was hydrophobic interaction chromatography (HIC) on phenylsepharose as described by Källenius et al. (1985) . Briefly, bacteria were suspended in 10 mM PBS (pH 7·4) to an OD470 of 1·0, applied to a phenylsepharose column and eluted with 4 M NaCl. The eluate was collected and its OD470 determined. The second method used was bacterial adherence to hydrocarbons (BATH) as described previously by Rosenberg et al. (1980) . Briefly, cells were washed twice in phosphate/urea/magnesium buffer (pH 7·1), suspended in the same buffer at an OD400 of 1·0 and vortexed with various volumes of hydrocarbon. The OD400 of the aqueous phase was expressed as a percentage of the OD400 of a standard volume of untreated cells.

Bacterial adherence assay.
The adherence assay using K. pneumoniae strains to uroepithelial cells (UEC) was done as described previously (Falkowski et al., 1986 ; Merino et al., 1997 ).

Urinary tract infections in rats.
The bacterial strains used to establish infection were grown overnight in LB agar supplemented with antibiotics when needed and gently resuspended in PBS to the appropriate concentration. In each experiment, 12 female Wistar rats (200–250 g; strain CFHB; Interfauna UK) were used. Ten animals were infected and two were used as controls. The infections were established and quantified as described previously (Camprubí et al., 1993 ).

Swimming and swarming motility assay.
Freshly grown bacterial colonies were transferred with a sterile toothpick into the centre of motility agar (1% tryptone, 0·5% NaCl, 0·25% agar) or swarm agar (LB base, 0·6% agar). The plates were incubated at 37 °C for 16–24 h and motility was assessed by examining the migration of bacteria through the agar from the centre towards the periphery of the plate.

Transmission electron microscopy.
Bacterial suspensions were placed on Formvar-coated copper grids and negatively stained with a 2% solution of potassium phospho-tungstate. Bacteria were then viewed on a Phillips EM 400 transmission electron microscope.

Biofilm formation.
Quantitative biofilm formation was performed in a microtitre assay as described by Pratt & Kolter (1998) with minor modifications. Briefly, bacteria were grown on trypticase soy agar (TSA) and several colonies were gently resuspended in trypticase soy broth (TSB) with or without the appropriate antibiotics; 100 µl aliquots were placed in a microtitre polystyrene plate and incubated for 48 h at 30 °C without shaking. After the bacterial cultures were poured out, the plate was washed extensively with water, fixed with 2·5% glutaraldehyde, washed once with water and stained with 0·4% crystal violet solution. After solubilization of the crystal violet with ethanol/acetone (80/20, v/v) the absorbance was determined at 570 nm.


   RESULTS
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ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
 
Construction and analysis of a K. pneumoniae 889 waaE mutant
We have reported previously the presence of the waaE gene in K. pneumoniae and Ser. marcescens N28b (Guasch et al., 1996 ; Regué et al., 2001 ). Monosaccharide composition analysis and complementation experiments with the Haemophilus ducreyi lgtF gene suggested that the waaE gene encodes the glucosyltransferase involved in the attachment of a Glc residue by a ß1,4 linkage to L,D-HeppI in the LPS inner core. To confirm this proposed function K. pneumoniae 889 (O8:K69) (Hansen et al., 1999 ) was used because its chemical core LPS structure has been recently updated using the non-encapsulated mutant NRC6121 (Fig. 1) (Vinogradov & Perry, 2001 ). To construct a K. pneumoniae 889 waaE non-polar mutant an in-frame tagged deletion was used. Plasmid pKO3{Delta}waaE, containing the engineered deletion, was used to introduce the waaE deletion into K. pneumoniae 889 by double recombination as described previously (Link et al., 1997 ; Regué et al., 2001 ). Candidate mutants were screened by PCR and one of them, strain 889-1, was proved to contain the desired mutation by DNA nucleotide sequence determination of the waaA-coaD region. LPS from strains 889 (wild-type) and 889-1 (waaE) were extracted and analysed by SDS-Tricine-PAGE. The core LPS from strain 889-1 migrates faster than that of the wild-type strain and it appears that the mutant LPS still contains O antigen, although in smaller amounts than wild-type LPS (Fig. 2, lanes 1 and 2). This behaviour is identical to that found for other K. pneumoniae waaE mutants (Regué et al., 2001 ). LPS samples from the wild-type strain and the waaE mutant were deacylated by mild acid hydrolysis and the core regions were recovered on Sephadex G50 chromatography. Monosaccharide analysis of the core oligosaccharides from the LPS of K. pneumoniae 889 revealed the presence of Glc, GalA, glucosamine (GlcN), L,D-Hep and Kdo. The absence of Glc was the most relevant change found in the core oligosaccharides from the LPS of K. pneumoniae 889-1.



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Fig. 1. Conserved region in the core LPS structure of K. pneumoniae (Vinogradov & Perry, 2001 ).

 


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Fig. 2. SDS-Tricine PAGE analysis of LPS samples from K. pneumoniae 889 (lane 1), K. pneumoniae waaE derivative (889-1; 2), K. pneumoniae 889-1(pGEMT-WaaEKp) (3), K. pneumoniae 889-1(pGEMT-WaaESm) (4), K. pneumoniae 889-1(pGEMT-WaaEPm) (5), K. pneumoniae 889-1(pGEMT-WaaEYe) (6), K. pneumoniae 889-1(pGEMT-WaaEEa) (7), Ser. marcescens wild-type N28b (8), Ser. marcescens waaE derivative (N28b-16; 9), Ser. marcescens N28b-16(pGEMT-WaaESm) (10), Ser. marcescens N28b-16(pGEMT-WaaEKp) (11), Ser. marcescens N28b-16(pGEMT-WaaEPm) (12), Ser. marcescens N28b-16(pGEMT-WaaEYe) (13) and Ser. marcescens N28b-16(pGEMT-WaaEEa) (14).

 
The core oligosaccharides were methylated as described by Ciucanu & Kerek (1984) and subjected to GLC-MS analysis. As expected from the known K. pneumoniae core structure (Vinogradov & Perry, 2001 ) 2,3,4-tri-O-methylglucitol, 2,3,4,6-tetra-O-methylgalactitol, 2,3,6-tri-O-methylgalactitol, 2,4,6-tri-O-methyl-L-glycero-D-manno-heptitol and 2,6,7-tri-O-methyl-L-glycero-D-manno-heptitol were identified among the methylation products obtained from the wild-type core LPS (Table 2). The waaE mutant core LPS fraction was characterized by the absence of 2,3,4-tri-O-methylglucitol and 2,3,4,6-tetra-O-methylgalactitol, and the presence of 2,4,6,7-tetra-O-methyl-L-glycero-D-manno-heptitol instead of 2,6,7-tri-O-methyl-L-glycero-D-manno-heptitol (Table 2). No differences in core LPS composition and methylated products were found between the wild-type strain and the waaE mutant complemented with the wild-type waaE gene (Table 2). These results show that the major effect of the waaE mutation is to preclude the attachment of a Glcp residue by a ß1,4 linkage to L,D-HeppI in the LPS inner core (Fig. 1). The waaE mutant LPS is devoid of Glcp and GalA1p residues, explaining its faster mobility in SDS-Tricine-PAGE. These results are in agreement with the initially proposed glucosyltransferase function for the K. pneumoniae WaaE protein (Regué et al., 2001 ).


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Table 2. Methylation analysis of core oligosaccharides released by mild acid hydrolysis from wild-type K. pneumoniae 889, 889-1 (waaE mutant) and 889-1 harbouring plasmid pGEMT-WaaEKp

 
Distribution of the waaE gene in members of the Enterobacteriaceae
In both K. pneumoniae and Ser. marcescens the waaE gene was found located between the waaA and coaD genes (Guasch et al., 1996 ; Regué et al., 2001 ). In the Enterobacteriaceae studied the coaD and fpg genes have been found just downstream from the waa gene cluster (Sofia et al., 1994 ; Heinrichs et al., 1998 ; Regué et al., 2001 ). To determine the extent of the presence of the waaE gene in clinical isolates of different Enterobacteriaceae, PCR DNA amplification experiments using genomic DNA and primers annealing to the waaA and fpg genes were used. Oligonucleotides NUE1 (5'-TACCGCCATCCGCTAAAAC-3') and NUE2 (5'-CGCTGGCAGAGTGTGAATT-3') were designed from the E. coli genomic sequence. Using genomic DNA from several E. coli strains, as well as several Sal. enterica serovars and Shi. dysenteriae, Shi. flexneri and Shi. sonnei strains as template, oligonucleotide pair NUE1/NUE2 generated PCR amplification DNA fragments of about 2·0 kb. The DNA nucleotide sequence of the amplified products was determined for at least one strain of each genus confirming the gene order waaA-coaD-fpg and the absence of the waaE gene in the tested strains of these species (Fig. 3).



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Fig. 3. Organization of the distal end of the waa gene cluster in members of the Enterobacteriaceae deduced from PCR genomic DNA amplification and determination of the nucleotide sequence of at least one strain from each species.

 
In addition, we designed oligonucleotides CNN5 (5'-AGGCGTGGTGGCAAACAAGAT-3') and FpgI (5'-AACCGTGGCTGATGGATAA-3') from the nucleotide sequence of the waa gene clusters from K. pneumoniae and Ser. marcescens. Using genomic DNA from Y. enterocolitica strains as template, primer pair CNN5-FpgI generated PCR amplification DNA fragments of about 3·0 kb. Oligonucleotide pairs KdtAF (5'-GGAAACAGAACTGTGGC-3') and FpgI, and Nuk9 (5'-TGCTTTATACCACCCTACT-3') and Nuk10 (5'-GATAAACGACCACTCTTTG-3'), generated PCR amplification DNA fragments of about 3·0 kb when using P. mirabilis and Ent. aerogenes DNA as template, respectively. The DNA nucleotide sequence of the amplified products was determined for P. mirabilis CECT170, Y. enterocolitica R102 and Ent. aerogenes CECT684. In these three strains waaE homologues were found between the waaA and coaD genes (Fig. 3).

An alignment of the known Enterobacteriaceae WaaA proteins was used to design degenerate oligonucleotide MRI (5'-ATGGRACYGARCTITGGCC-3'). The oligonucleotide pair MRI/Fpg1 allowed the amplification of the waaA-fpg region in all the strains of Enterobacteriaceae tested in this study, using genomic DNA as template. Amplified fragments of about 1·7 kb were obtained when using genomic DNA from strains of different serovars of Sal. enterica, E. coli, Shi. dysenteriae, Shi. flexneri and Shi. sonnei as template. In addition, amplified fragments of about 2·7 kb were obtained when using genomic DNA from different strains of K. pneumoniae, Ser. marcescens, P. mirabilis, Y. enterocolitica and Ent. aerogenes as template. Representative results of such amplification experiments are shown in Fig. 4. Thus, this oligonucleotide pair can be used to detect the presence of the waaE gene in members of the Enterobacteriaceae and to predict the presence of a Glcp residue attached via a ß1,4 linkage to the L,D-HeppI residue in the inner-core LPS (Fig. 1).



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Fig. 4. PCR-amplified DNA products obtained using oligonucleotides MRI and FPG1 and genomic DNA from E. coli (lane 1), Sal. enterica serovar Enteritidis (2), Shi. sonnei (3), K. pneumoniae (4), Ser. marcescens (5), P. mirabilis (6), Y. enterocolitica (7) and Ent. aerogenes (8). Lane 0, molecular mass standard (1 kb plus DNA ladder; Gibco-BRL).

 
Construction and preliminary analysis of Ser. marcescens N28b and P. mirabilis CECT170 waaE mutants
To construct waaE non-polar mutants in Ser. marcescens N28b and P. mirabilis CECT170, an in-frame tagged deletion was constructed using the previously mentioned plasmid pKO3. Plasmid pKO3{Delta}waaE was used to introduce the mutation into Ser. marcescens N28b and P. mirabilis CECT170 by double recombination (Link et al., 1997 ; Regué et al., 2001 ). Candidate mutants were screened by PCR and Ser. marcescens N28b-16 and P. mirabilis CECT170-8 were shown to contain the desired mutation by DNA nucleotide sequence determination of the waaA-coaD region. LPS from strains N28b (wild-type) and N28b-16 (waaE) were extracted and analysed by SDS-Tricine-PAGE. The core LPS from strain N28b-16 migrates faster than that of the wild-type strain and it appears that the mutant LPS still contains O antigen, although in smaller amounts than wild-type LPS (Fig. 2, lanes 8 and 9). This behaviour is identical to that found for K. pneumoniae waaE mutants (Regué et al., 2001 ). Similar results were obtained for P. mirabilis strains CECT170 (wild-type) and CECT170-8 (waaE).

Cross complementation among waaE homologues
Alignment of the known Enterobacteriaceae WaaE proteins revealed that all these proteins share a significant level of similarity (76%) and identity (53%). This similarity strongly suggests that these proteins perform the same function in the biosynthesis of the inner-core LPS. To confirm this functional relationship, the waaE gene homologues from Ent. aerogenes, Y. enterocolitica and P. mirabilis were cloned in vector pGEM-T. These recombinant plasmids were introduced into the waaE mutant strains K. pneumoniae 889-1 and NC16 (Regué et al., 2001 ), Ser. marcescens N28b-16 and P. mirabilis CECT170-8. The LPS from mutant strains 889-1 and N28b-16 transformed with pGEMT-WaaEKp, pGEMT-WaaESm, pGEMT-WaaEPm, pGEMT-WaaEYe and pGEMT-WaaEEa showed a wild-type migration pattern (Fig. 2). Similar results were obtained for K. pneumoniae NC16 and P. mirabilis CECT170-8. These results confirm that the waaE gene homologues perform the same role in inner-core LPS biosynthesis.

Contribution of the waaE gene to some pathogenic features
Since all the waaE homologues studied appear to have the same function, we decided to investigate the contribution of waaE genes to some pathogenic features involving surface molecules by using the set of isogenic mutants obtained. The waaE mutant strains K. pneumoniae 889-1 and NC16, Ser. marcescens N28b-16 and P. mirabilis CECT170-8 were sensitive to the bactericidal activity of NHS (non-immune human serum). The respective wild-type strains, as well as the mutants complemented with the waaE gene homologues, were resistant to this activity. As an example, Table 3 shows the results obtained for K. pneumoniae strains. Surface hydrophobicity of K. pneumoniae waaE mutant strain 889-1, measured by both the HIC and the BATH methods, was higher than the wild-type strain or the waaE mutant complemented with the waaE gene homologues. The Ser. marcescens and P. mirabilis waaE mutants were also more hydrophobic than their respective wild-type strains and the complemented waaE mutants.


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Table 3. Survival of exponential-phase K. pneumoniae strains in NHS

 
The physico-chemical properties of the bacterial surface are basic in the interactions (association or adhesion) between bacteria and the eukaryotic cells of the host tissues. Since K. pneumoniae, P. mirabilis and Ser. marcescens are important causes of nosocomial urinary tract infections, the adhesion of these strains to uroepithelial (UEC) cells was measured (Table 4). We found that waaE mutants showed an approximately two-fold reduction in their ability to adhere to UEC cells in comparison to their respective wild-type strains or the complemented mutants, again independent of waaE origin.


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Table 4. The adhesion of different K. pneumoniae, Ser. marcescens and P. mirabilis strains to UEC cells

 

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Table 5. Experimental urinary tract infection in rats by different K. pneumoniae, Ser. marcescens and P. mirabilis strains

 
To test if the changes in adhesion resulted in a lesser ability to infect and colonize, a urinary tract rat experimental infection model was used because it has been shown previously to be a good model to assay the in vivo colonization of some K. pneumoniae strains (Merino et al., 2000 ). In this assay the total number of infected animals and the viable bacteria found in kidney, bladder or urine were measured. The waaE mutants showed a drastic reduction in their ability to infect or colonize the urinary tract compared to the wild-type strains or the mutants complemented with the different waaE gene homologues (Table 4).

The role of the waaE gene in swarming motility and formation of biofilms
The results obtained in colonization of the rat urinary tract prompted us to examine if the lack of waaE was able to alter other surface molecules involved in bacterial colonization besides the LPS, for instance flagella. Using only the waaE mutants from Ser. marcescens and P. mirabilis, we studied their motility either on swimming or swarming agar plates. No differences could be observed in swimming plates, but a large swarming reduction of the waaE mutants in comparison to their wild-type strains or the mutants complemented with the different waaE gene homologues was found on the appropriate plates (see Fig. 5 for P. mirabilis results). However, when we studied the swarming cells using light or electron microscopy no differences were found between the waaE mutants and the wild-types or the waaE-complemented mutants. All the strains showed characteristic elongated and hyperflagellated cells.



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Fig. 5. The swarming motility phenotype of P. mirabilis strains CECT170 (wild-type, a) and CECT170-8 (waaE mutant, b) assayed as described in Methods.

 
In addition, the Ser. marcescens and P. mirabilis waaE mutants were also impaired in their ability to form biofilms when assayed by the method of Pratt & Kolter (1998) , with decreases of approximately 40% compared to their wild-types (Table 6). The introduction of the different waaE gene homologues into the mutants led to a full recovery of biofilm formation. In all these assays the same results were obtained for the waaE mutants and the corresponding mutants transformed with pGEMT.


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Table 6. Biofilm formation of Ser. marcescens and P. mirabilis strains quantified according to Pratt & Kolter (1998)

 

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ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
 
Structural studies of the core region of LPS from K. pneumoniae have revealed that all of them have very similar core structures with minor differences among different serovars (Vinogradov & Perry, 2001 ). In all the K. pneumoniae studied the core structure is characterized by the substitution of L,D-HeppI at the O-4 position by a Glcp [ß-D-Glcp-(1->4)-{alpha}-L,D-HeppI] and by the substitution of the L,D-HeppII at the O-3 position by an {alpha}-Kdo-(2->6)-{alpha}-D-GlcN-(1->4)-{alpha}-D-GalA trisaccharide (Vinogradov & Perry, 2001 ) (Fig. 1). Previously, we have suggested that the K. pneumoniae waaE gene encodes a glucosyltransferase involved in the transfer of a Glc residue by a ß1,4 linkage to L,D-HeppI in its inner-core LPS. This suggestion was based on the compositional analysis of a waaE mutant and complementation analysis of the waaE mutant with the Haemophilus ducreyi lgtF gene (Regué et al., 2001 ). In this work we confirmed waaE function by additional experimental evidence based on comparative composition and methylation analysis of the core region between wild-type K. pneumoniae 889 (O:8) and a non-polar waaE mutant (889-1). As expected from the K. pneumoniae core LPS structure, the major methylated products obtained from the core LPS of K. pneumoniae 889 corresponded to methylation products derived from residues L,D-HeppI (2,6,7-tri-O-methyl-D-glycero-L-manno-heptitol), L,D-HeppII (2,4,6-tri-O-methyl-D-glycero-L-manno-heptitol), L,D-HeppIII (2,3,4,6,7-penta-O-methyl-D-glycero-L-manno-heptitol), Glcp (2,3,4-tri-O-methylglucitol), GalpAI (2,3,4,6-tetra-O-methylgalactitol) and GalApII (2,3,6-tri-O-methylgalactitol) (Table 2). The presence of 2,3,4,6-tetra-O-methylgalactitol showed that strain 889 contains a substitution of Glcp at the O-6 position by a GalAp residue, a feature described for some strains of K. pneumoniae (Vinogradov & Perry, 2001 ). The absence of 2,3,4-tri-O-methylglucitol (Glcp) and 2,3,4,6-tetra-O-methylgalactitol (GalpAI) in core LPS obtained from strain 889-1 (waaE non-polar mutant) indicates that the major effect of the waaE mutation is to preclude the attachment of the Glcp residue to L,D-HeppI. Furthermore, 2,4,6,7-tetra-O-methyl-L-glycero-D-manno-heptitol, derived from O-4 non-substituted L,D-HeppI, instead of 2,6,7-tri-O-methyl-L-glycero-D-manno-heptitol, derived from O-4 substituted L,D-HeppI, was found in the core LPS of the waaE mutant. In addition, strain 889-1 transformed with pGEMT-WaaEKp generated core LPS methylated products identical to those obtained with core LPS isolated from the wild-type strain, as well as the presence of Glc. These results agree with a glucosyltransferase function for the WaaE protein.

Studies on the genetic organization of the Enterobacteriaceae genes involved in core LPS biosynthesis have showed that the waa gene cluster contains two conserved regions. One is in the proximal end of the cluster with three conserved genes, gmhD, waaF and waaC, encoding the last enzyme in the biosynthesis of L,D-Hepp, L-glycero-D-manno-heptosyltransferase II and L-glycero-D-manno-heptosyltransferase I, respectively (Heinrichs et al., 1998 ; Regué et al., 2001 ). There are two different models for the distal end of the waa gene cluster. In the five known E. coli core LPS types and in Sal. enterica serovar Typhimurium the waa clusters end with the waaA gene encoding the bifunctional Kdo transferase (Sofia et al., 1994 ; Heinrichs et al., 1998 ). In K. pneumoniae and Ser. marcescens waa clusters the waaE gene is found just downstream from the waaA gene (Guasch et al., 1996 ; Regué et al., 2001 ). In both models the coaD and fpg genes are found just downstream from the last waa gene, either waaA or waaE. A PCR assay for waaE homologues among clinical isolates of several species of Enterobacteriaceae suggested the presence of an extra gene in strains of P. mirabilis, Y. enterocolitica and Ent. aerogenes. No such waaE extra gene was detected among clinical isolates of E. coli, Sal. enterica serovars Typhimurium, Enteritidis and Infantis, and Shi. dysenteriae, Shi. flexneri and Shi. sonnei (Figs 3, 4). Furthermore, nucleotide sequence determination for a representative strain for each species confirmed the presence of waaE homologues flanked by the waaA gene and the coaD-fpg genes in P. mirabilis, Y. enterocolitica and Ent. aerogenes (Fig. 3). Analysis of the corresponding sequence strongly suggests that the waaA, waaE and coaD genes are cotranscribed and that fpg is transcribed in the opposite direction; these two features have also been found in K. pneumoniae (Regué et al., 2001 ). Genetic complementation studies showed that the identified waaE homologues are able to complement non-polar K. pneumoniae and Ser. marcescens waaE mutant strains, thus proving that all the waaE homologues perform the same function in inner-core LPS biosynthesis. These results agree with core LPS structural studies in P. mirabilis and Y. enterocolitica where a substitution of L,D-HeppI at the O-4 position by a Glcp [ß-D-Glcp-(1->4)-{alpha}-L,D-HeppI] has been found (Radziejewska-Lebrecht et al., 1994 ; Vinogradov et al., 2000 ). From the Enterobacteriaceae waa gene cluster distal end sequence data a set of primers was designed to amplify genomic DNA containing the 3' end of the waaA gene up to the 3' end of the fpg gene. These primers allow rapid screening for the presence of waaE homologues and can be used to predict the presence of a Glcp residue attached by a ß1,4 linkage to L,D-HeppI in the inner-core LPS of members of the Enterobacteriaceae, such as Ser. marcescens and Ent. aerogenes.

The contribution of the waaE gene to the different pathogenic features studied could be explained by the fact that these mutants showed a clear reduction in O-antigen LPS molecules (Fig. 2). It seems clear, at least in K. pneumoniae, that the O-antigen LPS is critical for serum resistance (Reid & Sobel, 1984 ; Figueroa & Densen, 1991 ; Merino et al., 2000 ), changes in surface hydrophobicity (Marshall, 1984 ; Merino et al., 2000 ), adhesion to UEC cells (Merino et al., 1997 , 2000 ), and urinary tract infection and colonization in rats (Nevola et al., 1987 ; Finlay & Falkow, 1989 ; Licht et al., 1996 ; Russo et al., 1996 ; Merino et al., 2000 ). We may conclude also that these facts could be extended to other enterobacteria, such as Ser. marcescens and P. mirabilis.

It has been reported that some Sal. enterica serovar Typhimurium mutants unable to show swarming motility (efficient colonization of the growth surface) with functional flagella (essential for this form of motility), showed defects in LPS biosynthesis (Toguchi et al., 2000 ). These authors suggested that the O-antigen LPS improves surface wettability required for swarm colony expansion. The role of the waaE gene in P. mirabilis and Ser. marcescens swarming phenotype could be attributed to O-antigen LPS reduction (surface wettability reduction) observed in the waaE mutants, while the flagella in both bacteria remain functional. Furthermore, we suggest that this altered swarming phenotype with a reduction of surface wettability could explain the reduced biofilm formation ability of the waaE mutants. Recently, Watnick et al. (2001) demonstrated that at least exopolysaccharide synthesis and flagella are needed for the efficient formation of a three-dimensional biofilm in Vibrio cholerae O:139.

All these roles that we attribute to the waaE gene are supported by the fact that well genetically characterized isogenic mutants were used, because all the waaE gene homologues were able to complement the waaE mutants, independently of their origin, and because the complemented mutants recovered the wild-type strain characteristics.

We cannot exclude that the inner-core change observed in the waaE mutants, the lack of substitution of L,D-HeppI at the O-4 position by ß-D-Glcp, could also play a role in swarming motility, biofilm formation and deficient pathogenic phenotypes showed by these mutants. Interestingly, both the WaaP and WaaE proteins are involved in substitution of L,D-HeppI at the O-4 position by a phosphate or pyrophosphoethanolamine (Yethon et al., 1998 , 2000 ) and by a ß-D-Glcp residue, respectively. The phenotype of the waaE mutants shares some similarities to E. coli F470 and Sal. enterica serovar Typhimurium waaP mutants. In all these cases a faster migrating LPS band is seen in Tricine-SDS-PAGE, the core LPS appears to be capped with O-antigen and the amount of O antigen is reduced in the mutants. In addition, in Sal. enterica serovar Typhimurium a waaP mutant showed reduced virulence in vivo (Yethon et al., 2000 ).


   ACKNOWLEDGEMENTS
 
This work was supported by Plan Nacional de I+D grants (Ministerio de Ciencia y Tecnología, Spain) and from Generalitat de Catalunya. L.I., N.A., N.C., B.H. and R.G. hold predoctoral fellowships from Generalitat de Catalunya, Ministerio de Ciencia y Tecnología (Spain) and Universitat de Barcelona. We also thank Maite Polo for her technical assistance.


   REFERENCES
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
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Received 21 March 2002; revised 16 July 2002; accepted 16 July 2002.