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Infection and Immunity, November 2007, p. 5390-5398, Vol. 75, No. 11
0019-9567/07/$08.00+0     doi:10.1128/IAI.00571-07
Copyright © 2007, American Society for Microbiology. All Rights Reserved.

Enterococcus faecalis Constitutes an Unusual Bacterial Model in Lysozyme Resistance

Laurent Hébert,¹ Pascal Courtin,² Riccardo Torelli,³ Maurizio Sanguinetti,³ Marie-Pierre Chapot-Chartier,² Yanick Auffray,¹ and Abdellah Benachour^1*

Laboratoire de Microbiologie de l'Environnement, USC INRA 2017, EA956, Université de Caen, 14032 Caen Cedex, France,¹ Unité de Biochimie Bactérienne, UR477, INRA, 78350 Jouy-en-Josas, France,² Institute of Microbiology, Catholic University of Sacred Heart, L. go F. Vito 1, 00168 Rome, Italy³

Received 19 April 2007/ Returned for modification 15 June 2007/ Accepted 14 August 2007

Top ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 
Lysozyme is an important and widespread compound of the host constitutive defense system, and it is assumed that Enterococcus faecalis is one of the few bacteria that are almost completely lysozyme resistant. On the basis of the sequence analysis of the whole genome of E. faecalis V583 strain, we identified two genes that are potentially involved in lysozyme resistance, EF_0783 and EF_1843. Protein products of these two genes share significant homology with Staphylococcus aureus peptidoglycan O-acetyltransferase (OatA) and Streptococcus pneumoniae N-acetylglucosamine deacetylase (PgdA), respectively. In order to determine whether EF_0783 and EF_1843 are involved in lysozyme resistance, we constructed their corresponding mutants and a double mutant. The EF_0783 mutant and EF_0783 EF_1843 double mutant were shown to be more sensitive to lysozyme than the parental E. faecalis JH2-2 strain and EF_1843 mutant were. However, compared to other bacteria, such as Listeria monocytogenes or S. pneumoniae, the tolerance of EF_0783 and EF_0783 EF_1843 mutants towards lysozyme remains very high. Peptidoglycan structure analysis showed that EF_0783 modifies the peptidoglycan by O acetylation of N-acetyl muramic acid, while the EF_1843 deletion has no obvious effect on peptidoglycan structure under the same conditions. Moreover, the EF_0783 and EF_1843 deletions seem to significantly affect the ability of E. faecalis to survive within murine macrophages. In all, while EF_0783 is currently involved in the lysozyme resistance of E. faecalis, peptidoglycan O acetylation and de-N-acetylation are not the main mechanisms conferring high levels of lysozyme resistance to E. faecalis.

Top ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 
Enterococcus faecalis is intrinsically not as virulent as other gram-positive organisms, such as Staphylococcus aureus, pneumococci, or group A streptococci. However, E. faecalis emerges as an opportunistic pathogen and one of the leading causes of hospital-acquired infections, such as urinary tract, surgical wound, abdominal, pelvic, and neonatal infections in the United States and northern Europe (26). Little is known about its virulence potential. This lack of information can be partly attributed to the fact that E. faecalis, which normally grows as a commensal organism in the gut, possesses very subtle virulence traits that are not easily identified (27). Several surface proteins, enzymes, and capsular polysaccharides likely involved in virulence and the ability of E. faecalis to survive inside polymorphonuclear leukocytes (36) and macrophages (25) may contribute to its pathogenicity. Besides, intrinsic physiological properties of E. faecalis, such as its exceptional resistance to harsh conditions (37) and its inherent antibiotic resistance (27), may also provide an advantage during the infection process. In order to survive and colonize the host, bacteria must overcome the constitutive or innate defense system and the host's phagocytic response to achieve infection. One of the most important and widespread compounds of the constitutive defense system is lysozyme. Lysozyme is a component of granules of neutrophils and the major secretory product of macrophages (29), found in mammalian secretions and tissues, and undoubtedly one of the most important enzymes of the human innate immune system. Lysozyme is a muramidase that cleaves peptidoglycan (PG) between the glycosidic beta-1,4-linked residues of N-acetylmuramic acid (NAM) and N-acetylglucosamine. In addition to the enzymatic lysis of the bacterial cell wall, lysozyme can also kill bacteria by a nonenzymatic mechanism in which its cationic and hydrophobic properties have been proposed to induce cell death by membrane perturbation (28). Therefore, pathogenic bacteria that colonize the host over a long period or that cause chronic infections must have developed mechanisms to avoid the lysozyme defense.

E. faecalis has an extraordinary capacity of resistance to the action of lysozyme, although variations in the lytic response of enterococcal cells were observed (16). However, the exact mechanism responsible for this resistance remains unknown hitherto. Two main mechanisms involved in lysozyme resistance have been characterized for different species of bacteria. The first mechanism is related to the modification of PG backbone structure via either O acetylation occurring on the C-6 hydroxyl moiety of the NAM residues (7) or the deacetylation of the N-acetylglucosamine residues (48). In gram-positive bacteria, the genes responsible for these activities encode OatA (O-acetyltransferase A) and PgdA (peptidoglycan N-acetylglucosamine deacetylase A) proteins, respectively. Two other O-acetyltransferase genes called aac(2')-Ia and pacA have also been identified in the gram-negative bacteria Providencia stuartii (24) and Neisseria sp. (22). These modifications act as a steric hindrance, preventing the binding of the lysozyme to the polysaccharide substrate, thus inhibiting its activity. The second mechanism is based on the production of lysozyme inhibitors which have a protective function. The first example was reported for Escherichia coli and consists of the periplasmic lysozyme inhibitor Ivy (inhibitor of vertebrate lysozyme) (20), and the second example is the Streptococcus pyogenes secreted factor Sic (streptococcal inhibitor of complement), identified as an extracellular virulence factors inhibiting lysozyme (8).

In this work we investigated the causes of the extraordinary resistance to lysozyme of E. faecalis. For this purpose, we mainly focused on the analysis of two proteins, the EF_0783 and EF_1843 proteins that display homology with OatA and PgdA, respectively. We studied the role of EF_0783 and EF_1843 in lysozyme resistance and their ability to survive within mouse peritoneal macrophages. In addition, we investigated the role of EF_0783 and EF_1843 on the PG structure of E. faecalis.

Top ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 
Bacterial strains, plasmids, and culture conditions. Bacterial strains and plasmids used in this study are listed in Table 1. E. faecalis strain JH2-2 (49) and its derivatives were grown at 37°C without shaking in M17 medium supplemented with 0.5% glucose (GM17) (43) or in tryptic soy broth (TSB) or in brain heart infusion (BHI). When required, erythromycin (100 µg/ml) was added. E. coli Top10 (Invitrogen, Groningen, The Netherlands) strain was cultured with vigorous shaking at 37°C in LB medium (40) with ampicillin (100 µg/ml) when required.

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TABLE 1. Bacterial strains, plasmids, and oligonucleotide primers and probes

DNA manipulations. General molecular methods, molecular cloning, and other standard techniques were performed essentially by the methods of Sambrook et al. (40). E. coli and E. faecalis were transformed by electroporation using Gene Pulser Apparatus (Bio-Rad Laboratories). Plasmids and PCR products were purified using QIAGEN kits (QIAGEN, Valencia, CA).

Construction of EF_0783, EF_1843, and EF_0783 EF_1843 mutants of E. faecalis. Plasmid pMAD, a thermosensitive, pE194^ts-based delivery vector system previously described by Arnaud et al. (4), was used to construct isogenic mutants of the E. faecalis JH2-2 strain. Construction of the deletion in EF_0783 and EF_1843 genes utilized DNA primers designed on the basis of the genome sequence of E. faecalis V583 strain (33) with flanking restriction sites (Table 1). For each gene, two DNA fragments consisting of 900-bp region were amplified from the upstream part of the gene (including the start codon and the 5' part of the coding sequence) and the downstream part (including the 3' part of the gene and the stop codon). Both PCR products were purified and digested by SalI and then self ligated in order to create a copy of the EF_0783 or EF_1843 gene carrying a deletion of 70% of the coding sequences. Ligated products were digested by appropriate restriction enzymes (Table 1), cloned into the pMAD vector, and finally transformed into electrocompetent cells of E. coli Top10. pMAD-EF0783 and pMAD-EF1843 recombinant plasmids were verified by restriction and PCR amplification to confirm gene deletion. These plasmids were subsequently used to transform E. faecalis strain JH2-2 cells. Gene replacement was performed via a double crossover event by a method based on the conditional replication of the pMAD plasmid as previously described (46). Briefly, after electroporation, the cell suspension was plated onto GM17 agar containing 100 µg/ml of erythromycin and 80 µg/ml of 5-bromo-4-chloro-3-indolyl-ß-D-galactopyranoside (X-Gal). After 48 h of incubation at 30°C, the resulting erythromycin-resistant blue colonies were selected and grown twice to log phase at 42°C in the presence of erythromycin. To induce the excision of the recombinant plasmid vector sequence, cultures were incubated for 4 h at 30°C, followed by incubation at 42°C overnight without antibiotics. This step was repeated four or five times. Erythromycin-sensitive white colonies, which indicated that the plasmid had been excised, were selected by plating on GM17 agar containing X-Gal (80 µg/ml) at 37°C. The gene deletion of the white erythromycin-sensitive colonies was verified by PCR. The EF_0783 EF_1843 double mutant was constructed by deleting the EF_0783 gene in the EF_1843 mutant, following the same procedure. Comparison of the growth of the wild-type (WT) strain and the isogenic EF_0783, EF_1843, and EF_0783 EF_1843 deletion mutant strains was performed by measuring the optical density at 600 nm (OD₆₀₀) of the cultures in GM17 medium, using a turbidimeter.

Lysozyme sensitivity assay. Lysozyme sensitivity assays were performed on 3 ml of Mueller-Hinton agar (pH 7.4) medium containing different lysozyme concentrations (0, 5, 20, 40, and 60 mg/ml) and potassium tellurite at 0.8 µg/ml disposed into five-by-five comparmented square culture plates. Ten microliters of a suspension containing 10⁶ cells/ml of each strain taken after overnight culture was then spotted onto the different media. Growth zones were photographed after 48 h of incubation at 37°C. The difference in color between the first column and the others is due to turbidity caused by lysozyme. Potassium tellurite was added in order to increase contrast between the colonies and media, and similar experiments were performed on medium deprived of potassium tellurite.

Survival assays in mouse peritoneal macrophages. Survival of E. faecalis in mouse peritoneal macrophages was tested by using an in vivo/in vitro infection model as described previously (25, 47). Briefly, E. faecalis EF_1843, EF_0783, and EF_0783 EF_1843 mutants and E. faecalis JH2-2 were grown aerobically at 37°C in BHI for 16 h. Then, bacteria were pelleted and resuspended in an adequate volume of phosphate-buffered saline for injection. Male BALB/c mice (10 weeks old) were infected with 10⁷ to 10⁸ cells of each strain by intraperitoneal injection. After a 8-h infection period, peritoneal macrophages were collected by peritoneal wash, centrifuged, and suspended in Dulbecco's modified Eagle's medium containing 10 mM HEPES, 2 mM glutamine, 10% bovine fetal serum, and 1 x nonessential amino acids supplemented with vancomycin (10 µg/ml) and gentamicin (150 µg/ml). The cell suspension was dispensed into 24-well tissue-culture plates and incubated at 37°C under 5% CO₂ for 2 h, and bacterial survival was monitored 24, 48, and 72 h after infection. All experiments were performed three times, and the results were subjected to statistical analysis using one-way analysis of variance with a Bonferroni's correction post test with GraphPad Prism version 5.00 for Windows (GraphPad Software, San Diego, CA).

PG structure analysis. E. faecalis PG structure was analyzed by reverse-phase high-performance liquid chromatography (RP-HPLC) and matrix-assisted laser desorption ionization-time of flight (MALDI-TOF) mass spectrometry as described previously for Lactococcus lactis (18). Briefly, PG was extracted from exponential- and stationary-phase cultures (OD₆₀₀ of 0.5 and 2, respectively) grown on GM17 or BHI with boiling sodium dodecyl sulfate and was deproteinized by treatment with pronase and trypsin. It was then treated with 48% hydrofluoric acid at 4°C for 16 h, washed several times with 0.25 M Tris-HCl, pH 8.0, and washed with H₂O before lyophilization. Purified PG (4 mg [dry weight] in 500 µl) was digested with mutanolysin (Sigma, Saint-Quentin, France; 2,500 U/ml) in 25 mM sodium phosphate buffer, pH 5.5, for 19 h at 37°C with shaking. The enzyme was inactivated by boiling the sample for 3 min, and insoluble material was removed by centrifugation. The soluble muropeptides were reduced with sodium borohydride. They were then separated by RP-HPLC using a Hypersil octyldecyl silane column (C₁₈; 250 x 4.6 mm; 5 µm; ThermoHypersil-Keystone) at 50°C by the method of Courtin et al. (18). Muropeptides were eluted for 5 min with 10 mM ammonium phosphate buffer (pH 5.6) (buffer A) and then with a 270-min methanol linear gradient (0 to 30%) in buffer A at a flow rate of 0.5 ml/min. Sodium azide was added (180 µg/liter of buffer A) to avoid baseline drift at 202 nm. Fractions were collected, and 1-µl portions of fractions containing the main peaks were analyzed by MALDI-TOF mass spectrometry with a Voyager DE STR mass spectrometer (Applied Biosystems, Framingham, MA) and -cyano-4-hydroxycinnamic acid matrix. It is noteworthy that O-acetyl groups are quite labile, since an incubation of 1 h at 37°C with NaOH (pH 9) reduces the quantity of O-acetylated muropeptides by about 30% (7). Thus, it is possible that the in vivo proportion of O-acetylated muropeptides was higher than that determined experimentally.

Quantitative real-time RT-PCR. Total RNA was extracted from exponential-phase enterococcal cultures or mouse peritoneal fluid samples with an RNeasy Protect Mini kit (QIAGEN) after the cells were suspended in RNAlater solution (QIAGEN) as described previously (21). Expression of the EF_1843 gene was quantitatively assessed by real-time reverse transcription-PCR (RT-PCR) in an iCycler iQ system (Bio-Rad Laboratories, Hercules, CA), using rpoB as the normalization gene (14). For each gene, a set of primer pairs and a TaqMan probe (Table 1) were designed with Beacon Designer 2 (version 2.06) software (Premier Biosoft International, Palo Alto, CA) and synthesized by MWG Biotech. rpoB primers and probe have been previously described (14).

PCRs were performed in a 50-µl volume containing the following reagents: 25 µl of the Platinum quantitative RT-PCR ThermoScript reaction mix (Invitrogen Inc., Milan, Italy), 1.5 U of ThermoScript Plus/Platinum Taq mix (Invitrogen), each primer pair and Taqman probe at a concentration of 0.5 µM, and 5 µl of total RNA sample and distilled water up to the final volume. Samples were subjected to an initial step at 52°C for 45 min for RT; 94°C for 5 min to inactivate the ThermoScript Plus reverse transcriptase and to activate the Platinum Taq polymerase; and 50 cycles, each consisting of 15 s at 94°C and 1 min at 59°C. Fluorescence data were collected during the 59°C annealing/extension step and analyzed with the iCycler iQ software. Each reaction was run in quadruplicate, and amplification efficiencies for the target gene were determined. The relative mRNA expression level of the target gene in each sample was calculated using the comparative cycle time as described previously (32).

Top ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 
Identification of EF_0783 and EF_1843 genes potentially involved in lysozyme resistance. E. faecalis has already been shown to acetylate its PG (35). In different bacteria, such as Neisseria gonorrhoeae and Proteus mirabilis, this modification has been shown to inhibit the action of lysozyme in a concentration-dependent manner (23, 38, 39, 42). The in silico analysis of the genome sequence of E. faecalis V583 strain (GenBank accession no. AE016830) revealed an open reading frame, EF_0783, showing significant similarity with an acetyltransferase gene of Staphylococcus aureus (7). The corresponding enzyme, OatA, acts to O acetylate at the C-6 OH group of NAM residues. The EF_0783 gene product (625 amino acids) shares 33% identity (55% similarity) with OatA over 613 amino acids. The predicted EF_0783 protein has several putative transmembrane segments, strongly suggesting its membrane association.

A similar search for a putative PG N-acetylglucosamine deacetylase (pgdA) involved in lysozyme resistance of Streptococcus pneumoniae (48) revealed that the EF_1843 protein product (301 amino acids) of E. faecalis shares 35% identity (53% similarity) with PgdA over 204 amino acids. PgdA, as well as the EF_1843 protein and other PG N-acetylglucosamine deacetylases previously identified in Listeria monocytogenes (12) and L. lactis (45), are members of the family 4 carbohydrate esterases as defined by the CAZy database (http://www.cazy.org/fam/acc_CE.html) and share conserved catalytic residues and metal ligand amino acids of the family 4 carbohydrate esterases identified by Blair et al. (9, 10) (Fig. 1). The EF_1843 gene of E. faecalis encodes a putative secreted protein with an N-terminal signal peptide typical for gram-positive bacteria (44). There is no predicted cleavage site for any known leader peptidase, suggesting that the protein remains likely anchored to the cytoplasmic membrane through its N-terminal membrane domain.

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FIG. 1. Sequence alignment of three N-acetylglucosamine deacetylases from L. monocytogenes (LmPgdA), L. lactis (LlPgdA), and S. pneumoniae (SpPgdA) and the EF_1843 (EF1843) protein product of E. faecalis. The sequences were aligned on the respect of the catalytic residues. Sequence manipulation was carried out using CLUSTALW software (http://align.genome.jp). The residues that are identical in all four proteins are indicated by the white letters on black background. Arrows and asterisks indicate the conserved catalytic residues and metal ligand amino acids characterized for S. pneumoniae PgdA, respectively. Amino acid numbers for each sequence are given on the right. Gaps introduced to maximize alignment are indicated by dashes. The GenBank accession numbers for the PgdA proteins of L. monocytogenes, L. lactis, S. pneumoniae, and for the putative EF_1843 polysaccharide deacetylase of E. faecalis are CAC98494, NP_266439, CAB96552, and AAO81606, respectively.

Nucleotide sequence analysis of the chromosomal DNA fragment harboring the EF_0783 gene revealed the presence of putative promoter sequences (TTGCCAN₁₇TATTAT) related to the ⁷⁰ consensus recognition motif (TTGACAN_16-18TATAAT). The analysis of EF_1843 promoter region previously performed (5) revealed the presence of a putative promoter sequence related to that of the extracytoplasmic sigma factor SigV of E. faecalis. The structural organization of the two loci showed that the EF_0783 gene is bordered by two inverted repeats, while the EF_1843 gene is preceded by a divergently transcribed gene and terminated by an inverted repeat located downstream of the stop codon (Fig. 2). These inverted repeats may form a secondary structure which could function as Rho-independent terminators. These observations strongly suggested a monocistronic operon structure of the EF_0783 and EF_1843 genes.

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FIG. 2. Schematic representation of the genetic organization of the EF_0783 (EF0783) and EF_1843 (EF1843) chromosomal loci in the E. faecalis JH2-2 strain. Large arrows represent the open reading frames, and their orientation shows the transcriptional direction. The putative promoter motifs (P) and terminators (T and Te) are indicated. Shaded boxes correspond to the deleted region in each mutant. The primers used for the mutagenesis experiments and PCR cloning are indicated by black arrows.

In order to investigate the possible phenotypic changes due to EF_0783 and EF_1843 gene inactivations, deletion of these genes was performed via a double crossover event using a method based on the conditional replication of the pMAD vector as described in Materials and Methods. This procedure led to obtaining EF_0783, EF_1843, and EF_0783 EF_1843 mutant strains (Table 1). Inactivation of the EF_0783 and EF_1843 genes had no consequences on the growth rate, morphology (photonic microscopy), and autolysis of the strains (data not shown).

In addition, in silico analysis of the genome sequence of E. faecalis V583 strain did not reveal any homologue for aac(2')-Ia, pacA, ivy, and sic genes.

Lysozyme resistance of EF_0783, EF_1843, and EF_0783 EF_1843 mutants. The parental strain JH2-2 and its derivative mutants were grown in liquid media (GM17, BHI, and TSB) in the presence of lysozyme concentrations up to 50 mg/ml. These attempts revealed no growth inhibition, neither for the WT JH2-2 strain, nor for the EF_0783, EF_1843, and EF_0783 EF_1843 mutants (data not shown). Thus, lysozyme sensitivity assays were performed on solid medium by plating 3 µl of a suspension containing 10⁶ cells/ml of each strain taken after overnight culture on Mueller-Hinton agar medium containing different lysozyme concentrations (0, 10, 20, 40, and 60 mg/ml). In the presence of 10 mg/ml of lysozyme, there was no significant differences between the spots of colonies of the four strains grown, whereas the EF_0783 and EF_0783 EF_1843 mutants showed a reduced intensity in the spots at 20 mg/ml of lysozyme and no growth at 40 mg/ml compared to the WT and the EF_1843 strains (Fig. 3). This experiment revealed that the EF_0783 mutant and the EF_0783 EF_1843 double mutant are more sensitive to lysozyme than the parental JH2-2 strain and its derivative EF_1843 mutant. The pHs of the media used for all these experiments correspond to physiological values (around pH 7) consistent with the optimal range (pH 6 to 9) of lysozyme activity.

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FIG. 3. Lysozyme sensitivity of E. faecalis JH2-2, EF_1843 (EF1843), EF0783 (EF0783), and EF_0783 EF_1843 (EF0783-EF1843) strains. The sensitivity to lysozyme was tested on Mueller-Hinton agar medium containing potassium tellurite at 0.8 µg/ml (to increase contrast between colonies and medium) and lysozyme at the indicated concentrations (in milligrams/milliliter).

Effects of EF_0783 and EF_1843 mutations on survival within mouse peritoneal macrophages. E. faecalis has already been shown to be able to persist for an extended period in mouse peritoneal macrophages by using a well-established infection model consisting of infecting BALB/c mice intraperitoneally, recovering infected macrophages, and then monitoring over a 72-h period the survival of intracellular bacteria within peritoneal macrophages maintained in vitro (25).

Since phagocytic cells produce lysozyme at high levels during phagocytosis (29), we compared the intracellular survival of the EF_0783, EF_1843, and EF_0783 EF_1843 mutants and the WT strain inside infected mouse peritoneal macrophages (Fig. 4). In this experiment, the nonpathogenic strain E. coli DH5 was used as a negative control, since this strain is known to be susceptible to killing by mouse peritoneal macrophages (25). No significant difference was observed in the levels of the E. faecalis strains recovered 8 h postinfection, suggesting that the strains possessed similar abilities to infect macrophages (Fig. 4). In contrast, E. faecalis JH2-2 was superior to the E. faecalis EF_1843 and EF_0783 EF_1843 mutants in its ability to survive intracellularly at the 24-, 48-, and 72-h time points (P < 0.05), whereas EF_0783 was shown to be more susceptible than strain JH2-2 to the macrophage killing only at 48- and 72-h time points (P < 0.05). At the end of the experiment, the survival of the parental strain JH2-2 was 1 to 2 log units higher than that of each mutant strain. As expected, E. coli DH5 was efficiently killed by the macrophages ever since 24 h postinfection.

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FIG. 4. Time course of intracellular survival of the E. faecalis and E. coli DH5 strains within murine peritoneal macrophages. The data are the mean numbers of viable intracellular bacteria per 105 macrophages ± standard deviations (error bars) for three independent experiments with three wells in each experiment. Symbols: , E. faecalis JH2-2; , EF_0783 mutant; •, EF_1843 mutant; , EF_0783 EF_1843 double mutant; , E. coli DH5.

Muropeptide composition of E. faecalis peptidoglycan. In order to study the functions of the EF_0783 and EF_1843 genes, we determined the PG structures of EF_0783, EF_1843, and EF_0783 EF_1843 mutants and compared them with the muropeptide profile of the E. faecalis parental strain JH2-2. After muramidase digestion and borohydride reduction of the obtained muropeptides, the PG from strain JH2-2 and its derivative mutants was analyzed by RP-HPLC and mass spectrometry (18). PG was extracted from the WT strain and the different mutants from exponential- and stationary-phase cultures (OD₆₀₀ of 0.5 and 2, respectively); the elution profiles of muropeptides extracted in both growth phase conditions were similar for each strain (data not shown). The mass of 29 muropeptides present in the WT strain was determined by MALDI-TOF mass spectrometry (Table 2 and Fig. 5). Taking into account the previously described E. faecalis PG structure (13, 30) (Fig. 6), we deduced the total number of Ala residues present in the C termini of the stem peptides and in the cross-bridges for each muropeptide. Our results showed nine muropeptides harboring a mass increase of 42 Da compared to nine other major muropeptides, suggesting the acetylation of these nine muropeptides. Furthermore, MALDI-post-source decay analysis of acetylated muropeptide 11 indicated that O acetylation occurs on NAM residues as described previously (17) (data not shown).

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TABLE 2. Molecular masses of muropeptides from E. faecalis JH2-2 strain

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FIG. 5. RP-HPLC separation of muropeptides from E. faecalis JH2-2 and the EF_0783 mutant (EF0783). See Table 2 for analysis of peaks. Asterisks indicate peaks identified as containing O-acetylated muropeptides.

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FIG. 6. Model structure for muropeptides from E. faecalis PG showing possible variations in the number of alanyl residues in the C terminus of peptide stems (a), in the cross-bridges (b), and in the free N-terminal side chains (c). R is the number of cross-bridges in oligomeric structures. G, N-acetylglucosamine; M, N-acetylmuramic acid; Ala, alanine; iGlx, isoglutamine or isoglutamate; Lys, lysine; O-Ac, O acetylation.

The muropeptide profiles of EF_0783 and EF_0783 EF_1843 mutants lacked the nine peaks corresponding to O-acetylated muropeptides (peaks 7, 11, 15, 18, 19, 24, 25, 27, and 29 in Table 2) present in the parental strain, JH2-2 PG (Fig. 5). These results confirmed our prediction, based on DNA homology, that the EF_0783 gene encodes the E. faecalis PG O-acetyltransferase.

In contrast, we did not observe any peak harboring a mass defect of 42 Da corresponding to de-N-acetylation in the parental JH2-2 strain, and no obvious differences were observed between the muropeptide profile of the E. faecalis parental strain and that of the EF_1843 mutant. Moreover, the muropeptide profile of the EF_0783 EF_1843 double mutant was similar to that of the EF_0783 mutant.

Following the observed survival sensitivity of the EF_0783, EF_1843, and EF_0783 EF_1843 mutants within murine macrophages, we assessed the comparison of the PG structure of the WT strain grown under rich medium (BHI) and infectious conditions. Since the PG analysis is technically inappropriate for the macrophage model, the cells were incubated instead in an acidic, low magnesium, and minimal nutrient medium (MgM) designed to roughly mimic the macrophage phagosomal environment and to up-regulate virulence gene expression (2). The PG from the stationary-phase culture on BHI or on BHI followed by 4-h incubation in MgM following the procedure of Adkins et al. (2) was treated and extracted as described above. Under these conditions, the muropeptide profiles obtained in BHI and MgM medium were similar to those obtained in GM17 medium.

Expression analysis of EF_1843 and its incidence on E. faecalis PG structure. The curious results obtained with the pgdA homologue raised the questions of whether EF_1843 is expressed and what is its potential effect on the PG structure. For this purpose, the comparative transcriptional analysis of EF_1843 on different media and with different infectious conditions (within mouse peritoneal macrophages) was performed. The quantitative RT-PCR assays showed that the expression of EF_1843 is weak and is expressed 15.1-fold ± 0.08-fold more in cells grown on BHI compared to those grown in macrophages. Moreover, to establish whether EF_1843 has a PG de-N-acetylase function, we incubated the purified histidine-tagged EF_1843 protein (lacking its putative signal sequence) with purified PG under the conditions allowing the PG de-N-acetylation by PgdA of S. pneumoniae (10). After the RP-HPLC analysis under these conditions, no significant modification of the PG structure was observed (data not shown).

Top ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 
It is becoming apparent that pathogens have developed many strategies to evade host control. For example, structural modifications of lipopolysaccharides or secondary polysaccharides and antigenic variation of virulence factors have been acknowledged for some time as mechanisms to escape the innate and adaptive immune systems. Since 1958, it has been known that bacteria have developed ingenious strategies, such as O acetylation (1, 15) and/or de-N-acetylation (3) of the PG, to counteract the hydrolytic activity of lysozyme. Recent studies show that PG is the pathogen-associated molecular pattern recognized by some recently discovered intracellular receptors of the innate immune system (such as Nod proteins and Toll-like receptors) (11, 19). Thus, we hypothesize that O acetylation and de-N-acetylation might be bacterial mechanisms that affect the capacity of the host to present these PG motifs and, therefore, to set up an appropriate response to infection.

In this study, the analysis of the muropeptides isolated from WT PG of E. faecalis showed different peaks with an additional mass of 42 Da (Table 2 and Fig. 5). This strongly suggests the presence of an additional acetyl group in the compounds of these peaks. PG of the EF_0783 mutant lacked these peaks, which indicated that EF_0783 is involved in PG acetylation as previously described for the S. aureus OatA homologue (7). This acetylation should occur at the C-6 OH group of NAM, as demonstrated by MALDI-post-source decay (data not shown) analysis and in accordance with other studies (17, 34, 42). All these data indicate that EF_0783 encodes an NAM-specific O-acetyltransferase, and therefore, we propose naming it the oatA gene.

Sequence alignment of the EF_1843 protein with three PG N-acetylglucosamine deacetylase proteins (Fig. 1) shows that conserved catalytic residues and metal ligand amino acids of the family 4 carbohydrate esterases identified by Blair et al. (9, 10) are conserved in all these proteins. In spite of this homology and although EF_1843 is expressed 15-fold more on BHI than in mouse peritoneal macrophages, no de-N-acetylated muropeptides were identified and no obvious difference was observed between the muropeptide profiles of the E. faecalis parental strain and EF1843 mutant in BHI, GM17, or MgM medium. Moreover, no evidence for de-N-acetylase activity was found for the histidine-tagged EF_1843 protein when incubated directly with the purified PG. These data strongly suggest that EF_1843 does not have a PG de-N-acetylase activity, which is in agreement with the literature data, since to our knowledge, no de-N-acetylated PG was reported during previous E. faecalis PG structure analysis.

As there is no cleavage site on the EF_1843 protein sequence for any known leader peptidase, the protein is most likely translocated across the cytoplasmic membrane by components of the general secretory pathway and remains anchored to the cytoplasmic membrane by its N-terminal membrane domain. Because of the location of the enzyme, the substrate of this member of the polysaccharide deacetylase family would be another extracellular polysaccharide, rather than PG.

It is conceivable that O acetylation of NAM plays a role in controlling the activity of enterococcal cell wall-hydrolyzing enzymes. It has been shown that E. faecalis produces different kinds of autolysins that preferentially hydrolyze either O-acetylated or non-O-acetylated PG (35). Vollmer and Tomasz (48) hypothesized that de-N-acetylation could also be involved in the control of autolysins. However, no difference in morphology (photonic microscopy), growth rate in exponential-phase cultures, and autolysis of stationary-phase cultures has been observed for EF_0783, EF_1843, and EF_0783 EF_1843 mutants, indicating that these genes do not have an obvious key role in the regulation of autolysin processes.

In numerous bacteria, resistance to lysozyme was shown to be directly proportional to the levels of O-acetylated and de-N-acetylated PG. Phenotypic analyses of the EF_1843 mutant did not show a decrease in lysozyme resistance. Only the EF_0783 and EF_0783 EF_1843 mutants have an obvious sensitivity to exogenous lysozyme (Fig. 3). It is noteworthy that the sensitivity of the non-O-acetylated strains remains relative because their growths are only slightly inhibited under 20 mg/ml of lysozyme. In comparison, the growth of S. aureus oatA deletion mutant is affected at 0.8 mg/ml in a lysozyme agar diffusion assay, while its parental strain resists lysozyme concentrations up to 50 mg/ml in liquid medium (6, 7). On the other hand, many bacteria are very sensitive to lysozyme; for example, Listeria monocytogenes and S. pneumoniae show lysozyme sensitivity at concentrations of 10 µg/ml and 80 µg/ml, respectively (12, 48). Collectively, these data suggest that EF_0783 is involved in lysozyme resistance but also that PG O acetylation and de-N-acetylation are not major determinants for lysozyme resistance of E. faecalis.

In this work, we demonstrated that the number of viable intracellular bacteria in infected murine macrophages was significantly reduced in EF_0783, EF_1843, and EF_0783 EF_1843 mutants over the 72-h infection period compared with the JH2-2 wild-type strain of E. faecalis. This suggests that the EF_0783 and EF_1843 genes may be part of the virulence mechanisms of E. faecalis. We could hypothesize that the diminution of persistence in mouse peritoneal macrophages observed for the EF_0783 strain is due to a decrease in lysozyme resistance. However, this hypothesis does not explain the EF_1843 mutant phenotype. As PG motifs are pathogen-associated molecular patterns recognized by the innate immune system, O acetylation may confer to E. faecalis the ability to evade this defense system, as shown recently for PG de-N-acetylation in Listeria monocytogenes (12). Even if the EF_1843 gene is expressed less in mouse peritoneal macrophages, this level of expression seems sufficient to confer a survival advantage to E. faecalis under infectious conditions.

In contrast to lysozyme, a small protein with a molecular mass of 14 kDa, it is assumed that small amounts (less than 50 µg/ml) of mutanolysin (a 23-kDa muramidase from Streptomyces globisporus, not affected by PG O acetylation) (41) usually lysed enterococci. This observation helps to rule out hampered access of lysozyme to its target substrate, i.e., the PG, and suggests that the resistance of E. faecalis to lysozyme may rather be due to a phenomenon that specifically inhibits lysozyme activity.

Moreover, several lines of evidence suggest that lysozyme possesses a bactericidal mode of action that is independent of its enzymatic activity and that does not cause cell lysis. This nonenzymatic inactivation mechanism has been attributed to cytoplasmic membrane destabilization due to the cationic properties of lysozyme, resulting in cell leakage, or to the induction of autolysins (31). This finding may explain why deletion of a PG modification enzyme, like the EF_0783 protein, has such a moderate effect on lysozyme resistance of E. faecalis. In fact, we could hypothesize that this bacterium possesses mechanisms other than PG modification that inhibit the nonenzymic and/or nonlytic mode of action of lysozyme. Such mechanisms might involve secreted proteinaceous lysozyme inhibitors, such as Ivy and Sic reported in E. coli (20) and S. pyogenes (8), respectively. If they exist, these genes remain to be identified, as no homologue for Ivy or Sic is present in the E. faecalis V583 genome sequence. Another way to explain the great lysozyme resistance of E. faecalis is the possible contribution of PG cross-linking and phosphoester-linked wall teichoic acid in the C-6 position of NAM (the same site as for O acetylation) to the resistance against the hydrolytic activity of lysozyme as recently shown for S. aureus (6).

Finally, we can add that EF_1843 is not responsible for the lysozyme sensitivity phenotype observed for the E. faecalis sigV mutant as first hypothesized by Benachour et al. (5), and unlike S. pneumoniae PgdA (48), it appears to not be a PG de-N-acetylase. Taken together, our results showed that E. faecalis lysozyme resistance constitutes an unusual model where EF_0783 PG O-acetyltransferase plays a role but is not the major determinant of lysozyme resistance.

 
L. Hébert was supported by a thesis grant awarded by Region Basse-Normandie and INRA.

We are grateful to François Lebreton for technical assistance and to Jean-Christophe Giard and Vianney Pichereau for critical review of the manuscript.

 
* Corresponding author. Mailing address: Laboratoire de Microbiologie de l'Environnement, USC INRA 2017, EA956, Université de Caen, 14032 Caen Cedex, France. Phone: 33 (0)2 31 56 66 18. Fax: 33 (0)2 31 56 53 11. E-mail: abdellah.benachour{at}unicaen.fr

Published ahead of print on 4 September 2007.

Editor: F. C. Fang

Top ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 

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Infection and Immunity, November 2007, p. 5390-5398, Vol. 75, No. 11
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