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Antimicrobial Agents and Chemotherapy, July 2003, p. 2072-2081, Vol. 47, No. 7
0066-4804/03/$08.00+0     DOI: 10.1128/AAC.47.7.2072-2081.2003
Copyright © 2003, American Society for Microbiology. All Rights Reserved.

Viridans Group Streptococci Are Donors in Horizontal Transfer of Topoisomerase IV Genes to Streptococcus pneumoniae

Luz Balsalobre,1 María José Ferrándiz,1 Josefina Liñares,2 Fe Tubau,2 and Adela G. de la Campa1*

Unidad de Genética Bacteriana (Consejo Superior de Investigaciones Científicas), Centro Nacional de Microbiología, Instituto de Salud Carlos III, 28220 Majadahonda, Madrid,1 Hospital de Bellvitge, L'Hospitalet de Llobregat, Barcelona, Spain2

Received 21 January 2003/ Returned for modification 3 March 2003/ Accepted 25 March 2003


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ABSTRACT
 
A total of 46 ciprofloxacin-resistant (Cipr) Streptococcus pneumoniae strains were isolated from 1991 to 2001 at the Hospital of Bellvitge. Five of these strains showed unexpectedly high rates of nucleotide variations in the quinolone resistance-determining regions (QRDRs) of their parC, parE, and gyrA genes. The nucleotide sequence of the full-length parC, parE, and gyrA genes of one of these isolates revealed a mosaic structure compatible with an interspecific recombination origin. Southern blot analysis and nucleotide sequence determinations showed the presence of an ant-like gene in the intergenic parE-parC regions of the S. pneumoniae Cipr isolates with high rates of variations in their parE and parC QRDRs. The ant-like gene was absent from typical S. pneumoniae strains, whereas it was present in the intergenic parE-parC regions of the viridans group streptococci (Streptococcus mitis and Streptococcus oralis). These results suggest that the viridans group streptococci are acting as donors in the horizontal transfer of fluoroquinolone resistance genes to S. pneumoniae.


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INTRODUCTION
 
Streptococcus pneumoniae (the pneumococcus) remains the leading bacterial cause of community-acquired pneumonia, meningitis, and otitis media. The emergence and spread of resistance to penicillin and macrolide antibiotics (17, 25, 41) have made the selection of the optimal antimicrobial therapy difficult. Parallel increases in the rates of resistance to those antibiotics have also been observed among the viridans group streptococci (VS) (1, 2, 7, 8), which are commensal organisms of the oropharyngeal tracts of healthy individuals but which are also a major cause of endocarditis (46) and bacteremia in neutropenic patients (3, 7, 8, 15). Fluoroquinolones with increased levels of activity against S. pneumoniae, such as levofloxacin, moxifloxacin, and gatifloxacin, are now being recommended for the treatment of patients with community-acquired pneumonia (5). Although the prevalence of ciprofloxacin resistance among S. pneumoniae strains is still low in Spain (3 to 7%) (32, 43) and Canada (2%) (9), higher prevalences have been found among the VS. Among 1,046 isolates of VS characterized as S. mitis and isolated from 1993 to 2001 at the Hospital of Bellvitge, the prevalence of ciprofloxacin resistance was 16.6% (unpublished data), a rate very similar to the rate of 11.4% reported in Canada (11). An increase in the rates of resistance to fluoroquinolones in both S. pneumoniae and VS would be expected as a consequence of the widespread use of these compounds. Prior fluoroquinolone administration is an important risk factor for the selection of resistant strains, as observed for respiratory tract infections caused by ciprofloxacin-resistant (Cipr) (44) and levofloxacin-resistant (10, 54) S. pneumoniae isolates. Likewise, the emergence of Cipr isolates of VS in the blood of neutropenic cancer patients that received fluoroquinolone prophylaxis has been reported (23, 55).

Bacterial resistance to fluoroquinolones occurs mainly by alteration of drug targets. The intracellular fluoroquinolone targets are DNA topoisomerase IV and DNA gyrase (gyrase), enzymes that function by passing a DNA double helix through another by use of a transient double-stranded break (14). DNA gyrase, an A2B2 complex encoded by gyrA and gyrB, catalyzes ATP-dependent negative supercoiling of DNA to relieve the topological stress generated during DNA replication and transcription. Topoisomerase IV, a C2E2 complex encoded by parC and parE, is essential in chromosome partitioning. The amino acid sequences of ParC and ParE are homologous to those of GyrA and GyrB, respectively (29).

Genetic and biochemical studies have shown that topoisomerase IV is the primary target for ciprofloxacin and that gyrase is a secondary target in S. pneumoniae (20, 28, 42, 52). Resistance mutations have been identified in a discrete region of ParC, ParE, and GyrA termed the quinolone resistance-determining region (QRDR). The VS share the same mechanism of ciprofloxacin resistance (23), and it has been possible to transform S. pneumoniae cells to ciprofloxacin resistance with DNA from Cipr VS in the laboratory (23, 27). The VS could act as a reservoir of fluoroquinolone resistance by acting as donors in the horizontal transfer of DNA to pneumococci, similar to the mechanism observed for penicillin resistance (50). The high level of intraspecies variation in the sequences of the DNA topoisomerase genes of VS (6, 23) and the mosaic structures of parC and gyrA in S. pneumoniae clinical isolates (6, 22, 57) led us to suggest that genetic interchange of the fluoroquinolone target genes occurs both among VS and between VS and pneumococci. In this work we present evidence supporting the hypothesis that VS are the donors in the recombination events yielding DNA topoisomerase genes with mosaic structures in S. pneumoniae Cipr clinical isolates.


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MATERIALS AND METHODS
 
Bacterial strains, serotyping, and susceptibility tests. The strains used in this work were isolated from the sputum of adult patients. Only one isolate per patient was evaluated. Identification was by a standard methodology by the following tests: colonial morphology, Gram staining, catalase reaction, optochin susceptibility, and bile solubility. The strains were serotyped at the Spanish Pneumococcus Reference Laboratory (Centro Nacional de Microbiología, Instituto de Salud Carlos III, Majadahonda, Madrid, Spain) by detection of the Quellung reaction with antisera provided by the Staten Seruminstitut (Copenhagen, Denmark). Identification of VS was done by standard methods (16, 48). MICs were determined by the microdilution method with cation-adjusted Mueller-Hinton broth supplemented with 2.5% lysed horse blood, as recommended by the National Committee for Clinical Laboratory Standards (40). The inoculum was prepared by suspension of several colonies from an overnight blood agar culture in Mueller-Hinton broth and adjustment of the turbidity to a 0.5 McFarland standard (ca. 108 CFU/ml). The suspension was further diluted to provide a final bacterial concentration of 104 CFU/ml in each well of the microdilution trays. The plates were covered with plastic tape and incubated in ambient atmosphere at 37°C for 20 to 24 h. The MIC was defined as the lowest concentration of drug that inhibited visible growth. Strains S. pneumoniae ATCC 49619 and S. pneumoniae R6 were used for quality control. Ciprofloxacin was kindly provided by Bayer.

Southern blot analysis. For identification of S. pneumoniae strains, plasmid pCE3 (18), which contains a 0.65-kb fragment coding for the N terminus of the major pneumococcal autolysin (amidase), was used as a source of the lytA DNA probe. Plasmid pJCP191 (51), which contains a 1.6-kb fragment coding for the complete pneumococcal pneumolysin gene, was used as a source of the pnl DNA probe and was kindly provided by S. Taira. Probes specific for parC and parE were obtained by PCR amplification of the R6 laboratory strain with oligonucleotides parCUP, parCDOWN, parEUP, and parEDOWN (Table 1). The ant-specific probe was obtained by amplification of strain 3870 DNA with oligonucleotides antUP and antDOWN (Table 1). All probes were labeled by use of the Phototope-Star detection kit (New England Biolabs). Southern blotting and hybridization were performed according to the instructions of the manufacturer.


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  		TABLE 1. Oligonucleotides used in this work

PCR amplification and DNA sequence determination. S. pneumoniae chromosomal DNA was obtained as described previously (19). DNA topoisomerase QRDRs were amplified from genomic DNA by the PCR described previously (23, 38). The parE, parC, and gyrA genes were amplified with the following primers, based on published sequences (4, 19, 21, 38): parEUP and parEDOWN, parCUP and parCDOWN, and gyrAUP1 and gyrADOWN, respectively (Table 1). PCR amplifications were performed with 0.5 U of Thermus thermophilus thermostable DNA polymerase (Biotools), 1 µg of chromosomal DNA, the synthetic oligonucleotide primers at a concentration of 0.4 µM each, deoxynucleoside triphosphates at a concentration of 0.2 mM each, and 2 mM MgCl2 in a final volume of 50 µl. Oligonucleotides atpWO and atpB56 (Table 1) were used to amplify the atpC and atpA genes. Amplification was achieved with an initial cycle of 1 min of denaturation at 94°C and 30 cycles of 30 s at 94°C, 90 s at 55°C, and a polymerase extension step for 80 s at 72°C, with a final extension step for 8 min at 72°C and slow cooling at 4°C. Electrophoresis of the PCR products was carried out in agarose gels as described previously (49). The DNA fragments were purified with MicroSpin S400 HR columns (Amersham Pharmacia Biotech), and both strands were sequenced with an Applied Biosystems Prism 377 DNA sequencer by the protocols provided by the manufacturer with both the primers used for the PCR amplifications and internal primers.

Phylogenetic analysis. Phylogenetic analysis was performed by using the MEGA program (version 2.1) (31), available at http://www.megasoftware.net. Dendrograms were constructed by the unweighted pair group method with arithmetic mean (UPGMA) method with the Kimura-2 parameter. The percentage of bootstrap confidence levels for internal branches, as defined by the MEGA program (31), was calculated from 1,000 random resamplings.

Nucleotide sequence accession numbers. The new DNA sequences reported in this paper have been assigned the following GenBank accession numbers: AY166963, AY166965, AY167641 to AY167643, AY168409 to AY168412, and AY184477 (ant regions); AY167637 and AY167640 (atpCA regions); AY157690 (parC of S. pneumoniae 4391); AY157689 (gyrA of S. pneumoniae 4391); and AY157687, AY157688, and AY167691 (parE sequences). The atpCA region of S. pneumoniae 4589 is identical to that of S. pneumoniae 3180 (GenBank accession number AF171000).


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RESULTS
 
Characterization of S. pneumoniae isolates. An epidemiological study performed at the Hospital of Bellvitge during an 11-year period (1991 to 2001) revealed that 2.3% (89 of 3,819) of the S. pneumoniae isolates were Cipr (MICs ≥ 4 µg/ml) (unpublished results). The parC, parE, gyrA, and gyrB QRDRs of a total of 46 Cipr strains were characterized. All strains showed low levels of variation (≤1%) in the nucleotide sequences of their gyrB QRDRs. However, although 41 of the 46 strains showed variations of ≤1% in the sequences of their parE, parC, and gyrA QRDRs, the sequences of at least one of the QRDRs from 5 strains (strains 3180, 3870, 4391, 4589, and 5237) exhibited unexpectedly high levels of nucleotide sequence variation (>4%). Since high levels of nucleotide sequence variation in the QRDRs have been associated with a mosaic structure in the parC and gyrA genes of strains 3180 and 3870 (21), we hypothesized that a gene showing a level of QRDR nucleotide sequence variation greater than 4% will have a mosaic structure, indicative of interspecies horizontal DNA transfer. Comparison of the parC, parE, and gyrA QRDRs of these five strains with mosaic gene structures with that of S. pneumoniae R6 and the Cipr S. pneumoniae 4638 typical isolate (which did not show variations in their QRDRs), the S. mitis and S. oralis type strains, and three Cipr S. mitis isolates was performed (Fig. 1; Table 2). Strains with mosaic gene structures showed levels of nucleotide sequence variation greater than 4% for parE (five of five strains), parC (five of five strains), and gyrA (four of five strains, with the exception being strain 5237) compared with the sequences of S. pneumoniae R6 and S. pneumoniae 4638. All Cipr strains of S. pneumoniae and VS showed typical mutations, yielding changes in ParC S79 (to F, N, or Y) and GyrA S81 (to F or Y) (Fig. 1; Table 2). Two additional amino acid changes (ParC K137N and ParE I460V) that are not involved in resistance and that are present in both Cips and Cipr S. pneumoniae strains, and which are consequently considered polymorphisms, were found in S. pneumoniae 4638. All five S. pneumoniae strains with high levels of nucleotide sequence variations in their parC QRDRs showed the same amino acid change (N91D) present in the ParC proteins of both Cips and Cipr strains of VS. When the GyrA QRDR was considered, the S114G amino acid change was observed in the four S. pneumoniae strains with high levels of nucleotide sequence variations in their gyrA QRDRs and in both Cips and Cipr strains of VS. The presence of ParC N91D and GyrA S114G in strains of VS and in Cipr S. pneumoniae strains with a mosaic gene structure suggests that strains with a mosaic gene structure have originated by recombination with VS. Additional amino acid changes were observed in S. pneumoniae 5237 (ParC I126V and E135D), S. mitis 181731-3 (GyrA M90G), and S. mitis 181732-2 (GyrA N150S and ParE P424A, I460L, A463E, K466N, and A468S). These changes are an indication of the high level of intraspecies variation in the VS.



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  		FIG. 1. Nucleotide sequence variations in the ParC, ParE, and GyrA QRDRs. The nucleotides present at each polymorphic site are shown for strain R6, but for the other strains, only the nucleotides that differ from those in R6 are shown. Changes yielding amino acid substitutions, including those involved in fluoroquinolone resistance, are shown in boldface. Codon numbers are indicated vertically above the sequences. Positions 1, 2, and 3 in the fourth row refer to the first, second, and third nucleotides in the codon, respectively. Strains whose sequences do not differ from that of R6 more than 1% are shaded in gray. SPN, S. pneumoniae; SOR, S. oralis; SMI, S. mitis.


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  		TABLE 2. Phenotypes of fluoroquinolone-resistant S. pneumoniae and S. mitis strains and amino acid changes in their DNA topoisomerase genes

Identification of strains as S. pneumoniae by use of molecular tools. We have previously characterized S. pneumoniae 3180 and S. pneumoniae 3870 (21) by hybridization of their DNA with pneumococcal pnl- and lytA-specific probes (18, 24, 45, 47, 56). In the present study identical tests were performed with the rest of the strains with mosaic gene structures. Tests with the pnl-specific probe detected hybridization with single fragments of about 5 kb of ClaI-digested DNA of the S. pneumoniae strains, while no hybridization was observed with the DNAs of the VS. As expected, a 1.2-kb HindIII-digested chromosomal fragment of all S. pneumoniae strains hybridized with the lytA-specific probe, while the DNA of the strains of VS did not. Sequencing of a region spanning 960 nucleotides, including the atpC and atpA gene sequences, which is responsible for the unique optochin susceptibility of the pneumococcus (19, 33, 39) allowed further characterization of the strains. The sequences of all S. pneumoniae strains showed a high degree of homogeneity (less than 0.7% nucleotide sequence variation), while the sequences of S. pneumoniae R6 and type strains of VS varied by greater than 20%. These values are in agreement with those obtained by comparison of amylomaltase gene sequences: ≤0.5% S. pneumoniae intraspecies variation (13) and 4 to 6% divergence between S. pneumoniae and S. oralis (12). A phylogenetic tree was constructed with the concatenated atpC and atpA genes of S. pneumoniae and strains of VS, with the sequence of Bacillus halodurans used as the outgroup (Fig. 2). The S. pneumoniae strains formed a monophyletic group within the tree.



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  		FIG. 2. UPGMA trees of concatenated atpC and atpA genes, full-length parE genes, and ant genes. Phylogenetic and molecular evolutionary analyses were conducted with the MEGA program (version 2.1) by the UPGMA method. Only bootstrap confidence intervals exceeding 90% are shown. The GenBank accession numbers for each nucleotide sequence are shown in parentheses.

Analysis of parC, parE, and gyrA gene sequences. To assess the recombinational origin of the strains with mosaic gene structures, the parE sequences of S. pneumoniae 4391 and the type strains of VS were determined. Oligonucleotides based on the S. pneumoniae R6 sequence were used to obtain and sequence the PCR products. Nucleotide sequence variations of between 8 and 12% were observed among the strains (Fig. 3). Similar variations were observed between the parE sequence of strain 4391 and that of S. pneumoniae R6 or S. pneumoniae TIGR4. However, the parE sequences of R6 and TIGR4 were almost identical (1% variation). The variations found could be organized into blocks with different degrees of relatedness. The limits of the blocks were determined by inspection, with the only limitation being at least a 4% difference in divergence between two contiguous blocks. Two blocks were detected in S. pneumoniae 4391, while no blocks were detected in the type strains of VS (Fig. 3). A WU-BLAST search of the Swiss-Prot sequence database with the S. pneumoniae 4391 ParE sequence was performed to select sequences to be used in the construction of the tree shown in Fig. 2. Only the nucleotide sequences of the more similar proteins, which corresponded to the ParE subunits of strains S. pneumoniae TIGR4 and R6, along with the sequences determined in this work and that of B. halodurans as an outgroup, were used. The parE sequences of S. pneumoniae 4391, S. mitis 12261, S. pneumoniae R6, and S. pneumoniae TIGR4 formed a statistically significant group within the tree.



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  		FIG. 3. Mosaic structures of the gyrA, parC, and parE genes of the indicated strains of S. pneumoniae and VS. The locations of the QRDRs are indicated at the tops of the gyrA and parC sequences. The positions of the active Tyr residues (Y120 in GyrA and Y118 in ParC) that bind to DNA and the Ser residues that are changed in strain 4391 (S81 in GyrA and S79 in ParC) and that are involved in resistance are marked. Blocks showing the percent sequence divergence from the corresponding regions of S. pneumoniae R6 are indicated. White box, region of the sequence that differed by ≤1.5%; gray boxes, regions that differed by more than 1.5% but less than 9%; black boxes, regions that differed by >9%. The strains used were S. pneumoniae (SPN) R6 (GenBank accession number AE008451), S. pneumoniae TIGR4 (GenBank accession number AE007391), S. pneumoniae 4391, S. mitis (SMI) NCTC 12261, and S. oralis (SOR) NCTC 11427.

The full-length parC and gyrA sequences of strain 4391 were also determined. The gyrA sequence showed 8.9% variation and the parC sequence showed 5.8% variation compared to the S. pneumoniae R6 sequences that could be organized into blocks of divergence (Fig. 3).

Characterization of parE-parC intergenic regions. Several PCR amplifications were performed to determine the sequences of the parE and parC genes of S. pneumoniae 4391. When oligonucleotides parEDOWNR (coding for the last 4 ParE residues) and parC26R (complementary to the strand coding for ParC residues 26 to 33) were used, a product of about 6 kb was obtained. Since the intergenic parE-parC region of S. pneumoniae R6 is 420 bp long (38), these results suggested a different genetic organization of the parE-parC chromosomal region in strain 4391. To determine whether this is the case for the rest of the Cipr S. pneumoniae strains with mosaic gene structures, the lengths of their parE-parC intergenic regions were determined and compared with those of S. pneumoniae R6, the typical Cipr strain S. pneumoniae 4638, and both Cips and Cipr strains of VS. PCR amplifications with four pairs of oligonucleotides whose sequences are specific for sequences located upstream of parE and in the parC N terminus (parEUP and parC26R), upstream of parE and downstream of parC (parEUP and parCDOWN), downstream of parE and in the parC N terminus (parEDOWNR and parC26R), and downstream of parE and downstream of parC (parEDOWNR and ParCDOWN) were performed. The sizes of these PCR products were compatible with the sizes of the intergenic regions for R6 and 4638 (0.4 kb), for strains of VS (range, 1 to 2.5 kb), and the S. pneumoniae Cipr strains with mosaic gene structures (range, 1.9 to 6.2 kb). Southern blotting experiments with parC- and parE-specific probes and digestion of the PCR products were performed (data not shown) to construct physical maps for EcoRV and NcoI in the parE-parC regions (Fig. 4). These experiments showed that, except for S. pneumoniae R6 and S. pneumoniae 4638, the estimated sizes of the intergenic parE-parC regions were in the range of 1 to 6.2 kb. Among the S. pneumoniae strains with mosaic gene structures, strains 3870 and 4589 shared the same physical maps for the parE-parC chromosomal region (Fig. 4). Among the strains of VS, the same parE-parC physical map was observed for S. mitis 12261T and S. mitis 181731-3.



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  		FIG. 4. Restriction map of the parE-parC region of S. pneumoniae strains and strains of VS and its genetic organization as deduced from Southern blotting experiments and nucleotide sequence analyses. E, EcoRV; N, NcoI. The parE and parC genes with mosaic structures are indicated by with striped arrows. SOR, S. oralis; SMI, S. mitis.

Characterization of ant gene. The nucleotide sequences of the parE-parC intergenic regions of S. oralis 10557T, S. pneumoniae 3870, and S. pneumoniae 4589 revealed the presence of an open reading frame, ant, recently described in the intergenic parE-parC region of S. mitis isolate CIP 103335T (27). To determine whether this gene was also present in other parE-parC intergenic regions, a probe specific for ant of S. pneumoniae 3870 was constructed by PCR with oligonucleotides antUP and antDOWN. This probe was used to hybridize the Southern blots of DNA cut with EcoRV and NcoI described above. These experiments (Fig. 5) confirmed the presence of ant in the intergenic parE-parC region of all strains of VS checked and in all S. pneumoniae strains with a suspected recombinational origin. However, no hybridization was observed with several S. pneumoniae strains, including R6 and strains ATCC 49619, ATCC 700669, and ATCC 700671; the last two strains are representative of clones Spain23F-1 and Spain9V-3, respectively (35). In addition, two pneumococcal clinical isolates with low-level Cipr (strains 3724 and 4837) and two pneumococcal clinical isolates with high-level Cipr (strains 4638 and 4235) and typical ParC, ParE, and GyrA QRDRs (data not shown) did not hybridize with the ant-specific probe (Fig. 5). These results show that the presence of ant is a characteristic of VS and that it is present in the parE-parC intergenic region of S. pneumoniae strains with a mosaic gene structure. The sequence of the ant open reading frame is homologous to those of ant genes encoding aminoglycoside adenylyltransferase enzymes from bacteria (36) but was not associated with any particular phenotype in the S. pneumoniae strains with mosaic gene structures or in the strains of VS (data not shown). Comparison of the ant nucleotide sequences of S. pneumoniae strains with mosaic gene structures and those of the strains of VS showed similarities between 99.7 and 65% (Fig. 2) and identity among S. pneumoniae strains 4589 and 3870 with mosaic gene structures. The ant sequences determined in this work were analyzed along with those of S. mitis CIP 103335T and B. halodurans and used in the construction of the phylogenetic tree shown in Fig. 2. The S. pneumoniae strains with mosaic gene structures and the strains of VS formed a separate group within the tree, with S. oralis 10557 being the only exception.




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  		FIG. 5. Southern blot hybridization of S. pneumoniae strains and strains of VS with an ant-specific probe. Chromosomal DNAs from the indicated strains were cleaved with EcoRV-NcoI, and the fragments were separated in 0.8% agarose gels. The gels were blotted, and the blot was probed with a biotinylated probe derived from S. pneumoniae 3870 containing positions 26 to 290 of the ant gene. The numbers on the bottom right are molecular sizes (in kilobases).


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DISCUSSION
 
The QRDRs of the DNA topoisomerase genes of 46 Cipr S. pneumoniae strains isolated during an 11-year period at the Hospital of Bellvitge were characterized. Four isolates showed high levels of nucleotide variation (>4%) in three genes (parC, parE, and gyrA), and one isolate showed high levels of nucleotide variation in two genes (parE and parC) (Fig. 1). We hypothesized that these unexpected variations in the QRDRs reflect the variations present in the whole gene. An isolate with a level of nucleotide variation of more than 4% in the QRDR of a specific gene compared with the sequence of S. pneumoniae R6 will likely have a mosaic gene structure that originated by recombination. This hypothesis was confirmed for all those genes that we have fully sequenced, such as parC of strains 3180 and 3870 (21) and strain 4391 (Fig. 3), parE of strain 4391 (Fig. 3), and gyrA of strains 3180 and 3870 (21) and strain 4391 (Fig. 3). Other investigators have reported results compatible with that hypothesis (6, 57). Among the five strains with mosaic gene structures, four would have a mosaic structure in their parC, parE, and gyrA genes and one strain would have a mosaic structure in its parC and parE genes. The genetic organization of the parE-parC chromosomal region of the five S. pneumoniae strains with mosaic gene structures isolated was different from that of typical S. pneumoniae strains, such as strains R6 and 4638. The size of the intergenic parE-parC region in the strains with mosaic gene structures and the strains of VS was longer than that in typical S. pneumoniae strains (Fig. 4). While the sizes of the intergenic regions in VS strains varied between 1 and 2.5 kb, those from the S. pneumoniae strains with mosaic gene structures studied in this work varied between 1.9 and 6.2 kb (Fig. 4). These values are compatible with an interchange of genetic material between VS and pneumococci. Supporting this hypothesis, the ant gene is present in the intergenic parE-parC region both in S. pneumoniae strains with mosaic gene structures and in strains of VS but is absent from typical S. pneumoniae strains (Fig. 5). In addition, the ant gene was not found in the S. pneumoniae R6 (26) or the S. pneumoniae TIGR4 (53) sequences in databases. Although the nucleotide sequences of the ant genes showed high levels of heterogeneity, comparison of the sequence of this gene from strains of VS and S. pneumoniae strains with mosaic gene structures and the ant gene sequences present in the databases showed that ant genes from S. pneumoniae strains with mosaic gene structures and strains of VS formed a separate group within the phylogenetic tree (Fig. 2). Similar results were obtained when the parE genes were compared (Fig. 2). Altogether these results show that ant is typical of VS, and its presence in the S. pneumoniae strains with mosaic gene structures indicates that a strain of VS, probably an S. oralis or S. mitis strain, was the donor in the recombination event that originated the mosaic parE and parC genes. S. oralis and S. mitis are the species the most closely related to S. pneumoniae on the basis of their 16S rRNA sequences, which exhibit more than 99% identity with the 16S rRNA sequence of S. pneumoniae, although the DNA-DNA similarity for the total chromosomal DNAs of S. oralis and S. mitis with the chromosomal DNA of S. pneumoniae is less than 50% (30). Analysis of the genetic structures of the five S. pneumoniae strains with mosaic gene structures (Fig. 4) suggests that the initial interchange that originated these strains with mosaic gene structures included the whole parE-ant-parC chromosomal region. However, further reorganizations by recombination with VS or S. pneumoniae probably occurred, as deduced from the analysis of the gyrA, parC, and parE genes of strain 4319 (Fig. 3) and of the gyrA and parC genes of strains 3180 and 3870 (21).

We have not found identity between the ant sequences of VS and S. pneumoniae strains with mosaic gene structures. However, identity was observed between the ant genes of S. pneumoniae strains 3870 and 4589 (Fig. 2). Although these strains had identical ant sequences, they were isolated from unrelated patients in different years, had different pulsed-field gel electrophoresis patterns (data not shown), and had different nucleotide sequences in their parC, parE, gyrA, and atpCA genes. However, both strains shared the same physical structure in their parE-ant-parC regions and ant sequences, suggesting that they have interchanged the parE-ant-parC region with that of a closely related VS strain.

Among the Cipr S. pneumoniae isolates, we have observed a low prevalence (11%; 5 of 46 strains) of strains with mosaic gene structures in the parE-parC region. These strains were of serotype 23F (two of five strains) or not typeable (three of five strains), and all of them were also resistant to penicillin and other drugs. Since serotype 23F is one of the most common among penicillin-resistant isolates in Spain and worldwide (17, 34, 37), it would be possible for these strains with mosaic gene structures to spread in the near future.


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ACKNOWLEDGMENTS
 
We thank A. Fenoll for checking the serotypes. The technical assistance of A. Rodríguez-Bernabé is acknowledged.

L.B. had a fellowship from Instituto de Salud Carlos III. This work was supported by grant BIO2002-01398 from the Ministerio de Ciencia y Tecnología and by grants 00/0258 and 01/1267 and Red Temática de Investigación Cooperativa G03/103 from Fondo de Investigación Sanitaria.


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FOOTNOTES
 
* Corresponding author. Mailing address: Unidad de Genética Bacteriana (Consejo Superior de Investigaciones Científicas), Centro Nacional de Microbiología, Instituto de Salud Carlos III, 28220 Majadahonda, Madrid, Spain. Phone: (34) 91-5097057. Fax: (34) 91-5097919. E-mail: agcampa{at}isciii.es. Back


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Antimicrobial Agents and Chemotherapy, July 2003, p. 2072-2081, Vol. 47, No. 7
0066-4804/03/$08.00+0     DOI: 10.1128/AAC.47.7.2072-2081.2003
Copyright © 2003, American Society for Microbiology. All Rights Reserved.



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