Role of the SaeRS two-component regulatory system in Staphylococcus epidermidisautolysis and biofilm formation
© Lou et al; licensee BioMed Central Ltd. 2011
Received: 14 February 2011
Accepted: 24 June 2011
Published: 24 June 2011
Staphylococcus epidermidis (SE) has emerged as one of the most important causes of nosocomial infections. The SaeRS two-component signal transduction system (TCS) influences virulence and biofilm formation in Staphylococcus aureus. The deletion of saeR in S. epidermidis results in impaired anaerobic growth and decreased nitrate utilization. However, the regulatory function of SaeRS on biofilm formation and autolysis in S. epidermidis remains unclear.
The saeRS genes of SE1457 were deleted by homologous recombination. The saeRS deletion mutant, SE1457ΔsaeRS, exhibited increased biofilm formation that was disturbed more severely (a 4-fold reduction) by DNase I treatment compared to SE1457 and the complementation strain SE1457saec. Compared to SE1457 and SE1457saec, SE1457ΔsaeRS showed increased Triton X-100-induced autolysis (approximately 3-fold) and decreased cell viability in planktonic/biofilm states; further, SE1457ΔsaeRS also released more extracellular DNA (eDNA) in the biofilms. Correlated with the increased autolysis phenotype, the transcription of autolysis-related genes, such as atlE and aae, was increased in SE1457ΔsaeRS. Whereas the expression of accumulation-associated protein was up-regulated by 1.8-fold in 1457ΔsaeRS, the expression of an N-acetylglucosaminyl transferase enzyme (encoded by icaA) critical for polysaccharide intercellular adhesin (PIA) synthesis was not affected by the deletion of saeRS.
Deletion of saeRS in S. epidermidis resulted in an increase in biofilm-forming ability, which was associated with increased eDNA release and up-regulated Aap expression. The increased eDNA release from SE1457ΔsaeRS was associated with increased bacterial autolysis and decreased bacterial cell viability in the planktonic/biofilm states.
The opportunistic pathogen Staphylococcus epidermidis has emerged as an important etiologic agent of nosocomial infections. The ability to form biofilms on the surfaces of medical devices is an important component of S. epidermidis pathogenicity. Biofilm resistance to antibiotics and host defense mechanisms are often regulated by two-component signal transduction systems (TCSs) .
Biofilm formation proceeds in two distinct developmental phases: primary attachment of staphylococcal cells to a polystyrene surface followed by bacterial accumulation in multiple layers . The initial adhesion of bacterial cells to a polymer surface is influenced by a variety of factors, including AtlE, Embp, and other staphylococcal surface-associated proteins. During the bacterial accumulation phase in S. epidermidis, biofilm formation is mediated by extracellular polysaccharides and proteins, such as polysaccharide intercellular adhesin (PIA)  and accumulation-associated protein (Aap) . In addition to extracellular polysaccharides and proteins, extracellular DNA (eDNA) is a matrix component that is critical for bacterial attachment during the initial stage of biofilm formation [5, 6]. Extracellular DNA release from S. epidermidis is related to AtlE-mediated bacterial autolysis . Another autolysin recently identified in S. epidermidis, Aae, also has bacteriolytic activities and adhesive properties .
TCSs regulate bacterial adaptation, survival, virulence and biofilm formation [9–12]. TCSs comprise a membrane-associated histidine kinase and a cytoplasmic response regulator. Overall, 16 or 17 TCSs have been identified in the genomes of S. epidermidis ATCC12228 or ATCC35984 [13, 14]. In S. epidermidis, the TCS agrC/agrA has been proven to negatively regulate biofilm formation [15, 16]. In a previous study of the S. epidermidis saeRS TCS, a saeR deletion mutant exhibited a lower anaerobic growth rate, a significantly reduced rate of nitrate utilization and a slightly higher biofilm-forming ability compared to the parental strain . In S. aureus, the saeRS TCS influences biofilm formation  and the expression of virulence-associated factors, such as protein A, α- and β-hemolysins, and coagulase . However, whether saeRS regulates S. epidermidis autolysis and biofilm formation remains unclear.
In the present work, we constructed a SE1457ΔsaeRS mutant with deletion of the genes that encode both the histidine kinase (SaeS) and the response regulator (SaeR) by homologous recombination. The effects of the saeRS deletion on S. epidermidis autolysis, eDNA release, bacterial cell viability, and biofilm formation were investigated.
Bacterial strains, plasmids, and media
Bacterial strains and plasmids used in the present study
Strain or plasmid
Relevant genotype or characteristic
Reference or source
E. coli DH5α
λ- ϕ80dlacΔM15 Δ(lacZYA-argF)U169 recA1 endA1 hsdR17 (rK- mK-) supE44 thi-1 gyrA relA1
Biofilm positive strain
S. aureus RN4220
Restriction-negative, modification-positive isolate
saeRS deletion mutant of strain 1457, Spcr
1457ΔsaeRS complemented with saeRS
Expression vector, KanR
Temperature-sensitive E. coli- Staphylococcus shuttle vector. Apr (E. coli) Cmr (Staphylococcus)
Derivate of pCX15
Escherichia coli/Staphylococcus Shuttle vector
Vector for allelic gene replacement of saeRS in S. epidermidis
Vector for complementation of saeRS in S. epidermidis 1457ΔsaeRS
Determination of the growth curves of S. epidermidisstrains
The aerobic growth curves of S. epidermidis strains were determined by measuring the optical density (OD600) as described previously . Briefly, overnight cultures were diluted 1:200 and incubated at 37°C with shaking at 220 rpm. The OD600 of the culture were measured at 60 min intervals for 12 h. At 6, 12, and 24 h time points, colony forming units on TSA plates were further counted with serial dilutions of each sample plated on 6 agar plates. For anaerobic growth conditions, bacteria were cultured in the Eppendorf tubes which were filled up with the TSB medium and sealed with wax.
Detection of biofilm formation
The biofilm-forming ability of S. epidermidis strains was determined by the microtiter-plate test as described by Christensen [19, 20]. Briefly, overnight cultures of S. epidermidis were diluted 1:200 and inoculated into wells of polystyrene microtiter plates (200 μL per well) at 37°C for 24 h. At different time points (0, 6, 12, and 24 h), DNase I (Takara Bio, Kyoto, Japan) was added at 28 U/200 μL. After incubation, the wells were gently washed three times with 200 μL PBS and stained with 2% crystal violet for 5 min. Absorbance was determined at 570 nm.
To determine whether saeRS affects cell death in biofilms, S. epidermidis cells were cultivated in FluoroDish (FD35-100, WPI, USA) as previously described . Briefly, overnight cultures of S. epidermidis grown in TSB medium were diluted 1:200, inoculated into dishes (2 mL per dish), and then incubated at 37°C for 24 h. The dishes were then carefully washed with PBS and stained with a LIVE/DEAD kit (containing SYTO9 and PI, Invitrogen Molecular Probes, USA) following the manufacturer's instructions. SYTO9 stains viable bacteria green while PI stains dead bacteria red. Biofilms of S. epidermidis 1457 and SE1457ΔsaeRS were observed under a Leica TCS SP5 confocal laser scanning microscope (CLSM) using a 63 ×(zoom ×3) objective lens and the Z-stack composite confocal photomicrographs of viable cells, dead cells, and both cells (viable & dead) were generated by Leica LAS AF softwear (version 1.8.1). The fluorescence quantity of each stack was determained using ImageJ software.
For scanning electron microscopy (SEM), biofilms were grown in TSB for 24 h at 37°C with fragments of an introvenous catheter, rinsed with PBS three times, fixed with a 2% (w/v) solution of glutaraldehyde prepared in phosphate-buffered saline, and then observed under a TECNAI- 12 field emission source instrument (Philips, Eindhoven, The Netherlands).
For transmission electron microscopy (TEM), bacteria grown for 24 h were stained by mixing with a 1% (w/v) solution of uranyl acetate on an electron microscope grid covered with a carbon-coated Formvar film. S. epidermidis cells were observed using a Hitachi S-520 electron microscope (Hitachi, Tokyo, Japan).
RNA extraction and microarray analysis
Overnight cultures of S. epidermidis 1457 and 1457 ΔsaeSR were diluted 1:200 into fresh TSB and grown at 37°C to an OD600 of 3.0 (mid-exponential growth). Eight millilitres of bacterial cultures were pelleted, washed with ice-cold saline, and then homogenized using 0.1 mm Ziconia-silica beads in Mini-Beadbeater (Biospec) at a speed of 4800 rpm. The bacterial RNA was isolated using a QIAGEN RNeasy kit according to the standard QIAGEN RNeasy protocol.
The microarray was manufactured by in situ synthesis of 14,527, 60-mer long oligonucleotide probes (Agilent, Palo Alto, CA, USA), selected as previously described . It covers > 95% of all ORFs annotated in strains ATCC12228 (GeneBank accession number NC_004461), ATCC35984 (GeneBank accession number NC_002976), SE1457 (unpublished sequence). Preparations of 10 μg of total S. epidermidis RNA were labeled by Cy-3 dCTP (Perkin-Elmer) using the SuperScript II (Invitrogen, Basel, Switzerland) and purified as previously described . Pool of purified genomic DNA from the reference sequenced strains used for the design of the microarray was labeled with Cy-5 dCTP  and used for microarray normalization . Mixtures of Cy5-labeled DNA and Cy3-labeled cDNA were hybridized and scanned as previously described  in a dedicated oven. Fluorescence intensities were quantified using Feature Extraction software (Agilent, version 8). Green (Cy3) and red (Cy5) feature extraction processed data were imported in the Partek genomics suite software (Partek Incorporated. St. Louis, USA). Data were normalized to baseline using red channel data as control  and mean to estimate baseline. Variance analysis of three biological replicates was processed with a false discovery rate value of 5% (P value cutoff; 0.05) and an arbitrary threshold of 3.0 fold for defining significant differences in expression ratios. The complete raw microarray dataset has been posted on the Gene Expression Omnibus database (http://www.ncbi.nlm.nih.gov/geo/), accession number GPL13532 for the platform design and GSE29309 for the original dataset.
Quantitative real-time PCR analysis
GenBank accession no.
Oligonucleotide primers used for RT real-time PCR
Oligonucleotide primers used for eDNA quantification
Determination of Triton X-100-induced autolysis
Triton X-100-induced autolysis was performed to determine the potential role of saeRS in autolysis regulation in S. epidermidis, as described elsewhere [24–26]. SE1457ΔsaeRS, SE1457, and SE1457saec cells were diluted in TSB containing 1 M NaCl, grown to mid-exponential phase (OD600 = ~0.6-0.8), washed twice in cold sterile distilled water, resuspended in the same volume of 0.05 M Tris-HCl containing 0.05% Triton X-100 (pH 7.2), and incubated at 30°C. OD600 was measured every 30 min. The Triton X-100-induced autolysis rate was calculated as follows: Ra = OD0-ODt/OD0.
The murein hydrolase activities of SE1457, SE1457ΔsaeRS, SE1457saec, and SE1457ΔatlE were detected by zymographic analysis as described elsewhere [26, 27]. Extracts from lysostaphin- and SDS-treated S. epidermidis (Ex-Lys and Ex-SDS, respectively) and the concentrated supernatants of the bacterial culture (Ex-Sup) were used to analyze the murein hydrolase activities of each strain. Ex-Lys were obtained by treating S. epidermidis cells with 30 μg/mL of lysostaphin for 2 h at 37°C and subsequently centrifuged at 8,000 g for 30 min. Ex-SDS were obtained by treating S. epidermidis cells in 100 μL of 100 mM phosphate buffer containing 4% SDS at 37°C for 30 min and centrifuged (10,000 g) for 10 min. Ex-Sup were acquired by concentrating supernatants of overnight S. epidermidis cultures to 10% initial volume using a centrifugal filter device (Millipore, Billerica, MA).
S. epidermidis cell extracts were separated on a SDS-PAGE gel (10% acrylamide, pH 8.8) containing 0.2% (wt/vol) lyophilized Micrococcus luteus (M. luteus) or S. epidermidis cells. After electrophoresis, the gels were washed four times with distilled water for 30 min at room temperature, incubated in 25 mM Tris-HCl containing 1% Triton X-100 (pH 8.0) at 37°C for 6 h, and then stained with methylene blue.
Quantification of eDNA
Extracellular DNA isolation from biofilms was performed as described by Rice et al. [7, 19, 28]. Briefly, SE1457, SE1457ΔsaeRS, and SE1457saec biofilms (grown for 24 h) were chilled at 4°C for 1 h and treated with 1.0 μL of 0.5 M EDTA. Supernatants were discarded, and the unwashed biofilms were resuspended in 50 mM TES buffer (Tris-HCl (pH 8.0), 10 mM ETDA, 500 mM NaCl). Extracellular DNA was extracted with phenol/chloroform/isoamyl alcohol (25:24:1), precipitated with 100% ethanol, and dissolved in 20 μL of TE buffer.
Extracellular DNA was quantified by qPCR using gyrA (gyrase A), serp0306 (ferrichrome transport ATP-binding protein A), lysA (diaminopimelate decarboxylase A), and leuA (2-isopropylmalate synthase) primers as listed in Table 2. Each sample was diluted to 1:10, and PCRs were performed with SYBR Premix Ex Taq TM (TaKaRa, Japan) and primers (2 μM), according to the manufacturer's recommendations. The average OD600 of each unwashed biofilm was determined for calculating potential differences in biomass. The amount of eDNA per relative biomass of each biofilm was then calculated as follows: total eDNA (ng)/ relative OD600.
Initial bacterial attachment assays
Initial cell attachment was detected as described by Heilmann et al. . Briefly, mid-exponential phase cells were diluted to OD600 = 0.1 in PBS and then incubated in wells (1 mL per well) of cell-culture polystyrene chambers (Nunc, Denmark) with DNase I (140 U/mL) for 2 h at 37°C. Numbers of attached cells were counted under a microscope. Three independent experiments were carried out.
Detection of Aap expression
Concentrations of lysostaphin-treated whole bacterial proteins from SE1457ΔsaeRS, SE1457, and SE1457saec were determined by the Bradford method. For the detection of Aap in all samples by Western blot assay, proteins were separated on a 7% SDS-PAGE gel and then transferred to polyvinylidene fluoride (PVDF) membranes (Whatman, D-37586 Dassel, Germany) by electroblotting with a Mini-Transfer system (Bio-Rad, Mississauga, Canada) at 200 mA for 2 h (4°C). Monoclonal antibodies against the Aap B-repeat region (prepared by Abmart, Shanghai, China) were diluted 1:6000, and horseradish peroxidase-conjugated goat anti-mouse IgG antibodies (Sino-American Biotech) were diluted 1:2000. The gray scale of the bands corresponding to Aap was quantified using the Quantity-one software (Bio-Rad, USA).
Semi-quantitative detection of PIA
PIA was detected as described elsewhere [30–32]. Briefly, S. epidermidis strains were grown in 6-well plates (Nunc, DK-4000 Roskitde, Denmark) under static conditions at 37°C for 24 h. The cells were scraped off and resuspended in 0.5 M EDTA (pH 8.0). The supernatant was treated with proteinase K (final concentration 4 mg/mL; Roche, MERCK, Darmstadt, Germany) for 3 h (37°C). Serial dilutions of the PIA extract were then transferred to a nitrocellulose membrane (Millipore, Billerica, MA) using a 96-well dot blot vacuum manifold (Gibco). The air-dried membrane was blocked with 3% (wt/vol) bovine serum albumin and subsequently incubated with 3.2 μg/mL wheat germ agglutinin coupled to horseradish peroxidase (WGA-HRP conjugate; Lectinotest Laboratory, Lviv, Ukraine) for 1 h. Horseradish peroxidase (HRP) activity was visualized via chromogenic detection. The gray scale of the spots corresponding to PIA was quantified using the Quantity-one software.
Experimental data were analyzed with the SPSS software and compared using the Student's t-test. Differences with a P value of < 0.05 were considered statistically significant.
Effect of saeRS deletion on S. epidermidisbiofilm formation
Effect of saeRS deletion on the autolysis of S. epidermidis
Effect of saeRS deletion on S. epidermidisviability in planktonic and biofilm states
The viability of SE1457ΔsaeRS and the wild-type strain in 24 h biofilm was determined by confocal laser scanning microscopy (CLSM) with LIVE/DEAD staining . More dead cells were observed in the SE1457ΔsaeRS biofilm compared to the wild-type strain (Figure 5B).
Effect of saeRS deletion on eDNA release from S. epidermidis
When DNase I (28 U/200 μL/well) was added prior to biofilm formation, the biomass of the SE1457ΔsaeRS biofilms was decreased by 4-fold (P < 0.05); in contrast, the biomasses of SE1457 and SE1457saec biofilms were decreased by 1.5-fold (Figure 1).
Effect of eDNA release on SE1457ΔsaeRS primary attachment of SE1457ΔsaeRS
Effect of saeRS deletion on PIA production and Aap expression of S. epidermidis
PIA in the extracellular matrix of biofilms was detected using a dot blot assay with the WGA-HRP conjugate. PIA production levels were not significantly different in the SE1457ΔsaeRS strain compared to the SE1457 and SE1457saec strains (Additional file 2: Fig. S2). When assessed by comparative proteomic analysis, expression of accumulation-associated protein (Aap), an important factor for intercellular adhesion, was up-regulated in SE1457ΔsaeRS compared to the wild-type strain (Additional File 3: Fig. S3). Aap in lysostaphin-treated whole bacterial lysates of SE1457ΔsaeRS, SE1457 and SE1457saec strains was detected by Western blot using an anti-Aap monoclonal antibody. The SE1457ΔsaeRS strain expressed more Aap (1.85-fold up-regulation) compared to the wild-type and the complementation strains (Additional file 4: Fig. S4).
Analysis of the autolysis-related gene transcription in SE1457ΔsaeRS
Genes expression regulated by saeRS in S. epidermidis
Genbank accession no.
Expression ratio mutant/WT
two-component sensor histidine kinase LytS
2.33 ± 0.35
Negatively modulating the expression of murein hydrolases and positively regulates the expression of the lrgAB operon in S. aureus
holin-like protein LrgA
2.75 ± 0.05
Encoding a murein hydrolase exporter similar to bacteriophage holin proteins; may be required for the activity or transport of this cell wall-associated murein hydrolase in S. aureus
2.25 ± 0.20
Having lysozyme activity in peptidoglycan catabolic process in S. aureus
glycerophosphoryl diester phosphodiesterase GlpQ, putative
1.80 ± 0.20
Having glycerophosphodiester phosphodiesterase activity in lipid and glycerol metabolic process in S. aureus
DNA-binding response regulator
3.20 ± 0.45
Regulating extracellular proteolytic activity; may be involved in the modulation of expression of genes associated with growth and cell division; positively regulating a two-component system lytRS in S. aureus
S. epidermidis autolysin
1.45 ± 0.10
Having amidase activity to cleave the amide bond between N-acetyl muramic acid and L-alanine; mediating lysis of a subpopulation of the bacteria and extracellular DNA release in S. epidermidis
S. epidermidis autolysin/adhesin
2.32 ± 0.38
Having bacteriolytic activity and binding to fibrinogen, fibronectin and vitronectin in S. epidermidis
Biofilm-forming related genes
a gene of ica operon
1.22 ± 0.13
Encoding N-acetyglucosaminyltransferase for synthesis of polysaccharide intercellular adhesin (PIA) which is important for biofilm formation of S. epidermidis
1.62 ± 0.06
Contributing to intercellular adhesion and biofilm formation of S. epidermidis
sensor histidine kinase SaeS
Encoding a histidine kinase; involving in the tight temporal control of virulence factor expression in S. aureus
DNA-binding response regulator SaeR
The response regulator SaeR binding to a direct repeat sequence in S. aureus; involving in anaerobic growth and nitrate utilization in S. epidermidis
conserved hypothetical protein
Encoding a membrane protein, function unknown in S. epidermidis
Encoding a lipoprotein, function unknown in S. epidermidis
As Staphylococci biofilm formation is influenced by external factors such as glucose, NaCl, temperature, aerobiosis-anaerobiosis, static-dynamic conditions, and pH [36–39], it suggests that there are mechanisms that can sense environmental signals and regulate bacterial biofilm formation. In S. epidermidis, the agrC/A TCS has been proven to negatively regulate biofilm formation [15, 16], while the lytS/R TCS has been shown to positively regulate bacterial autolysis . In S. aureus, the saeRS TCS influences biofilm formation  and the expression of virulence-associated factors , whereas in S. epidermidis, a mutant with saeR deletion showed a slightly higher biofilm-forming ability compared to the parental strain .
In the present study, SE1457ΔsaeRS, a saeR and saeS deletion mutant from S. epidermidis 1457, was constructed by homologous recombination. Although saeRS in S. epidermidis ATCC 35984 and S. aureus Newman are similar both at nucleotide sequence level (75% for saeR and 67% for saeS) and at the amino acid level (84% for SaeR and 70% for SaeS), both biofilm formation and autolysis were up-regulated in SE1457ΔsaeRS, suggesting that saeRS in S. epidermidis plays a different role from that in S. aureus. Additionally, when examined by SEM, increased quantities of extracellular polymeric substances (EPSs) were observed in the SE1457ΔsaeRS biofilm compared to the SE1457 and SE1457saec biofilms (Figure 2A).
Aap expression and PIA synthesis are important for biofilm formation. Therefore, we examined the contribution of Aap and PIA to SE1457ΔsaeRS biofilm formation. In S. epidermidis, Aap plays an important role in biofilm formation, and biofilm-positive strains that express aap show higher biofilm forming abilities than strains that lack the Aap protein . In SE1457ΔsaeRS, Aap up-regulation was detected using 2-DE and confirmed by Western blot, suggesting that Aap is a factor associated with the enhanced biofilm formation capacity of SE1457ΔsaeRS. PIA plays a major role in intercellular adhesion in S. epidermidis biofilms . However, no obvious differences in either PIA production or transcription of icaA, the gene that encodes an N-acetylglucosaminyl transferase enzyme critical for PIA synthesis, were observed between SE1457ΔsaeRS and SE1457 (Table 3). These results are consistent with the findings reported for a saeR deletion mutant by Handke et al. .
The enhanced S. epidermidis biofilm formation may be correlated with the increased amounts of eDNA released in the biofilm matrix [19, 25, 28]. Quantitative PCR revealed that eDNA release from S. epidermidis 1457ΔsaeRS was up-regulated (Figure 6). Furthermore, the biomass of SE1457ΔsaeRS biofilms was markedly decreased compared to SE1457 and SE1457saec biofilms when DNase I was added prior to biofilm formation.
Extracellular DNA is known to be released following bacterial autolysis . SE1457ΔsaeRS showed a higher level of Triton X-100-induced autolysis compared to the wild-type strain in TSB medium containing 1 M NaCl. In accordance with the enhanced autolysis of SE1457ΔsaeRS, extracts from SDS-treated SE1457ΔsaeRS cells exhibited more bacteriolytic bands compared to extracts from the wild-type strain. These results indicate that saeRS influenced the activity of autolysins that bind non-covalently to the S. epidermidis cell wall. In S. aureus, autolysis is a complicated process regulated by the lytSR TCS  and global regulators such as mgrA and sarA [44, 45]. Autolysis is influenced by a variety of different factors such as NaCl, pH, temperature, and growth phase, suggesting the existence of a mechanism that can sense environmental conditions [36–39]. However, Zhu et al. have demonstrated that the lytSR TCS in S. epidermidis is not involved in Triton X-100-induced autolysis and does not alter the zymogram profile , indicating that a different mechanism for autolysis regulation exists in S. epidermidis. The findings in the present study suggest that the saeRS TCS may regulate S. epidermidis autolysis.
The increased autolysis rate observed in SE1457ΔsaeRS may also be associated with the up-regulated expression of autolysins. In S. epidermidis, AtlE and Aae are important autolysins [8, 46]. AtlE is expressed as a 138 kDa precursor protein that is proteolytically processed to release the GL (51 kDa) and AM domains (62 kDa) [13, 14, 23]. Aae, a 35 kDa protein, contains three repetitive sequences in its N-terminal portion. These repeats comprise features of a putative peptidoglycan binding domain (LysM domain) found in several enzymes that are involved in cell-wall metabolism. Aae from S. epidermidis O-47 exhibited bacteriolytic activity in zymographic analysis using S. carnosus or S. epidermidis cells as a substrate. In the present study, atlE and aae transcription was up-regulated in SE1457ΔsaeRS (Table 3), which may account for the increase in bacteriolytic bands in the zymogram assay. In addition, expression of numerous autolysis-related genes in SE1457ΔsaeRS, such as lytS, lrgA, arlR, serp0043 and glpQ, were also up-regulated, suggesting that S. epidermidis autolysis mediated by saeRS may be influenced by other factors that remain to be defined.
Transcriptional profile analysis of the saeRS mutant and the wild-type strain found 135 differentially expressed genes in the present study, whereas in the Handke's study, only 65 genes in the saeR mutant were differentially expressed compared to the wild-type strain. The deletion of saeRS in S. epidermidis affects genes with a variety of functions, including bacterial autolysis (lrgA, arlR, lytS), biofilm formation (ebhA), leucine biosynthesis (leuD), protein hydrolysis (clpP), stress resistance (asp23), and cell viability (yycH). Three genes with increased expression, pflB (formate acetyltransferase), pflA (formate acetyltransferase-activating enzyme) and lrgA (holin protein) in SE1457ΔsaeRS, overlapped with the saeR deletion mutant. The discrepancies of the microarray data between the saeR mutant and the saeRS mutant may result from crosstalk between saeS and the response regulators of other TCSs. When the transcriptional profiles of the saeRS deletion mutant was compared to the S. aureus strains N315, COL, and Newman, only three differentially expressed genes, geh (glycerol ester hydrolase), efb (fibrinogen-binding protein) and lrgA (holin-like protein LrgA), were found to overlap [18, 47]. Taken together, these results suggest a different role for saeRS in S. epidermidis from that in S. aureus.
Genes containing the direct repeat sequence with no more than one mismatch
ATP:guanido phosphotransferase family protein
DNA-binding response regulator SaeR
phenylalanyl-tRNA synthetase, alpha subunit
3-hydroxyacyl-CoA dehydrogenase family protein
The deletion of saeRS in S. epidermidis resulted in the alteration of bacterial autolysis, increased eDNA release, and decreased bacterial cell viability in the planktonic/biofilm states. Further, Aap expression and the transcription of autolysin genes such as atlE and aae were up-regulated. Overall, these alterations were associated with the increased biofilm-forming ability of the saeRS deletion mutant. The present study suggests that in S. epidermidis, the saeRS TCS plays an important role in regulating bacterial autolysis, which is related to biofilm formation.
We thank Prof. Friedrich Götz (University of Tübingen) for his academic advice regarding zymogram analysis, PIA detection, and microarray analysis. We appreciate the suggestions and support of Prof. Søren Molin (Technical University of Denmark) regarding biofilm CLSM observation. We also thank Prof. Michel Débarbouillé (Institut Pasteur) for providing the pMAD plasmid for the construction of the SE1457ΔsaeRS strain.
This work was supported by the National High Technology Research and Development Program (863 Program) (2006AA02A253), the Scientific Technology Development Foundation of Shanghai (10410700600, 09DZ1908602, 08JC1401600), the National Natural Science Foundation of China (30800036, J0730860), National Science and Technology Major Project (2009ZX09303-005, 2008ZX10003-016, 2009ZX10004-502), the Program of Ministry of Science and Technology of China (2010DFA32100), and the IBS Open Research Grant (IBS09064).
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