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1 Dipartimento di Patologia Sperimentale, Biotecnologie Mediche, Infettivologia ed Epidemiologia, Università di Pisa, via San Zeno 37, 56127 Pisa, Italy
2 Dipartimento di Biologia, Sezione di Microbiologia, Università di Pisa, via San Zeno 35, 56127 Pisa, Italy
3 Dipartimento di Morfologia Umana e Biologia Applicata, Università di Pisa, via Roma 55, 56126 Pisa, Italy
Correspondence
Sonia Senesi
ssenesi{at}biologia.unipi.it
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMETHODSRESULTSDISCUSSIONREFERENCES |
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The GenBank/EMBL/DDBJ accession number for the flhF sequence reported in this study is AM180264.
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONREFERENCES |
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; Dasgupta et al., 2000; Kusumoto et al., 2006). flhF has been recognized to be necessary for the placement of flagella at a cell pole in Pseudomonas putida (Pandza et al., 2000) and P. aeruginosa (Murray & Kazmierczak, 2006), as well as for flagellation in Vibrio cholerae (Correa et al., 2005), Campylobacter jejuni (Hendrixson & DiRita, 2003) and Helicobacter pylori (Niehus et al., 2004). In Bacillus subtilis, a newly identified gene, swrAA, governs the degree of cell flagellation (Calvio et al., 2005), while the role of flhF, also present in this peritrichous species, remains to be clarified. In fact, depending upon the genetic background of B. subtilis, flhF disruption has been reported to differently affect the motility phenotype of this species (Carpenter et al., 1992; Zanen et al., 2004). In all bacterial species studied so far, the protein encoded by flhF shares substantial homology with proteins of the GTP-binding signal recognition particle (SRP) family (Carpenter et al., 1992; Kim & McCarter, 2000; Pandza et al., 2000), such as Ffh and FtsY. However, while Ffh and FtsY are required for targeting many secretory and membrane proteins to the plasma membrane (for a review see Halic & Beckmann, 2005), an involvement of FlhF in protein secretion has been demonstrated only in P. putida (Pandza et al., 2000).
Despite the observation that the arrangement of bacterial flagella is conserved over generations, this feature can vary following cell adaptation to different environmental conditions. For several flagellated bacteria, surface sensing is an environmental signal inducing swarming differentiation, a specialized form of flagellum-driven motility that is exhibited by a wide range of species when transferred from liquid to solid growth media (Allison & Hughes, 1991; Ghelardi et al., 2002; Kearns & Losick, 2003; McCarter, 1999; Senesi et al., 2002). This surface-induced motility is dependent on the production of differentiated swarm cells that are aseptate, multinucleate and elongated, and express a higher number of peritrichously placed flagella than the corresponding oligoflagellated swimmer cells (for a review see Fraser & Hughes, 1999). The swarm cells possess the unique ability to migrate away from the colony in organized groups of tightly bound cells that coordinately move across solid surfaces, including host mucosal surfaces during colonization by swarming-proficient infectious agents (for a review see Fraser & Hughes, 1999). In Bacillus cereus, the ability to swarm contributes to the severity of experimental endophthalmitis in rabbits, being responsible for a very rapid invasion of the anterior segment of the eye (Callegan et al., 2006).
The interest in studying bacterial flagella has been substantially increased following the demonstration that the flagellum should not be regarded only as a locomotion organelle. Indeed, it contains a sophisticated secretory apparatus, whose components share substantial homology with those of the type III secretion system described exclusively in Gram-negative bacteria (for a review see Hueck, 1998). In Escherichia coli, Yersinia enterocolitica and C. jejuni the flagellar export apparatus is required for the secretion of specific virulence-associated proteins (Givskov & Molin, 1993; Konkel et al., 2004; Young et al., 1999a). In Bacillus thuringiensis, the closest relative of B. cereus (Helgason et al., 2000), the flagellar type III secretion system is required for secretion of haemolysin BL (HBL), a tripartite heat-labile toxin showing haemolytic, enterotoxic and dermo-necrotizing activities, as mutants lacking flagella do not export the intracellularly produced toxin (Ghelardi et al., 2002). In this study, we describe the flhF gene in the peritrichous flagellate species B. cereus and give evidence showing its role in regulating degree of cell flagellation, motility behaviour and protein secretion.
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METHODS |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONREFERENCES |
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. Bacteria were grown at 37 °C in either tryptone-NaCl (TrB; 1 % tryptone, 0.5 % NaCl), brain heart infusion supplemented with 0.1 % glucose (BHIG), Luria–Bertani (LB) or HBL sheep blood agar (Beecher & Wong, 1994). Media were routinely solidified with 1 % bacteriological agar, unless otherwise specified. For selection of B. cereus transformants, appropriate antibiotics (5 µg erythromycin ml–1 or 30 µg kanamycin ml–1) were added to the media. The E. coli strain TOP10 (Invitrogen) was used for general cloning strategies while the strain SCS110 (Stratagene) was used as an intermediary host in B. cereus transformation experiments. E. coli strains were grown at 37 °C in LB supplemented with ampicillin (100 µg ml–1) when required.
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flhF1 or pRNflhF2 in flhF of B. cereus ATCC 14579, thus producing strains MP05 and MP06, respectively (Table 1
and Fig. 2c). For plasmid construction, B. cereus ATCC 14579 chromosomal DNA was amplified using the primers flhFEcoU1/flhFHindL2 for pRNflhF1 and flhFBamU2/flhFHindL4 for pRNflhF2 (Table 2, Fig. 2a). PCR fragments were purified, digested, and separately cloned in the thermosensitive plasmid pRN5101 (Lereclus et al., 1995), thus producing pRNflhF1 and pRNflhF2, which were used to transform B. cereus ATCC 14579 by electroporation. Erythromycin-resistant transformants were isolated by growing cells at 28 °C on LB agar plates containing erythromycin. Transformants were pooled, transferred into LB broth, and incubated at a non-permissive temperature for plasmid replication (40 °C). flhF disruption was verified by PCR with the primers flhFU2 and flhFL2 (Table 2, Fig. 2a) and by DNA sequencing.
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, Fig. 2a). The 1351 bp PCR product was cleaved with XbaI and SphI and ligated into the expression vector pDG148 (Stragier et al., 1988). The recombinant plasmid (pDGflhF), carrying the flhF coding region under the transcriptional control of the inducible spac promoter (Pspac), was used to transform strain MP06 or MP05. After selection for kanamycin resistance, strains MP07 and MP09 were respectively isolated. For overexpression experiments, pDGflhF was introduced into B. cereus ATCC 14579 to produce the strain MP08. Pspac was induced by adding IPTG (final concentration, 2 or 4 mM) to bacterial cultures.
Motility and chemotaxis assays.
Swimming and swarming motility were examined as previously described (Senesi et al., 2002). Briefly, for swimming motility assays, 0.5 µl of an overnight culture (approx. 2x108 cells ml–1) was spotted onto the centre of TrB plates solidified with 0.25 % agar (TrM), and growth halo diameters were measured after 6–8 h of incubation at 37 °C in a humidified chamber. Swimming motility was also evaluated under a phase-contrast microscope (BH-2; Olympus) by measuring the time of the smooth swimming phase between two consecutive tumblings. Isolated cells (n=200) were analysed in each sample for a total of five experiments performed on separate days. Flagellar staining for light microscopy was performed as described by Harshey & Matsuyama (1994) and several samples were analysed at 1000x magnification using an optical microscope (BH-2; Olympus). The ability of bacteria to undergo chemotaxis was determined by recording, at different time intervals during incubation, the growth halo diameter (Da) on TrM plates supplemented with 2 mM mannitol in comparison to the growth haloes measured in TrM without attractant (Dc). Each assay was repeated five times on separated days and the chemotaxis index (CI) for each strain was calculated as follows: CI=(Da–Dc)/Dc .
Assays for swarming motility were initiated by spreading 50 µl of a culture containing approximately 2x104 cells ml–1 onto TrB plates containing 1 % agar (TrA) and incubating plates at 37 °C. Formation of swarm cells was evaluated by analysing cell length and the amount of cell surface flagellin at different time intervals during growth (Senesi et al., 2002). Cell length was measured by phase-contrast microscopy of bacteria stained with a solution containing 10 % (w/v) crystal violet. The extent of cell flagellation was measured as previously described by subjecting purified extracellular flagellin to protein gel electrophoresis (Calvio et al., 2005). The intensity of the flagellin band, at different time intervals during growth, was measured by densitometric analysis using the Image Master 1D software (Pharmacia Biotech). For each strain the increase was calculated as follow: IF=(It–It0)/It0, where It is the intensity of the flagellin band at different times (2, 4, 6 and 8 h) post-inoculation and It0 is the intensity of the band at 2 h post-inoculation.
Electron microscopy.
Bacteria were grown overnight in 5 ml LB broth, diluted (1 : 250) into fresh LB medium and then grown for 6 h at 150 r.p.m. Two drops of bacterial culture were subsequently placed onto the same Formvar-coated copper grid. After 1 min, excess liquid was wicked off without completely drying the grid to avoid flagellum shearing. The grids were negatively stained with 2 % (w/v) phosphotungstic acid for 3 min and air-dried. Samples were analysed using a transmission electron microscope (JEOL 100SX). To quantify flagella per cell, 200 cells of each strain were examined; flagella were counted only in well-isolated cells.
Aldolase activity.
The fructose-1,6-bisphosphate aldolase (EC 4 . 1 . 2 . 13) activity was assayed in culture supernatants and cell lysates as described by Warth (1980).
RNA isolation and RT-PCR analysis.
Total RNA was extracted from B. cereus cultures with the RNeasy Mini Kit (Qiagen) modified as previously described (Ghelardi et al., 2002). The concentration, integrity and purity of RNA samples were determined spectrophotometrically and by visual inspection of agarose gels. Purified RNA (100 ng) was used as a template in one-step RT-PCR with the cMaster RTplusPCR system (Eppendorf), according to the manufacturer's instructions. The primers used in different reactions, listed in Table 2, were: flhFU2/flhFL2, for the amplification of flhF cDNA; flhFU2/flhFHindL2, to amplify the first 1120 bp of the flhF cDNA; flgGU1/flgGL1, for the amplification of flgG cDNA; fAint1/flhFHindL4, for the amplification of a cDNA fragment containing both flhA and flhF; and flhFU3/flgGL2, for the amplification of a cDNA fragment containing both flhF and flgG. The RT reactions were carried out at 42 °C for 70 min in a thermal cycler, immediately followed by 30 cycles of PCR, consisting of 15 s at 94 °C, 20 s at 58 °C and 1 min at 68 °C, and ending with a 7 min incubation at 68 °C. To detect DNA contamination in RNA samples, reactions in which RT enzyme was replaced by RNase-free water (negative controls) were performed. Positive controls were obtained using genomic DNA as template. The RT-PCR products were analysed by agarose gel electrophoresis.
Protein samples and gel electrophoresis.
Protein samples were prepared by growing bacterial cells to the late exponential growth phase in BHIG at 200 r.p.m. for 6 h at 30 °C. Cultures were normalized to the same OD600 and aliquots of the resulting bacterial suspensions were pelleted by centrifugation at 10 000 g. Culture supernatants were collected and cell lysates prepared as previously described (Ghelardi et al., 2002). Protein concentration was determined by the bicinchoninic acid (BCA) assay (Smith et al., 1985), with BSA as a standard. Protein samples were suspended in NuPAGE LDS sample buffer (Invitrogen), heated at 72 °C for 10 min, and subjected to SDS-PAGE. Gels were either silver stained or electrotransferred to nitrocellulose membranes, using the XCell II Blot Module system (Invitrogen), for immunoblot analysis.
Detection of phosphatidylcholine-preferring phospholipase C (PC-PLC), HBL and non-haemolytic enterotoxin (NHE).
For detection of virulence factors, whole or diluted protein samples were used. PC-PLC activity was measured by a gel-diffusion assay with a gel containing crude phosphatidylcholine (PC), as previously described (Ghelardi et al., 2002). Different amounts of pure PC-PLC (Sigma-Aldrich) were used to generate a standard calibration curve. NHE was detected by using the commercial Bacillus Diarrhoeal Enterotoxin (BDE) visual immunoassay kit (Tecra Diagnostics) according to the manufacturer's instructions. Sample absorbance was measured at 405 nm (A405) using a plate reader. HBL activity was assessed on sheep blood agar plates by the formation of a discontinuous zone of haemolysis around colonies (Beecher & Wong, 1994), while individual HBL components were detected by immunoblot analysis. Nitrocellulose membranes (prepared as described above) were probed with rabbit polyclonal antibodies recognizing individual HBL components, followed by incubation with a secondary antibody conjugated with horseradish peroxidase. The peroxidase activity was visualized by diaminobenzidine colorimetric reaction in accordance with standard procedures (Sambrook et al., 1989).
Statistical analyses.
Statistical analyses were performed with InStat (GraphPad Software). A P value <0.05 was considered significant. Values were expressed as the mean±SD from five independent experiments.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONREFERENCES |
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; Correa et al., 2005; Dasgupta et al., 2000; Hendrixson & DiRita, 2003; Kusumoto et al., 2006; Niehus et al., 2004; Pandza et al., 2000) prompted us to evaluate whether homologues of such genes were present in B. cereus. In silico analysis was performed on the B. cereus ATCC 14579 genome with the ERGO Genome and Discovery System (Overbeek et al., 2003) from Integrated Genomics and the BacMap Bacterial Genomes Atlas (Stothard et al., 2005). An flhF homologue was identified, but no homologues of fleN/flhG and swrAA were found. The B. cereus flhF nucleotide sequence available in databases showed homologies with flhF of other organisms; however, a stop codon (codon 301) internal to the B. cereus flhF ORF was identified. We presumed that this unexpected stop codon derived from an erroneous annotation of the sequence and therefore decided to revisit the sequence of the B. cereus flhF ORF. A chromosomal region comprising flhF was sequenced from B. cereus ATCC 14579 and compared with the flhF sequence present in the database. Sequence analysis revealed that two additional base pairs were present in the annotated sequence (nucleotides 889 and 1001), thus leading to a frame-shift that produced an erroneous stop codon. The new sequence of the B. cereus ATCC 14579 flhF gene was deposited in the EMBL database (GenBank accession no. AM180264).
B. cereus flhF encodes a putative protein of 439 aa, having an estimated molecular mass of 50.2 kDa and an iso-electric point of 5.93. No predicted signal peptide sequence or transmembrane domains were found. As shown by sequence alignments and domain searches, B. cereus FlhF contains a conserved C-terminal and consensus GTP-binding motif (G-domain), which is typical for the proteins belonging to the SRP family (Fig. 1). The N-terminal sequence of B. cereus FlhF is less conserved compared to other bacteria, but a basic lysine-rich region (B-domain), described also for B. subtilis FlhF (Zanen et al., 2004), was found from aa 125 to aa 173 (Fig. 1).
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). The flhA gene, which encodes a membrane protein that is part of the flagellar export apparatus (Ghelardi et al., 2002; Minamino & Macnab, 1999), is located upstream of flhF in B. cereus, as well as in many other bacteria (Carpenter et al., 1992; Dasgupta et al., 2000; Kim & McCarter, 2000; Kusumoto et al., 2006; Pandza et al., 2000). In contrast, only in B. cereus, the gene downstream of flhF is flgG, which encodes the flagellar basal body rod protein in many micro-organisms (Minamino & Macnab, 1999; Zuberi et al., 1991).
To evaluate whether flhF was part of a transcriptional unit also containing flhA or flgG, RT-PCR analysis was carried out on total RNA purified from B. cereus ATCC 14579. No amplification products were obtained using the primer pairs fAint1/flhFHindL4 or flhFU3/flgGL2, that were designed on the sequence of flhA/flhF and flhF/flgG, respectively (Fig. 2b, lanes 1 and 5). An amplification product of the predicted size was obtained using the primer pair flhFU2/flhFL2, designed on the sequence of flhF (Fig. 2b, lane 3). Thus, differently from other bacteria in which flhF is part of a large operon including flagellar and chemotaxis genes (Carpenter et al., 1992; Kim & McCarter, 2000), the B. cereus flhF constitutes a monocistronic transcriptional unit.
Disruption of flhF causes defects in the number and localization of flagella
flhF has a crucial role in flagellar biogenesis of polar flagellate bacteria (Correa et al., 2005; Hendrixson & DiRita, 2003; Kusumoto et al., 2006; Niehus et al., 2004; Pandza et al., 2000), whereas in the peritrichous species B. subtilis, flhF has been reported to be either dispensable (Zanen et al., 2004) or required (Carpenter et al., 1992) for cell flagellation, mainly depending upon the genetic background of the strain. To evaluate whether flhF was involved in the control of cell flagellation in B. cereus, an flhF insertional mutant (MP06) and a complemented strain (MP07) carrying a plasmid harbouring flhF under the Pspac transcriptional control were generated. Disruption of flhF in MP06 did not cause polar effects on the expression of flgG, which is immediately downstream of flhF, as demonstrated by the presence of a flgG transcript detected by RT-PCR analysis with the primers flgGU1 and flgGL1 (Table 2) (data not shown).
Phenotypic analysis of bacteria propagated in liquid media revealed that strain MP06 was characterized by a remarkable reduction in the number of flagella (1–3 per cell) in comparison to wild-type (10–12 per cell) (Fig. 3a); moreover, electron microscopy showed that in mutant MP06, flagella were always localized in polar or peri-polar positions compared to the peritrichous distribution observed in the wild-type (Fig. 3c). The wild-type phenotype was completely restored in strain MP07, after induction of the Pspac promoter with 2 mM IPTG (Fig. 3a). The altered ability of MP06 to produce flagellar filaments was also confirmed by SDS-PAGE analysis of purified flagella: the protein band corresponding to extracellular flagellin was less abundant in the mutant than in the wild-type and complemented strains (Fig. 3b). These observations indicated that disruption of B. cereus flhF determined a loss in the ability to control the arrangement of flagella on the cell surface in a peritrichous bacterial species.
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). Thus we addressed the question of whether a lack of flhF caused metabolic or growth defects leading, directly or indirectly, to an impairment in flagella assembly. Therefore, both the biochemical profile and the ability to assemble flagella at fixed time intervals during growth were compared in B. cereus ATCC 14579, MP06 and MP07. No difference was revealed in the ability to use carbohydrates or amino acids for growth by using the API 50-CHB and API 20E strip tests (bioMérieux). All strains showed overlapping growth curves (Fig. 4) and comparable percentages of flagellated cells at mid-exponential, late-exponential or stationary phases (Fig. 4). Remarkably, no cell harbouring peritrichous flagella or more than three flagella was observed in mutant MP06.
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). Microscopic examinations of bacteria by the hanging-drop method revealed that mutant MP06 maintained the directional movement exhibited by the peritrichous flagellated wild-type. However, smooth swimming phases were shorter in the mutant strain than in the wild-type, the time between two consecutive tumblings being 2.96±0.35 s and 7.36±1.17 s, respectively (P=0.0013). Despite exhibiting an altered motility phenotype, MP06 was not defective in chemotactic response, since similar chemotaxis indexes were obtained for all strains toward mannitol, which acts as a strong attractant for B. cereus (Fig. 5b). Taken together, these results suggest that the altered motility of MP06 was exclusively due to the reduced number and defective arrangement of flagella displayed on the cell surface.
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To evaluate whether flhF was required for B. cereus to undergo surface-induced swarming differentiation, we measured cell length and the amount of cell-surface flagellin in strains ATCC 14579, MP06, and MP07 at fixed time intervals during growth on TrA plates (Fig. 6). SDS-PAGE analysis of purified flagella revealed an increase in the level of extracellular flagellin in the wild-type and MP07 strains during the first 8 h post-inoculation (Fig. 6a). In contrast, levels of flagellin from strain MP06 remained unchanged at all time points examined (Fig. 6a). Moreover, microscopic examination of cells collected from growing colonies revealed a progressive increase in the percentage of elongated cells in both the wild-type and MP07, while the mutant never exhibited cells with an elongated phenotype (Fig. 6b). These observations indicate that MP06 is unable to undergo swarming differentiation and suggest that flhF is involved in a common pathway controlling the number of flagella in swimmer as well as in swarmer cells.
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), while in B. subtilis, such a gene appears to be dispensable for its secretory properties (Zanen et al., 2004). To evaluate whether flhF could influence protein secretion in B. cereus, culture supernatants of B. cereus ATCC 14579 and MP06 (see Methods for details) were analysed by SDS-PAGE. The levels of proteins secreted by mutant MP06 appeared increased compared to the wild-type strain (Fig. 7). Protein quantification by the BCA assay revealed that the proteins in the supernatants of MP06 were about 20 % higher than that of the parental strain. To rule out that MP06 was subjected to cell lysis, supernatants and cell lysates of both strains were assayed for the presence of fructose-1,6-bisphosphate aldolase, a cytoplasmic enzyme that is not released by intact cells. No aldolase activity was detected in culture supernatants, while identical enzyme activity was observed in cell lysates (0.08 U ml–1). This result indicates that the higher protein amount in the supernatant of MP06 was not due to cell lysis during growth.
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) and did not vary when flhF expression was hyper-stimulated using higher IPTG concentrations (up to 4 mM; data not shown). The effect of flhF overexpression was also analysed in strain MP08, obtained by introducing pDGflhF into strain ATCC 14579. Overexpression of flhF in the wild-type background had no detectable effect on protein secretion, even when IPTG was used at the highest concentration (Fig. 7). Therefore, while overexpression of flhF did not alter the amount of secreted proteins, under the conditions tested, lack of flhF led to protein hyper-secretion.
Disruption of flhF differently affects the export of secretory virulence factors
The higher level of proteins secreted in the flhF null mutant of B. cereus raised the question of whether the lack of flhF altered the export of secretory virulence factors, such as NHE, HBL and PC-PLC.
Secretion of NHE was tested in the strains ATCC 14579, MP06 and MP07 of B. cereus. Immunoassays of culture supernatants showed that the level of extracellular NHE was higher in the mutant MP06 (A405 1.02±0.08) compared to the wild-type (A405 0.78±0.06) and complemented (A405 0.87±0.09) strains (P<0.05). This result was in agreement with the increase in total protein secretion observed for MP06.
Secretion of the enterotoxic tripartite HBL was evaluated by growing bacteria on sheep blood agar plates. Production of the typical HBL discontinuous pattern of haemolysis around colonies (Beecher & Wong, 1994) was revealed for the mutant, parental, and complemented strains, thus indicating that all strains were able to secrete an active toxin (data not shown). To evaluate whether different levels of HBL were secreted by the strains, immunoblot analysis of supernatants from cultures normalized as described was performed with polyclonal sera specific to the individual HBL proteins (B, L1, L2). Although a positive signal was obtained for each HBL component in all strains, an almost threefold reduction in the amount of HBL proteins was registered in mutant MP06, compared to the wild-type and MP07 (as shown in Fig. 8 for the L1 component of HBL). As observed for HBL, the amount of PC-PLC activity secreted by MP06 (0.060±0.016 U ml–1) was also lower than that detected for the wild-type (0.124±0.018 U ml–1) and the complemented strain (0.134±0.011 U ml–1) (P<0.001).
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) showed that the C-terminal half of FlhF is highly conserved in all flagellated bacteria studied so far. However, a difference in this region emerged between B. cereus and B. subtilis, the only two peritrichous species in which flhF has been studied, with B. subtilis lacking the last C-terminal domain (referred to as domain no. 518651, as following ProDom analysis). To evaluate the functional role of such protein domain in B. cereus, an ATCC 14579 derivative (MP05) carrying a truncation in the 3' end of flhF was constructed by Campbell-type recombination of pRNflhF2 at the flhF locus. RT-PCR analysis on total RNA extracted from strain MP05 showed the absence of an flhF full-length mRNA (Fig. 9, lane 2); expression of a flhF transcript lacking the last 198 nt was demonstrated (Fig. 9, lane 4) following amplification of total RNA with the primers flhFHindL2, which was designed on a sequence immediately upstream of the pRNflhF2 insertion site, and flhFU2 (Table 2, Fig. 2a).
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). Restoration of a wild-type pattern of protein secretion was observed in strain MP09 (Fig. 7), which was obtained after transformation of MP05 with the plasmid pDGflhF, harbouring a copy of wild-type flhF gene. Among the virulence-associated proteins analysed, NHE and PC-PLC levels were found to be respectively higher (A405 of 0.97±0.03) and lower (0.057±0.015 U ml–1) in strain MP05 than in the parental strain. In contrast, no difference in the levels of secreted HBL was noted between MP05 and the wild-type strain (data not shown). |
DISCUSSION |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONREFERENCES |
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; Hendrixson & DiRita, 2003; Niehus et al., 2004; Pandza et al., 2000). In these bacteria, the protein encoded by flhF displays a high degree of sequence conservation, particularly in its C-terminal half (Fig. 1), while the functions attributed to FlhF vary in the species analysed so far. Indeed, while flhF mutants of P. aeruginosa and P. putida display peritrichous distribution of flagella (Murray & Kazmierczak, 2006; Pandza et al., 2000), lack of FlhF produces non-flagellated phenotypes in C. jejuni, H. pylori and V. cholerae (Correa et al., 2005; Hendrixson & DiRita, 2003; Niehus et al., 2004). The absence of flhF homologues in E. coli, together with the doubtful role of flhF in B. subtilis (Carpenter et al., 1992; Zanen et al., 2004), addressed the question of whether this gene played a role in cell flagellation of peritrichous bacteria.
In this investigation, we demonstrate that the number and distribution of flagellar filaments in the peritrichous species B. cereus depend on the integrity of flhF. The flhF insertional mutant of B. cereus described in this study (MP06) showed a remarkable reduction in the number of flagella (from 10–12 to 1–3 per cell) (Fig. 3a) that, moreover, were localized in polar or peri-polar positions (Fig. 3c). This reduction was not due to a delay in the assembly of flagella during growth, since the percentage of flagellated cells of MP06 was comparable to that of the wild-type strain (Fig. 4) at all stages during growth. The abnormal flagellar arrangement exhibited by MP06 causes a significant reduction in its ability to swim (Fig. 5a). The motility impairment appeared to be a direct consequence of shorter directional smooth swimming phases the mutant exhibited in comparison to the wild-type strain (2.96±0.35 s vs 7.36±1.17 s; P=0.0013). It is of some interest to highlight that a non-peritrichous distribution of flagella in a naturally peritrichously flagellate species does not determine an impairment in the mechanism enabling cells to coordinate directional movement. The maintenance of directional movement by MP06 explains why disruption of flhF does not lead to an impairment in the ability to undergo chemotaxis (Fig. 5b).
The B. cereus mutant MP06 was found to be completely impaired also in swarming differentiation, and restoration of the swarming phenotype was observed in the complemented strain MP07 (Fig. 6). This finding is in agreement with a robust literature demonstrating that deletions in flagellar genes severely impair or abolish the ability to undergo swarming differentiation in swarming-proficient species (Gygi et al., 1995; Harshey & Matsuyama, 1994; O'Rear et al., 1992; Senesi et al., 2002; Young et al., 1999b). In addition, flhF has been recently demonstrated to be required for swarming motility in the Gram-negative species P. aeruginosa (Murray & Kazmierczak, 2006). Lack of hyperflagellated cells in strain MP06 suggests that B. cereus flhF is involved in a common pathway controlling degree of flagellation in the swim as well as in the swarm cell-state.
The molecular mechanisms by which flhF governs the flagellar arrangement in B. cereus remain to be clarified. In many polar flagellates, such as Pseudomonas spp. and Vibrio spp., flhF resides in a large region concerned with flagellar synthesis, chemotaxis and motility, and FlhF has been described to act as a positive regulator for flagellar gene transcription (Correa et al., 2005; Hendrixson & DiRita, 2003; Niehus et al., 2004; Pandza et al., 2000). In B. cereus, the expression of flhF is independent from that of the upstream (flhA) and downstream (flgG) genes (Fig. 2b), and flhF overexpression does not appear to influence cell flagellation in both the mutant (MP07) and wild-type (MP08) backgrounds. In H. pylori, the absence of FlhF leads to a downregulation of the expression of flagellar class 3 genes, which is dependent on the antisigma factor FlgM (Niehus et al., 2004). In Vibrio parahaemolyticus, Kim & McCarter (2000) suggested that FlhF could co-operate with FlhG in determining site selection of flagellar insertion. Also in B. subtilis, ylxH, a fleN/flhG homologue, has been reported to cooperate with flhF for correct motility (Zanen et al., 2004). All the interactions described for FlhF in other bacteria appear not to be applicable to B. cereus FlhF, since no homologues of fleN/flhG and flgM have been found. In this context, we can only suggest that the sequence predicted to encode the GTP-binding domain of FlhF is involved in the activity exerted by this protein in regulating B. cereus motility. Indeed, in the B. cereus mutant MP05, which harbours a flhF deletion in a region downstream of the G-domain, no defect in motility or flagellar arrangement was evidenced.
The demonstration that flhF mutants of B. cereus display increased levels of extracellular proteins (Fig. 7) further supports the involvement of FlhF in the regulation of protein secretion, as already reported for P. putida (Pandza et al., 2000). FlhF is a member of the SRP-GTPase family (Carpenter et al., 1992; Pandza et al., 2000) and shows substantial homology with Ffh and FtsY, two proteins required for extracellular accumulation of proteins in E. coli and B. subtilis (Sijbrandi et al., 2003; Zanen et al., 2006). Similarly to flhF disruption in B. cereus, Ffh or FtsY depletion in B. subtilis determines an increased level of various extracellular proteins (Zanen et al., 2006). However, the mechanisms whereby FlhF is involved in the regulation of protein secretion in B. cereus are presently not understood. Nevertheless, it appears that the lack of flhF, but not its overexpression, is responsible for the altered protein secretion in B. cereus, since overexpression of flhF does not apparently cause secretory defects in the wild-type background (Fig. 7).
Detection of virulence determinants known to be secreted by B. cereus highlighted that export of individual proteins is differently influenced by an flhF deletion. In fact, reduced amounts of both HBL (about 70 %) and PC-PLC (about 50 %) were observed in mutant MP06, together with an increase in the amount of secreted NHE. Notably, the increase in NHE secretion was as high as that observed for the total proteins secreted by the flhF mutant (about 20 %), suggesting that lack of FlhF causes a dysregulation in a secretory pathway also needed for NHE export.
The reduced secretion of HBL observed in strain MP06 was interpreted with the lower number of flagella carried by this strain. Indeed, HBL components have already been shown to be secreted only through the flagellar type III export machinery in B. thuringiensis (Bouillaut et al., 2005; Ghelardi et al., 2002), the closest relative of B. cereus. The reduced amount of extracellular PC-PLC activity in mutant MP06 was more difficult to interpret; in fact, it could not be considered as a consequence of the reduced number of flagella, since (i) a lower amount of PC-PLC was also detected in the MP05 mutant, which carries a wild-type flagellar arrangement, and (ii) a B. thuringiensis mutant completely lacking flagella was found to be able to secrete this toxin (Ghelardi et al., 2002). However, whatever the mechanism accounting for the reduced amount of PC-PLC, it is interesting that a deficiency in a single flagellar gene influences the virulence phenotype exhibited by B. cereus.
Finally, the identification of an FlhF domain that appears to be essential for correct protein secretion in B. cereus is of some relevance. The mutant MP05, lacking the last C-terminal domain of FlhF, secretes an abnormal amount of proteins (Fig. 7), while it shows a wild-type flagellar arrangement and motility phenotype. The correct flagellar arrangement of MP05, together with the dependence of HBL secretion on the flagellum channel, explains why no difference in HBL export was detected between MP05 and the wild-type strain.
Taken together, the results described in this study indicate that B. cereus FlhF plays a critical role in regulating flagellar arrangement, motility behaviour and protein secretion, thus placing FlhF as a promising candidate for connecting motility/flagella with other cellular functions.
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ACKNOWLEDGEMENTS |
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Edited by: P. W. O'Toole
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REFERENCES |
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Received 23 December 2006;
revised 16 March 2007;
accepted 29 April 2007.
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