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Infection and Immunity, April 2002, p. 1791-1798, Vol. 70, No. 4
0019-9567/02/$04.00+0 DOI: 10.1128/IAI.70.4.1791-1798.2002
Copyright © 2002, American Society for Microbiology. All Rights Reserved.
Instituto de Bioquímica y Biología Molecular,1 Centro de Investigación y Desarrollo en Fermentaciones Industriales, Facultad de Ciencias Exactas, Universidad Nacional de La Plata, 1900 La Plata, República Argentina,2 Unité des Bordetella, Institut Pasteur, 75724 Paris Cedex 15, France3
Received 1 August 2001/ Returned for modification 1 November 2001/ Accepted 10 January 2002
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ABSTRACT |
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INTRODUCTION |
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B. bronchiseptica is currently acquiring relevance because of its increased importance as a human pathogen (20, 48). This microorganism colonizes the ciliated epithelium of the respiratory tract of the host and establishes chronic infections (20). It has been speculated that the development of such chronic infections may partially depend on the ability of bacteria to develop adaptive phenotypic changes in response to variable stimuli. This bacterial capacity could be related to two members of the two-component family of signal transduction protein, the bvg (Bordetella virulence gene) (4, 13, 16, 34) and ris (regulator of intracellular stress response) loci (5, 9, 20, 26). In agreement with this hypothesis, it was reported that in a patient with bronchopneumonia in whom B. bronchiseptica persisted over a period of 2 years, bacteria isolated from the initial period of infection produced toxins and adhesins (14, 22, 27), while successive isolates produced only adhesins (20).
Concomitant with this variation, the lipopolysaccharide structure shifts from a smooth to a rough/deep rough phenotype (21). The same LPS phenotypic change along the course of the infection was observed in another human patient with persistent B. bronchiseptica infection (unpublished results). Bacterial isolates obtained from different hosts also present different lengths of LPS molecules (29), pointing out that the LPS variation in vivo might be common in long-term infections. In addition, the observation that LPS structure varies in vivo suggests that this molecule plays different roles during the different stages of B. bronchiseptica infection (6, 23).
In other gram-negative bacteria, it was demonstrated that the LPS structure strongly influences the synthesis and/or secretion of certain proteins (7, 10, 15, 40, 44). In the case of Bordetella, current evidence indicates that the expression of both components, LPS and protein virulence determinants, is modified by the environment (13, 32, 34, 42). At present, however, the linkage between the different structures of the LPS, in particular the deep rough phenotype, with the expression of virulence factors and its significance in the whole infection process remain to be established.
In order to gain an insight into the role of LPS in B. bronchiseptica infection, we constructed the deepest possible rough LPS phenotype by site-specific insertional mutagenesis on the waaC gene, which codes for the glycosyltransferase responsible for the addition of the first heptose residue to 3-deoxy-D-manno-octulosonic acid (15, 31, 36). The B. bronchiseptica waaC mutant obtained was characterized within the framework of virulence determinant production and its in vitro and in vivo behavior.
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MATERIALS AND METHODS |
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(Bethesda Res. Lab) and S17-1 (37) were cultured in Luria-Bertani (LB) medium supplemented, when appropriate, with ampicillin or kanamycin at a final concentration of 200 or 25 µg ml-1, respectively. B. bronchiseptica strain 9.73 (Collection de l'Institut Pasteur designation) was grown on Bordet Gengou agar (Difco) supplemented with 15% (vol/vol) defibrinated fresh sheep blood (BGA medium) at 36°C for 48 h. Then it was replated in the same medium for 24 h. For mutant selection, BGA was supplemented with streptomycin (200 µg ml-1) and kanamycin (75 µg ml-1). For LPS extraction, subcultures were grown in Stainer-Scholte (SS) liquid medium (39) for 20 h at 36°C until the optical density measured at 650 nm reached 1.0. In order to label bacteria to be able to study bacterial infection of mice, we introduced plasmid pGB5P1 (45), which codes for kanamycin resistance and for the green fluorescent protein, into the wild-type B. bronchiseptica strain by conjugation. This plasmid was kindly provided by Alisson Weiss (Department of Molecular Genetics, Biochemistry and Microbiology, University of Cincinnati). For immunoblot analysis, bacteria grown in BGA were resuspended in saline at a concentration of 2 x 1010 CFU ml-1, diluted in Laemmli buffer (28), and boiled for 15 min.
PCR and recombinant DNA techniques. Based on previously reported consensus sequences (2, 3, 7, 16, 36) (B. bronchiseptica cosmid BbLPS1 AJ007747), we designed two single-stranded oligonucleotide primers, waaCpf (5'-TTCAICAGCCCCTG-3', where I is inosine) and waaCpr (5'-CCAGATTGACGGGT-3'). These two oligonucleotides were provided by DNAgency, Inc. (Malvern, N.Y.). We then amplified a 200-bp fragment corresponding to an internal sequence of the waaC gene. This PCR product was cloned into the shuttle plasmid pGem T-Easy (Apr, lacZ; Promega). After ligation, a recombinant fragment from this plasmid was released with EcoRI and cloned into the EcoRI site of the Bordetella suicide plasmid pK18mob (Kmr) (35).
We performed conjugation incubations in BGA plus 10 mM MgCl2, using B. bronchiseptica 9.73 Smr as the recipient and E. coli S17-1 containing the recombinant suicide plasmid as the donor, and thereafter colonies were selected for single genetic crossovers on kanamycin plus streptomycin. We then analyzed the detergent sensitivity of the resultant colonies of transconjugants on Stainer-Scholte solid medium (agar 1.5% [wt/vol]; Sigma Chemical Co.) supplemented with 0.02% (wt/vol) sodium dodecyl sulfate (SDS). Genomic DNAs from the SDS-sensitive colonies, hereafter referred to as B. bronchiseptica waaC mutants, were also probed by Southern hybridization for the presence of the expected DNA structure.
Southern hybridization. We performed Southern hybridizations (33) using a probe labeled with digoxigenin-conjugated waaCpf- and waaCpr-primed chain elongation products. This probe was synthesized by PCR as described above, except for the substitution of digoxigenin-dUTP (Boehringer Mannheim) for dTTP. For hybridizations, DNA extracted from the wild-type B. bronchiseptica 9.73 and from the B. bronchiseptica waaC mutant was digested and transferred to nitrocellulose strips (Hybond N; Amersham), as described by Chomczynski (11). We then hybridized the digoxigenin-labeled DNA probe to the membranes under the conditions specified by the manufacturer and, after blocking nonspecific binding sites, exposed the reacted strips to an antibody against the digoxigenin ligand (Boehringer Mannheim). In order to visualize the positive bands, the final color reaction was initiated at alkaline pH by the addition of colorless X-phosphate (Boehringer Mannheim) plus tetrazolium blue.
LPS extraction and SDS-PAGE. Cells grown at 36°C in Stainer-Scholte medium were centrifuged (10,000 x g, 15 min, 4°C) and washed twice in distilled water. After adjusting the bacterial concentration, we extracted the LPS either by the hot phenol-water method (46) along with the modifications previously described (24), or by affinity chromatography (41). The procedure employed is indicated in the legends to the figures. In both cases the isolated LPS was solubilized by heating at 100°C for 5 min in Laemmli sample buffer (28). We then applied the LPS suspensions to SDS gels. Gel acrylamide concentrations are indicated in the legends to the figures.
SDS-polyacrylamide gel electrophoresis (SDS-PAGE) was performed at room temperature and constant voltage. The LPS was visualized by the Bio-Rad silver-staining technique.
Detection of adenylate cyclase, pertactin, filamentous hemagglutinin, and LPS in Western immunoblots. Cells corresponding to 108 CFU of the B. bronchiseptica parental strain and the B. bronchiseptica waaC mutant were treated with Laemmli sample buffer (28), and the extracts were run on 8 to 25% (wt/vol) polyacrylamide-SDS gradient gels. Electrophoresis was performed at room temperature and constant voltage, with molecular weights being estimated by means of the Pharmacia calibration kit. After electrophoresis, the proteins were transferred to nitrocellulose membranes (Hybond C-Super; Amersham) and incubated with either a 1:10,000 dilution of hamster polyclonal immune serum directed against pertactin or a 1:5,000 dilution of mice polyclonal immune serum directed against adenylate cyclase or filamentous hemagglutinin of B. bronchiseptica. Mouse serum (1:500) collected 10 months after infection with wild-type B. bronchiseptica 9.73 was also assayed to test its reactivity against LPS isolated from either B. bronchiseptica 9.73 or B. bronchiseptica waaC mutants. In all cases, we used alkaline phosphatase-labeled sheep anti-mouse immunoglobulins to detect the presence of immune complexes.
To obtain the sera, the virulence factors were purified and subsequently inoculated into BALB/c mice as previously described (19, 20).
Complementation analysis. Parental strain B. bronchiseptica 9.73 DNA was partially digested with EcoRI and BamHI. The EcoRI- and BamHI-digested fragments were ligated into pJB3Tc (8), digested with EcoRI and BamHI, and introduced into competent E. coli S17-1 by transformation. Transformants were grown on LB containing tetracycline (6 µg/ml). Conjugation incubations were performed in BGA plus 10 mM MgCl2, using the B. bronchiseptica waaC (Smr, Kmr) mutant as the recipient and E. coli S17-1 containing the recombinant replicative plasmid (Tcr) as the donor and thereafter selected on BGA plus tetracycline and streptomycin.
We then analyzed the detergent sensitivity of the resultant colonies of transconjugants on Stainer-Scholte solid medium supplemented with 0.02% (wt/vol) SDS. From an SDS-resistant colony, recombinant plasmids, hereafter referred to as pJB3FS, were isolated. The presence of the waaC gene in such plasmids was analyzed by PCR and Southern hybridization.
To clone the waaC gene, the whole gene was amplified by PCR from recombinant plasmid pJB3FS using primers totalwaaCf (5'-TGC GAA TTC CCA GCA TGT CGC TGA G-3') and totalwaaCr (5'-GCA TGC ACC CAG ACC GAA TTC C-3'). The 1,529-bp PCR product was ligated first into pGem-T-Easy (Promega) and then into pJB3Tc. The last recombinant plasmid was introduced into competent E. coli S17-1 by transformation, and the transformants were grown on LB containing tetracycline (6 µg ml-1). Conjugation incubations were performed as described above using the B. bronchiseptica waaC (Smr, Kmr) mutant as the recipient and E. coli S17-1 containing the recombinant replicative plasmid (Tcr) as the donor. The transconjugants were thereafter selected on BGA plus tetracycline (18 µg ml-1) and streptomycin (200 µg ml-1). We then analyzed the detergent sensitivity of transconjugants on Stainer-Scholte solid medium supplemented with 0.02% (wt/vol) SDS and the electrophoretic profile in SDS-PAGE.
Murine respiratory infection model. Female BALB/c mice 3 to 4 weeks of age were used as a model of in vivo respiratory infection by B. bronchiseptica. Bacteria grown on BGA were resuspended and adjusted to approximately 107 CFU ml-1 in phosphate-buffered saline (PBS). Fifty microliters of bacterial suspension was delivered intranasally to each mouse via an air displacement pipette. At different times postinoculation, three mice from each group were sacrificed, and their lungs were removed aseptically. Lungs were homogenized in PBS, and appropriate dilutions were plated onto BGA to determine the number of viable bacteria present in the lungs. All the experiments were repeated three times and gave consistent results.
Tissue culture and determination of bacterial adhesion and persistence. The human alveolar cell line A549 (ATCC CCL185) was used in this study. Cells were cultured in Dulbecco's modified Eagle's medium (Gibco Laboratories) supplemented with 10% fetal calf serum, streptomycin (100 µg ml-1), and ampicillin (100 µg ml-1) to 70 to 80% confluence. Twenty-four-well Nunclon Delta tissue culture plates (Nunc, Roskilde, Denmark) were seeded with approximately 8 x 104 cells per well 18 h before the assay. For adhesion assays, cells were seeded on glass coverslips previously placed in the selected wells.
Bacterial strains (either wild type or mutant) were grown for 16 h on SS medium, washed, and suspended in Dulbecco's modified Eagle's medium supplemented with 10% fetal calf serum without antibiotics to an optical density of 0.5 at 650 nm. Then 8 x 106 bacteria in 0.5 ml were added to the wells (ratio of bacteria to cells, 100:1) and centrifuged onto adherent cells at 300 x g. After 2 h at 37°C in 5% CO2, the monolayers were washed at least five times with PBS (pH 7.2).
To determine bacterial adhesion, the cells grown on glass coverslips with attached bacteria were fixed in methanol, and the staining of cells and bacteria was done with crystal violet (0.07% in water). Examination was carried out using phase-contrast microscopy at 1,000x magnification. Approximately 50 cells were examined to calculate the number of adherent bacteria per epithelial cell. All experiments were done at least three times in duplicate.
To determine bacterial survival within the alveolar cell, after the 2-h incubation period and the extensive washing described above, the monolayers were further incubated for 3 h at 37°C in 5% CO2 to allow bacterial invasion. The medium was then replaced with 0.5 ml of complete medium containing 100 µg of polymyxin B ml-1 and then incubated for 1 h at 37°C in 5% CO2 to kill extracellular bacteria (9). When B. bronchiseptica alone was treated with this level of polymyxin B, 99.999% of the bacteria were killed over a 1-h period.
Following incubation, the polymyxin B was removed by extensive washing and replaced by 0.5 ml of Dulbecco's modified Eagle's medium supplemented with 10% fetal calf serum, streptomycin (100 µg ml-1), and ampicillin (100 µg ml-1). The medium was changed every 24 h. At selected time periods, cells containing viable intracellular organisms were recovered from the trypsin-treated and stripped monolayers. Trypan blue dye exclusion was used to check eukaryotic cell viability. Intracellular bacteria were counted by plating appropriate dilutions onto BGA. The number of CFU per alveolar cell was then calculated. Each strain was tested in triplicate.
Statistical analysis. Means and standard deviations were calculated from log10-transformed numbers of CFU. Differences between means were assessed by two-tailed Student's t tests, with significance accepted at the P < 0.05 level.
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RESULTS |
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We generated a deep rough mutant by effecting a site-specific integration through conjugation with E. coli containing the recombinant plasmid pK18mob waaC (Fig. 1A). Since detergent sensitivity should also become altered in such a deep rough mutant (36), we confirmed that the resulting streptomycin- and kanamycin-resistant genomic transconjugants were unable to grow on Stainer-Scholte solid medium supplemented with 0.02% (wt/vol) SDS.
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For the following studies, we selected one of the confirmed waaC mutants, hereafter referred to as the B. bronchiseptica LP39 mutant.
LPS electrophoresis and immunoblotting. We analyzed the LPS phenotype of B. bronchiseptica LP39 by SDS-PAGE using silver staining (Fig. 2A). The LPS profile from B. bronchiseptica LP39 exhibited no O-antigen band; moreover, only a single LPS band was present, which migrated considerably faster than the corresponding wild-type species. These electrophoretic properties are consistent with the deep rough phenotype. Furthermore, when we carried out immunoblots of these LPS species using sera from B. bronchiseptica-infected mice, we observed that the wild-type LPS was serologically recognized, whereas the mutant LPS was not (Fig. 2B).
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We then performed a semiquantitative analysis of the expression of virulence determinants by Western blotting, using mouse polyclonal immune sera directed against purified adenylate cyclase, pertactin, and filamentous hemagglutinin (Fig. 3). Although a decrease in both filamentous hemagglutinin and pertactin could be observed in the B. bronchiseptica LP39 mutant lysates, in the case of pertactin the decrease in intensity was especially notable (Fig. 3).
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To determine whether the first-inoculated strain precludes the colonization of the second one, we performed an independent coinoculation experiment in which the second inoculation was performed with a labeled parental strain carrying plasmid pGB5P1 as a marker, to differentiate it from the first inoculum. In this case, the colonization kinetics of labeled B. bronchiseptica exhibited the observed pattern of parental strain infection (data not shown).
We also varied the delay (5 to 20 days) between the first and second inoculum. Interestingly, we did not detect significant changes in the results obtained with either the mutant or the labeled wild-type strains even in those experiments, in which the second inoculation was performed as late as 20 days after the first one. These data seem to indicate that under our experimental conditions, no protection was induced after 20 days of infection.
Adhesion and intracellular survival of parental and B. bronchiseptica LP39 mutant strains. In vitro assays demonstrated that the adherence of B. bronchiseptica LP39 to human pulmonary epithelial cells (5 ± 4 bacteria/epithelial cell) was significantly lower than that exhibited by the parental strain (62 ± 25 bacteria/epithelial cell) (Fig. 6). The waaC mutant also showed a lower invasion rate than the parental strain. This result was more likely to be caused by the reduced number of attached bacteria than by a defect in bacterial invasive ability. However, this last possibility could not be ruled out in our system.
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DISCUSSION |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
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Here we have approached the construction of a waaC mutant of B. bronchiseptica on the basis of genetic alterations that in other gram-negative bacteria have been shown to produce a deep rough LPS structure compatible with bacterial viability. The resulting waaC mutation of B. bronchiseptica was not lethal, and the altered bacteria were still able to produce detectable levels of the major virulence factors adenylate cyclase, filamentous hemagglutinin, and pertactin (Fig. 3) and responded to well-known modulators of the bvgAS system, such as low temperature and sulfate ion. These observations indicate that the bvgAS sensor and response regulator functions remain unaltered even within the context of a deep rough LPS mutation.
We observed that the mutation in the waaC gene of B. bronchiseptica reduced the expression of pertactin and to a lesser extent of filamentous hemagglutinin. Since all of these proteins are related to the outer membrane, the alteration of this cellular compartment by changes in the structure of the LPS could explain the observed protein reduction levels. This decrease in virulence factors might favor the persistence of the bacteria within the host during chronic manifestations of B. bronchiseptica infection. Indeed, the production of bvg-activated gene products may be a disadvantage in terms of the intracellular survival of the bacteria, since the expression of certain virulence factors is highly toxic to the infected eukaryotic cells (1, 5, 17).
Interestingly, the LPS from the mutant B. bronchiseptica LP39 was immunologically different from the wild-type LPS. Accordingly, the waaC mutation that exhibits no O-antigen determines a complete loss of LPS serologic reactivity (Fig. 2B), including that of the higher-mobility component, which is strongly reactive within the LPS of the parental strain (Fig. 2B). These results resemble those observed during chronic infections in which sera that still react with intact B. bronchiseptica LPS isolated from early stages of infection failed to immunologically recognize the deep rough LPS of bacteria isolated from the same human infections (data not shown).
Our results from the murine respiratory infection model clearly demonstrated that the deep rough phenotype of the LPS diminishes B. bronchiseptica's ability to colonize mice. The B. bronchiseptica LP39 mutant was cleared from lungs within 5 days, whereas the parental strain persisted for at least 30 days postinfection (Fig. 5). This decrease in colonization ability could not be overcome by coinfection experiments with the parental strain, indicating that the waaC phenotype cannot be complemented with wild-type LPS, which has been shown to be released into the surrounding medium (25).
These data point out that the B. bronchiseptica LP39 mutant, either alone or in combination with the wild-type strain, is unable to colonize mouse lungs and suggest that the observed deep rough LPS phenotype isolated from chronic infections (20, 21) would not come from mutants that infected the host from the beginning. By contrast, these deep rough bacteria would have arisen from already established infections.
In agreement with this inability to colonize, the B. bronchiseptica LP39 mutant attaches to pulmonary cells less efficiently than the wild-type strain. However, persistence kinetics of both strains within the eukaryotic cells showed similar patterns over the time period studied. Figure 7 shows an initial decrease in bacterial survival, which could suggest a lag period in which the bacteria have to adapt to environmental conditions, followed by a significant increase in the number of live bacteria per eukaryotic cell over time.
These results suggest that both strains of B. bronchiseptica are able not only to survive but also to replicate inside pulmonary cells. After 14 days, significant eukaryotic cell death was detected. A similar cell death rate was found in both infected and noninfected (control) cells, indicating that death was not caused by B. bronchiseptica infection. Due to these circumstances, in vitro studies could not be performed for more than 2 weeks, which excludes the possibility of a long-term survival comparison of the strains.
According to our results, the truncated form of B. bronchiseptica LPS isolated from chronic human infections and animal infections (20, 21, 29) can only be explained if we assume that they derive from smooth forms which were able to colonize. The concomitant modification of the expression pattern of the principal virulence factors and the complete loss of LPS serologic reactivity in the waaC mutant underscore the idea that structural changes that could occur during infection help bacteria to persist within the host.
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ACKNOWLEDGMENTS |
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We are grateful to Donald F. Haggerty and Christina McCarthy for critical reading and editing of the manuscript.
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FOOTNOTES |
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