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Antimicrobial Agents and Chemotherapy, March 2008, p. 822-830, Vol. 52, No. 3
0066-4804/08/$08.00+0     doi:10.1128/AAC.00731-07
Copyright © 2008, American Society for Microbiology. All Rights Reserved.

Fitness of Streptococcus pneumoniae Fluoroquinolone-Resistant Strains with Topoisomerase IV Recombinant Genes

Luz Balsalobre and Adela G. de la Campa^*

Unidad de Genética Bacteriana, Centro Nacional de Microbiología and Ciber Enfermedades Respiratorias, Instituto de Salud Carlos III, 28220 Majadahonda, Madrid, Spain

Received 6 June 2007/ Returned for modification 20 August 2007/ Accepted 19 December 2007

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The low prevalence of ciprofloxacin-resistant (Cp^r) Streptococcus pneumoniae isolates carrying recombinant topoisomerase IV genes could be attributed to a fitness cost imposed by the horizontal transfer, which often implies the acquisition of larger-than-normal parE-parC intergenic regions. A study of the transcription of these genes and of the fitness cost for 24 isogenic Cp^r strains was performed. Six first-level transformants were obtained either with PCR products containing the parC quinolone resistance-determining regions (QRDRs) of S. pneumoniae Cp^r mutants with point mutations or with a PCR product that includes parE-QRDR-ant-parC-QRDR from a Cp^r Streptococcus mitis isolate. The latter yielded two strains, T6 and T11, carrying parC-QRDR and parE-QRDR-ant-parC-QRDR, respectively. These first-level transformants were used as recipients in further transformations with the gyrA-QRDR PCR products to obtain 18 second-level transformants. In addition, strain Tr7 (which contains the GyrA E85K change) was used. Reverse transcription-PCR experiments showed that parE and parC were cotranscribed in R6, T6, and T11; and a single promoter located upstream of parE was identified in R6 by primer extension. The fitness of the transformants was estimated by pairwise competition with R6 in both one-cycle and two-cycle experiments. In the one-cycle experiments, most strains carrying the GyrA E85K change showed a fitness cost; the exception was recombinant T14. In the two-cycle experiments, a fitness cost was observed in most first-level transformants carrying the ParC changes S79F, S79Y, and D83Y and the GyrA E85K change; the exceptions were recombinants T6 and T11. The results suggest that there is no impediment due to a fitness cost for the spread of recombinant Cp^r S. pneumoniae isolates, since some recombinants (T6, T11, and T14) exhibited an ability to compensate for the cost.

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Streptococcus pneumoniae (the pneumococcus) is an important cause of morbidity and mortality and is a major etiological agent of community-acquired pneumonia, meningitis, and acute otitis media. Pneumococcal resistance to antimicrobials (including β-lactams, macrolides, tetracyclines and co-trimoxazole) has become a worldwide problem (21). The new fluoroquinolones have been recommended for use for the treatment of community-acquired pneumonia in adults (30). Although the current prevalence of fluoroquinolone resistance among pneumococci is less than 3% (8, 12, 13, 27), prevalence values greater than 13% (10, 41) have been reported for the viridans group streptococci of the mitis group (SMG), which are considered the donors of resistance genes to pneumococci (3).

Resistance to fluoroquinolones in streptococci occurs mainly by alteration of their intracellular drug targets, the essential enzymes DNA topoisomerase IV (topo IV; ParC₂ ParE₂) and DNA gyrase (gyrase; GyrA₂ GyrB₂). Resistance mutations have been identified in the quinolone resistance-determining regions (QRDRs) of ParC, ParE, and GyrA, located either in the N-terminal domains of ParC and GyrA or in the C-terminal domain of ParE. Genetic and biochemical studies have shown that for fluoroquinolones, such as ciprofloxacin (CIP) and levofloxacin (LVX), topo IV and gyrase are the primary and the secondary targets, respectively (15, 23, 33, 35, 43). However, for other fluoroquinolones, such as moxifloxacin and gemifloxacin, gyrase is the primary target (20). Low-level-CIP-resistant (Cp^r) isolates had mutations that altered the QRDRs of one of the two subunits of topo IV, while high-level-Cp^r isolates had additional changes in GyrA. Clinical resistance to fluoroquinolones is achieved only with mutants with double mutations, for which CIP MICs can reach 16 µg/ml (11). Resistance can be acquired by point mutation, intraspecific recombination (42), or interspecific recombination with the SMG (3, 5, 11, 16, 42, 46). Although the acquisition of resistance by interspecific recombination could be much more common than that by point mutation, considering the frequencies of these events under laboratory conditions, S. pneumoniae Cp^r recombinants account for less than 11% of Cp^r isolates (3, 11, 42). An initial study from our group detected 2.3% Cp^r (MICs 4 µg/ml) isolates among 3,819 pneumococci collected at Bellvitge Hospital from 1991 to 2001. Among 46 isolates analyzed, 86.9% had point mutations and 13.1% were recombinants (3, 16; unpublished results). An additional study of 2,882 pneumococci collected during 2002 at the Spanish Reference Laboratory showed 2.6% Cp^r isolates, with 93.3% having point mutations and 6.7% being recombinants (11). However, under laboratory conditions, the frequency of mutation to Cp^r is about 10^–9 (36), while the frequency of transformation with chromosomal DNA from Cp^r Streptococcus mitis isolates is about 10^–3 (19, 22). Besides other factors, such as the availability of DNA in the natural environment and the competence state of the recipient cells, one cause that could account for the low frequency of the Cp^r recombinant isolates is the fitness cost imposed by the DNA interchange. It is well known that the development and dissemination of antibiotic resistance in bacteria depend on the balance between antibiotic use and the cost that resistance imposes on bacterial fitness (1). A direct relation between the consumption of fluoroquinolones and an increase in the prevalence of resistance among S. pneumoniae isolates has been observed (8, 27). In addition, the emergence of resistance during the treatment of pneumococcal pneumonia with fluoroquinolones has been described (9, 37, 38, 44). Moreover, long-term CIP therapy of Pseudomonas aeruginosa infections caused the emergence of Cp^r pneumococci that were colonizing a patient with bronchiectasis (12).

It has been shown, however, that fitness depends on the specific drug and on the specific mutation (14, 17, 25, 28) and that compensatory mutations can ameliorate the fitness loss (6, 7). Although a few reports have related the fitness cost of fluoroquinolone resistance mutations in S. pneumoniae (18, 24, 39), there are no reports on the fitness cost for recombinant strains. The Cp^r pneumococcal recombinant isolates studied by our group have acquired portions of either parE (unpublished results), parC (11), or parE plus parC (3, 11, 16) from the SMG. In the last case, given the presence of the ant gene in the intergenic parE-parC region of the SMG but not in S. pneumoniae, recombinants acquired an extra gene in the recombination process and, consequently, had larger intergenic parE-parC regions (1.1 to 7.2 kb) than nonrecombinant pneumococci (0.4 kb). It is unknown how the parE and parC genes are transcribed, but it could be assumed that transcription should be coordinated given the tetrameric (ParC₂ ParE₂) structure of topo IV. If the transcription of parE-parC occurs from a single promoter in S. pneumoniae, the acquisition of ant could affect transcription and, consequently, the fitness of recombinant isolates. Another factor that could influence fitness is the existence in recombinants of parE and parC genes of different origins (pneumococci and the SMG).

In this work we made a parallel analysis of the transcriptional characteristics of and the fitness cost for isogenic Cp^r strains, including recombinant Cp^r strains.

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MIC determination. Antimicrobial susceptibility testing was performed by the agar dilution method. The MICs of CIP (Bayer, Barcelona, Spain) and LVX (Sanofi-Aventis, Barcelona, Spain) were determined according to the testing conditions proposed by the Clinical and Laboratory Standards Institute (11). Sparfloxacin (SPX; kindly provided by Rhône-Poulec Rorer, Antony, France). Mueller-Hinton agar plates (Difco) supplemented with 5% defibrinated sheep blood were used to grow the strains. S. pneumoniae strains ATCC 49619 and R6 were used for quality control.

Bacterial strains and growth and transformation of bacteria. The S. pneumoniae Cp^r strains used as DNA donors in transformation experiments were laboratory mutant CMJ1 (31) and Cp^r clinical isolates CipR-49, CipR-55, CipR-59, CipR-60, CipR-73, and 5237 (3, 11). Cp^r S. mitis isolate 181731-3 (3) was also used as a donor. In addition, strain Tr7, a strain R6 derivative carrying the GyrA E85K change (45), was kindly provided by E. Varon and L. Gutmann. Strain Tr7 has a CIP MIC equal to that of R6, but its SPX MIC is 2 µg/ml, while the SPX MIC of R6 is 0.25 µg/ml. S. pneumoniae was grown in a casein hydrolysate-based medium with 0.2% sucrose (AGCH medium) as the energy source and was transformed as described previously (26). Cultures containing 9 x 10⁶ CFU per ml of strain R6 or its derivatives were treated with 0.1 µg/ml DNA for 40 min at 30°C and then at 37°C for 90 min before they were plated on AGCH medium plates containing 1% agar and 1 µg/ml of CIP (first-level transformants) or 8 µg/ml of CIP (second-level transformants). The PCR products used for the construction of isogenic transformants were obtained either from S. pneumoniae with oligonucleotides parC50 and parC152R (parC-QRDRs) or gyrA44 and gyrA170R (gyrA-QRDRs) or from S. mitis 181731-3 with oligonucleotides parE398 and parC152R (parE-QRDR-ant-parC-QRDR). In the competition experiments, the colonies were counted after 24 h of growth at 37°C in a 5% CO₂ atmosphere on AGCH medium with 1% agar plates. The Escherichia coli strain used for plasmid transformation was XL1-Blue (Stratagene). E. coli was grown in Luria-Bertani broth and was transformed as described previously (39).

Plasmid construction, PCR amplification, and DNA sequence determination. Chromosomal DNA was obtained as described previously (19). PCR amplifications were usually performed with 1 U of Thermus thermophilus DNA polymerase (Biotools, Madrid, Spain), 0.1 µg of chromosomal DNA, 1 µM (each) of the oligonucleotide primers (Table 1), and 0.2 mM of each deoxynucleoside triphosphate in a final volume of 50 µl in the buffer recommended by the manufacturers. Amplification was achieved with an initial cycle of 1 min of denaturation at 94°C and 25 cycles of 1 min at 94°C, 45 s at 55°C, and a polymerase extension step for 1 to 5 min at 72°C, with a final extension step for 8 min at 72°C and slow cooling at 4°C. To amplify DNA fragments longer than 5 kb, the Expand Long Template PCR system (Roche, Germany) and the Certamp long amplification kit (Biotools, Madrid, Spain), both of which are able to amplify up to 20 kb, were used according to the manufacturers' instructions. The PCR products were purified and sequenced with an Applied Biosystems Prism 377 DNA sequencer. Agarose gel electrophoresis of the PCR products was carried out as described previously (40). The plasmids used for primer extension were constructed as follows. To construct pLPARE, a 2,528-bp PCR fragment (parE positions –858 to 1670) obtained with the parE–853 and phosphorylated parE549R oligonucleotides from the chromosomal DNA of strain R6 was cut with HindIII (a target included in parE–853) and ligated to pLS1 treated with EcoRI plus T4 DNA polymerase and HindIII. To make pLPPARC, a PCR fragment of 3,366 bp (position 1612 of parE to position 141 downstream of parC) amplified with the phosphorylated parE538 and parCDOWN oligonucleotides from R6 was cut with HindIII and ligated to pLS1, treated as described above. To construct pUPANT in E. coli XL1-Blue, a PCR fragment from S. mitis 181731-3 amplified with the oligonucleotides parE583 and parCDOWN was cut with HindIII-EcoRI to obtain a 1,662-bp fragment (positions –158 to 1504 of ant) that was ligated to pUC18 cut with the same enzymes. S. pneumoniae and E. coli transformants were selected in 1 µg/ml tetracycline and 100 µg/ml ampicillin, respectively.

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TABLE 1. Oligonucleotides used in this work

RNA extraction, primer extension, and RT-PCR experiments. The total RNA of the exponential cultures was extracted with an RNeasy midi kit (Qiagen), which included a DNase treatment, according to the manufacturer's instructions. S. pneumoniae cultures were previously lysed for 15 min at 37°C in 10 mM Tris, 1 mM EDTA (pH 8.0), and 0.1% sodium deoxycholate. Primer extension analysis was performed as described previously (2) by using 20 µg of RNA from S. pneumoniae R6 carrying plasmid pLPPARC or pLPPARE or from Escherichia coli XL1-Blue carrying plasmid pUPANT. The oligonucleotides (1 pmol) used were parE26R, parC26R, and ant8R. Synthesis of cDNAs in the reverse transcription-PCR (RT-PCR) experiments were carried out in 20-µl reaction mixtures containing 0.5 µg of RNA, 0.5 mM of each deoxynucleoside triphosphate, 2 pmol of each gene-specific primer, 40 U of the RNaseOUT RNase inhibitor, and 200 U of SuperScript III RNase H^– reverse transcriptase (Invitrogen), which can generate cDNA up to 12 kb, in the buffer recommended by the manufacturer.

The reaction mixtures were incubated for 60 min at 55°C, and the reactions were terminated by subjecting the reaction mixtures to heat (70°C, 15 min). The template RNA was removed by 20 min of incubation at 37°C with 20 U of RNase H (Amersham Biosciences). The cDNAs obtained were subjected to quantitative real-time PCR (LightCycler 2.0 instrument) in 20-µl reaction mixtures containing 2 µl of cDNA, 2 µM of each primer, a variable amount of MgCl₂ (2 to 3 mM), and 2 µl of LightCycler FastStart DNA Master SYBR green I (Roche). Amplification was achieved with 42 cycles of a three-segment program: denaturation (10 s at 95°C), annealing (15 s at 50 to 55°C), and elongation (6 to 11 s at 72°C). To check the purity of the amplification product, a melting curve program (65 to 95°C with a heating rate of 0.1°C/s and continuous fluorescence measurement) was performed. For relative quantification of the fluorescence values, a calibration curve was made with the PCR products of each amplicon obtained from strain T11 genomic DNA. To normalize the four independent cDNA replicate samples, the values were divided by those obtained from the amplification of an internal fragment of 142 bp of the rpoB gene with oligonucleotides rpoB428 and rpoB474R.

Determination of bacterial fitness. The cost of a resistance mutation was determined by direct competition against the susceptible R6 strain. Individual strains were growth exponentially to an optical density at 600 nm of 0.25. The cultures were diluted 2,000-fold, and mixed cultures containing equivalent amounts of R6 and each resistant strain (about 5 x 10⁴ CFU/ml) were incubated in antibiotic-free medium for 6 h (ca. 10 to 12 generations). The mixed cultures were then diluted 1,000-fold to avoid the typical lysis of S. pneumoniae cultures at a high optical density and were regrown for an additional 6 h. The number of viable cells was determined at 0 h, at the end of the first 6-h cycle (6 h), and after the second 6-h cycle (12 h) by plating aliquots of the culture on AGCH agar plates containing either 1 µg/ml CIP (0.25 µg/ml SPX when strain Tr7 was used) or no drug. The number of susceptible cells was calculated by subtracting the number of resistant cells from the total cell number revealed by the CFU counts of the plates without drug. For the determination of CFU numbers, the mean of two or three plates was used. The number of generations of the resistant strain and strain R6 in the mixed culture was calculated by using the following formula: (log B – log A)/(log 2), where A is the number of CFU/ml at time zero and B is the number of CFU/ml at the end of each cycle (6 h and 12 h). The relative fitness of each strain was determined from the ratio of the number of generations of the resistant strain and strain R6. The means of four to nine replicate competition assays were determined. The 95% confidence intervals (CIs) were calculated on the basis of the t distribution with N degrees of freedom, where N refers to the number of replicates. Statistical tests were performed with GraphPad Prism (version 4) software.

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Generation of Cp^r isogenic strains. A series of isogenic mutants derived from strain R6 carrying different combinations of mutations in parC, parE, and gyrA were obtained by transformation (Table 2). Six first-level transformants (CIP MICs, 2 to 4 µg/ml; LVX MICs, 2 µg/ml) carrying parC mutations were obtained. Four of them (strains T1, T16, T21, and T26) were obtained by using the parC-QRDR PCR products from Cp^r S. pneumoniae isolates with point mutations, and two of them (strains T6 and T11) were obtained by using the products that contained parE-QRDR-ant-parC-QRDR from Cp^r S. mitis 181731-3. Sequence analysis revealed different recombination regions in T6 and T11. While T6 underwent recombination upstream of parC (positions –84 to –59) and into parC (positions 625 to 641), T11 did it in parE (positions 1211 to 1233) and parC (positions 385 to 407) and acquired the ant gene (Fig. 1A and B). All six first-level transformants were used as the recipients in further transformation experiments with PCR products containing the gyrA-QRDRs from genetically characterized Cp^r S. pneumoniae isolates. A total of 18 second-level transformants, whose CIP MICs were 16 to 64 µg/ml and whose LVX MICs were 8 to 32 µg/ml, were obtained.

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TABLE 2. Generation of Cpr mutants derived from strain R6 by genetic transformation

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FIG. 1. Transcriptional analysis of the parE and parC genes in S. pneumoniae R6, T6, and T11. (A) Genetic structure of the parE-parC region. SPN, S. pneumoniae; SMI, S. mitis. Oligonucleotides are indicated by black arrows (not drawn to scale). (B) Nucleotide sequences of the regions surrounding the recombination points in strains T6 (a and b) and T11 (c and d). (C) RT-PCR analysis. cDNAs (synthesized with total RNAs from R6, T6, and T11 and oligonucleotide parC119R) and control RNAs (cDNAs prepared in the absence of reverse transcriptase) were subjected to PCRs with three pairs of oligonucleotides, named 1 (parE592 and antUpR), 2 (antUP and parC26R), and 3 (parE592 and parC26R), that would render products indicative of common mRNAs for parE-ant, ant-parC, and parE-parC, respectively. The products were run in 1% agarose gels and stained with 0.5 µg/ml ethidium bromide. Lanes Mw, molecular weight standard consisting of HindIII- and EcoRI-cut bacteriophage lambda DNA. (D) Quantitative real-time RT-PCR. The results (means ± standard errors of the means) of four independent replicates are represented.

Transcriptional analysis of parE, ant, and parC. To determine if the transcription of parE and parC occurs from a single mRNA and if there is a disbalance in the amount of their respective mRNAs in strain T11, RT-PCR experiments were performed. The cDNAs used in the PCRs were synthesized with the total RNAs from strains R6, T6, and T11 and oligonucleotide parC119R. Three oligonucleotide pairs (Fig. 1), named 1 (parE592-antUpR), 2 (antUP-parC26R), and 3 (parE592-parC26R), that would render products indicative of common mRNAs for parE-ant, ant-parC, and parE-parC, respectively, were used. As expected, no amplification was observed with R6 and T6 cDNAs with pairs 1 and 2 (Fig. 1C), in accordance with the absence of ant in these strains. cDNAs from strains T11 and S. mitis 181731-3 rendered products of about 0.2 and 1 kb with pairs 1 and 2, respectively (Fig. 1C), suggesting cotranscription for parE and ant and for ant and parC. To confirm these results and to determine if parE and parC are transcribed in a single mRNA in R6, T6, and T11, a PCR with oligonucleotide pair 3 was performed. With all cDNAs used, a fragment of a size concordant with the cotranscription of parE and parC was obtained (Fig. 1C). The same RT-PCR experiments were performed with total RNA from six S. pneumoniae Cp^r recombinant clinical isolates (Fig. 2). The PCR products were obtained with pairs 1, 2, and 3 and cDNAs from S. pneumoniae 3870, CipR-73, and CipR-75, which had intergenic parE-parC regions of 1.9, 1.2, and 1.7 kb, respectively. No products were obtained with pairs 1 and 3 and cDNAs from S. pneumoniae 3180, 4391, and 5237, which had intergenic parE-parC regions of 6.3, 6.2, and 6.4 kb, respectively. However, with pair 2, PCR products of about 1.9 and 2.4 kb were obtained with the cDNAs of S. pneumoniae 4391 and 5237 (Fig. 2), suggesting that ant and parC are cotranscribed in those strains. These results suggest that while parE and parC are cotranscribed in recombinant isolates with intergenic regions smaller than 2 kb, this could not be the case for strains with larger intergenic regions.

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FIG. 2. Transcriptional analysis of the parE and parC genes in S. pneumoniae recombinant clinical isolates. (A) Genetic structure of the parE-parC region. SPN, S. pneumoniae. Oligonucleotides are indicated by black arrows (not drawn to scale). (B) cDNAs (synthesized with total RNAs from the indicated isolates with oligonucleotide parC119R) and control RNAs (cDNAs prepared in the absence of reverse transcriptase) were subjected to PCRs with three pairs of oligonucleotides, named 1 (parE592 and antUpR), 2 (antUP and parC26R), and 3 (parE592 and parC26R), that would render products indicative of common mRNAs for parE-ant, ant-parC, and parE-parC, respectively. The products were run in 1% agarose gels and stained with 0.5 µg/ml ethidium bromide. Lanes Mw, molecular weight standard (Biotools DNA marker, 1-kb ladder).

To confirm the RT-PCR data, the relative amounts of the mRNAs of parE and parC were quantified in real time RT-PCR experiments. cDNAs from strains R6, T6, and T11 were obtained with a mixture of random hexanucleotides; and the transcripts were quantified by real-time PCR with the appropriate oligonucleotides (parE214 and parE274R for parE, parC214 and parC278R for parC) to give PCR fragments of equivalent sizes (206 and 212 bp for parE and parC, respectively). These experiments showed that the amounts of mRNAs for parE and parC were equivalent in strains R6, T6, and T11 (Fig. 1D).

Primer extension experiments were performed to detect promoters and determine the initiation of transcription of parE, ant, and parC. Total RNA was extracted from S. pneumoniae R6 carrying plasmid pLPPARC, pLPPARE, or pUPANT, which contained the R6 parC region from positions –752 to 2138, the R6 parE region from positions –858 to 1670, and the S. mitis 181731-3 ant region from positions –201 to 1684, respectively. While no runoff products were observed for parC or ant, a 97-nucleotide product was observed for parE with oligonucleotide parE26R. This result permitted mapping of the initiation of transcription of parE in R6 to the G residue (position –23 of parE) that is 8 bp downstream of a –10 sequence (Fig. 3).

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FIG. 3. Localization of the transcription initiation site of parE. Sequenase reactions with plasmid pLPPARE as the template and parE26R as the primer provided a reference sequence ladder: G, A, T, and C indicate the dideoxynucleotides used. For the primer extension experiments, 20 µg of RNAs obtained from S. pneumoniae R6 carrying pLPPARE (the top line) was used. The double-stranded DNA sequence of the 5' parE region is shown with the principal features: the –35 and –10 regions, the first nucleotide of the mRNA (+1), and the start codon of parE.

Fitness assays. The relative fitness of CIP-susceptible strain R6 was compared to that of its isogenic resistant progeny and with that of strain Tr7 by the use of competitive growth experiments. When a single cycle (6 h in Table 3) of competitive growth was considered, no significant fitness cost was observed for most of the first-level transformants, with all assays giving mean relative fitness greater than 1 (Table 3). The only exception was strain Tr7, with a GyrA E85K change and with a relative fitness of 0.85. Among the second-level transformants, only some strains carrying the GyrA E85K change, in addition to the ParC change S79F, S79Y, D83N, or D83Y, showed a fitness burden, with mean relative fitness values of 0.92, 0.96, 0.93, and 0.92, respectively (Table 3). Although second-level transformants T9 and T14 carried the same GyrA E85K and ParC S79F changes, they showed different fitness costs. The mean relative fitness value for recombinant T9 was 0.93, a value compatible with that of the rest of the second-level transformants carrying the GyrA E85K change (strains T4, T19, T24, and T29). In contrast, recombinant T14 had a relative fitness of 1.03, which indicates a compensation with respect to the fitness of strain T9.

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TABLE 3. Competitive fitness of fluoroquinolone-resistant mutants of S. pneumoniae

When two cycles (12 h in Table 3) of competitive growth were considered, a significant fitness cost was observed in three of six of the first-level transformants, while transformants T21, T6, and T11 did not show a fitness cost. Transformant T21 carried the ParC D83N change and showed a mean relative fitness of 0.95 (95% CI, 0.90 to 1.00). Strains T6 and T11 are recombinants carrying the same change (ParC S79F) that strain T1 carries. While T1 did show a significant fitness cost (mean, 0.94; 95% CI, 0.89 to 0.99), T6 (mean, 0.97; 95% CI, 0.92 to 1.02) and T11 (mean, 0.97; 95% CI, 0.91 to 1.03) did not. These results suggest the existence of compensation in the recombinants. Among the second-level transformants, the only strain that did not show a fitness cost was the T14 recombinant strain (mean, 0.95; 95% CI, 0.90 to 1.00).

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The lower-than-expected prevalence of recombinant S. pneumoniae Cp^r isolates that have acquired resistance mutations from the SMG among all Cp^r isolates (3, 11) could be attributed, at least in part, to the lower fitness of these kinds of isolates. Among the 10 recombinant clinical isolates studied by our group, 2 have acquired portions of parC (11) and 8 have acquired both parE and parC (3, 11, 16) from the SMG. In the latter case, given the presence of the ant gene in the intergenic parE-parC region of the SMG but not in S. pneumoniae, recombinants acquired an extra gene in the recombination process and, consequently, had larger intergenic parE-parC regions (1.2 to 6.4 kb) than the nonrecombinant pneumococci (0.4 kb). Two factors would influence the fitness of these recombinants. One is the putative discoordination of parE-parC transcription, in the case of joint transcription, and the acquisition of ant; and the other is the existence in some of the isolates of parE and parC genes of different origins. Isolates with a parE gene of pneumococcal origin and a parC gene of SMG origin are represented in this work by strains T6 to T9, and isolates that have acquired the ant gene are represented by strains T11 to T14 strains (1.1-kb parE-parC intergenic region). In addition, to discern the effect of point mutations in the fitness of the strains, a series of mutants with point mutations (Table 3) were studied.

With respect to the putative discoordination of the transcription of the parE-parC region, our results showed that transcription of both genes occurs from a single promoter in S. pneumoniae strains R6, T6, and T11 (Fig. 1 and 3), allowing the synthesis of equivalent amounts of the mRNAs of both genes, as shown by real-time RT-PCR (Fig. 1D) and in accordance with the ParC₂ ParE₂ composition of topo IV. Then, at least in strain T11, the presence of ant in the 1.1-kb intergenic parE-parC region does not have any effect on transcription. In isolates with intergenic regions smaller than 2 kb (S. pneumoniae 3870, CipR-73, and CipR-75), cotranscription of parE and parC was observed (Fig. 2). However, no cotranscription was observed in recombinant isolates with larger intergenic regions (up to 7.2 kb; S. pneumoniae 3180, 4391, and 5237). Future work would be necessary to ascertain if these kinds of recombinants would have a fitness cost due to some discoordination in the transcription of their topo IV genes.

With respect to their fitness costs, strains can be categorized as those with no, low, and high fitness costs. High-fitness-cost strains showed a relative mean fitness value of less than 1 both in the one-cycle experiments (95% CI, 0.88 to 0.99) and in the two-cycle experiments (95% CI, 0.63 to 0.89). Low-fitness-cost strains showed a relative mean fitness value less than 1 only in the two-cycle experiments (95% CI, 0.81 to 0.99), while for the no-fitness-cost strains the 95% CI of the mean relative fitness included the value of 1.

The high-fitness-cost strains included first-level transformant Tr7 and second-level transformants T4, T19, T24, T29, and T9, which carried the GyrA E85K change. The E85 GyrA residue is located within the QRDR, close to the catalytic cleavage residue Y120 (equivalent to Y122 of E. coli GyrA). The three-dimensional structure of the breakage-reunion domain of E. coli GyrA reveals that the active-site tyrosines (Y122) are on loops at either end of the dimer interface and sit at the ends of strongly basic grooves created by the dimer-related monomers for binding to the G segment (32). Residues S83 and D87 (equivalent to S81 and E85, respectively, of S. pneumoniae GyrA) are solvent exposed and probably interact with the C-8 group of the fluoroquinolone. We have found that strains with substitutions in GyrA S81 did not show a fitness cost in the one-cycle experiments (Table 3), but the E85K substitutions imposed a burden. In accordance, a Ser83A substitution has no effect on E. coli gyrase activity; however, a D87A substitution causes a 60% reduction in supercoiling activity, probably due to stabilization of enzyme-DNA interactions (4). It is tempting to speculate that the burden imposed by the E85K substitution could be also attributed to a stronger interaction of gyrase with DNA due to the more positive charge of the K residue. Given that topo IV and gyrase have complementary activities (34), compensation by the recombinant T14 topo IV enzyme (parC and parE recombinant genes) for the fitness cost imposed by the GyrA E85K change (Table 3) could be attributed to a mechanism of bypass (29) due to the putatively more efficient T14 topo IV enzyme.

The low-fitness-cost strains included single ParC mutants carrying the S79F, S79Y, or D83Y change; all mutants with double mutations carrying a ParC change plus a GyrA change (except E85K; Table 3); and recombinant strains T7, T8, T12, and T13. Other authors have reported that some of these changes confer a fitness cost in one-cycle experiments (18, 24, 39). However, in general, there is no agreement among the results from different laboratories, which suggest that one-cycle experiments are not enough to detect a low fitness cost, which would be masked by experimental errors, and that it is necessary to perform two-cycle experiments to improve the accuracy of the results.

The no-fitness-cost strains included the single ParC mutant carrying D83N and recombinant strains T6 and T11. Although the last two strains carried the parE and parC genes of different species (S. pneumoniae or the SMG), this feature was not associated with a fitness cost, even though the ParE changes of strain T11 mapped in the C-terminus region, which is involved in the interaction of ParE with ParC. Nevertheless, T6 and T11 compensated for the fitness cost imposed by the ParC S79F change, which could be attributed to the presence in these strains of a more efficient topo IV. However, when this feature was associated with mutations in gyrA, a fitness cost was detected in the two-cycle experiments (Table 3) in all strains except strain T14 (see above).

From the results presented here it could be assumed that there is no impediment to the spreading of recombinant isolates that have acquired Cp^r resistance mutations from the SMG due to a fitness cost, since recombination compensates for the fitness cost imposed by specific mutations.

 
We thank M. J. Ferrándiz and C. López-Galíndez for critical reading of the manuscript.

This study was supported by grants BIO2005-02189 from the Dirección General de Investigación Científica y Técnica, MPY 1278/05 from the Instituto de Salud Carlos III, and COMBACT-S-BIO-0260/2006 from the Comunidad de Madrid.

 
* Corresponding author. Mailing address: Unidad de Genética Bacteriana, Centro Nacional de Microbiología and Ciber Enfermedades Respiratorias, Instituto de Salud Carlos III, 28220 Majadahonda, Madrid, Spain. Phone: (34) 91 509 7057. Fax: (34) 91 509 7919. E-mail: agcampa{at}isciii.es

Published ahead of print on 26 December 2007.

Top ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 

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Antimicrobial Agents and Chemotherapy, March 2008, p. 822-830, Vol. 52, No. 3
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