Fluoroquinolone resistance is a poor surrogate marker for type II topoisomerase mutations in clinical isolates of Streptococcus pneumoniae

Detection of parC mutations in Streptococcus pneumoniae by Real-time PCR and Taqman–MGB probes

A Real-time PCR assay was developed by using Taqman-MGB probes to screen mutations at codons Ser79 and Asp83 of Streptococcus pneumoniae parC. One hundred and thirty levofloxacin-susceptible and forty-two levofloxacin-resistant clinical strains were assayed. Mutations at codon 79 were found among all the levofloxacin-resistant strains. Mutations at codon 79 or 83 were found in ten levofloxacin-susceptible strains. This procedure is a reliable method for a rapid detection of mutations in the QRDRs of parC gene of S. pneumoniae and could be carried out in a diagnostic laboratory for some high-risk patients or in epidemiological surveys.

Optimal pharmacological therapy for community-acquired pneumonia: the role of dual antibacterial therapy

The optimal pharmacological therapy of community-acquired pneumonia (CAP) is one of the most ardently debated issues in medicine. Presently, most guidelines recommend either a fluoroquinolone alone or dual therapy with a third-generation cephalosporin plus a macrolide in patients hospitalised with CAP, but few provide clinicians with specific considerations for selecting from these agents. Despite a similar spectrum of activity and favourable resistance patterns (for fluoroquinolones and third-generation cephalosporins) against CAP pathogens, there is emerging evidence that dual therapy may be superior to monotherapy in certain populations.In patients with non-severe CAP, the evidence supports the use of either monotherapy or dual therapy in most patients; however, patients with severe CAP or bacteraemic pneumococcal CAP experience improved survival when treated with dual therapy. It is unclear from this evidence if any specific combination of agents is the most effective, but the combination of a third-generation cephalosporin plus a macrolide is the most extensively studied. Dual therapy was superior to monotherapy irrespective of the susceptibility of the aetiological pathogen, thus insufficient antimicrobial spectrum does not explain the disparity. The most likely explanation for improved outcomes with dual therapy is the combined effect of optimised antimicrobial spectrum (including atypicals), decreased impact of resistance to a single agent and the immunomodulatory effects of macrolides. Increasing resistance in patients with non-severe CAP warrants the consideration of dual therapy and perhaps a reappraisal of agents usually reserved for second-line therapy, including doxycycline, in these populations as well. In light of the available evidence, dual therapy should be strongly considered in all patients with severe CAP, especially when complicated by pneumococcal bacteraemia.

Antimicrobial Resistance amongStreptococcus pneumoniaein the United States: Have We Begun to Turn the Corner on Resistance to Certain Antimicrobial Classes?

BACKGROUND

Antimicrobial resistance has emerged as a major problem in Streptococcus pneumoniae in the United States during the past 15 years. This study was undertaken to elucidate the current scope and magnitude of this problem in the United States and to assess resistance trends since 1994-1995.

METHODS

A total of 1817 S. pneumoniae isolates obtained from patients with community-acquired respiratory tract infections in 44 US medical centers were characterized during the winter of 2002-2003. The activity of 27 antimicrobial agents was assessed. In addition, selected isolates were examined for the presence of mutations in the quinolone-resistance determining regions (QRDRs) of parC and gyrA that resulted in diminished fluoroquinolone activity. The results of this survey were compared with the results of 4 previous surveys conducted in a similar manner since 1994-1995.

RESULTS

Overall rates of resistance (defined as the rate of intermediate resistance plus the rate of resistance) were as follows: penicillin, 34.2%; ceftriaxone, 6.9%; erythromycin, 29.5%; clindamycin, 9.4%; tetracycline, 16.2%; and trimethoprim-sulfamethoxazole (TMP-SMX), 31.9%. No resistance was observed with vancomycin, linezolid, or telithromycin; 22.2% of isolates were multidrug resistant; 2.3% of isolates had ciprofloxacin MICs of >or=4.0 microg/mL. It was estimated that 21.9% of the isolates in this national collection had mutations in the QRDRs of parC and/or gyrA, with parC only mutations occurring most often (in 21% of all isolates). Trend analysis since 1994-1995 indicated that rates of resistance to beta -lactams, macrolides, tetracyclines, TMP-SMX, and multiple drugs have either plateaued or have begun to decrease. Conversely, fluoroquinolone resistance among S. pneumoniae is becoming more prevalent.

CONCLUSION

It appears that, as fluoroquinolone resistance emerges among S. pneumoniae in the United States, resistance to other antimicrobial classes is becoming less common.

The Changing Face of Antibiotic Prescribing: The Mutant Selection Window

OBJECTIVE

To describe the mutant selection window, discuss supporting evidence and limitations, and suggest potential applications for clinical practice.

DATA SOURCES

A MEDLINE search (1990–December 2003) of the English-language literature was conducted using the key words antibiotic, antimicrobial, resistance, mutant, selection window, prevention, MPC, and MSW in various combinations. Original investigations and reviews evaluating the mutant selection window, including abstracts and proceedings, were considered for inclusion. Published articles were also cross-referenced, and experts were contacted to locate additional pertinent data.

STUDY SELECTION AND DATA EXTRACTION

All data sources identified were evaluated and all information deemed relevant was included.

DATA SYNTHESIS

Until recently, physicians have had few ways to preserve antimicrobials from resistance other than by prescribing the agents less often. The mutant selection window hypothesis may modify this paradigm by shifting the focus to dosing strategies that reduce the growth of resistant mutants. Conventional dosing strategies have been formulated on the likelihood of curing an individual patient. Unfortunately, doses that cure patients appear to enrich resistant subpopulations of bacteria, thus promoting resistance. Antimicrobial—pathogen combinations can be identified that minimize mutant selection and cure patients while possibly restricting the progression of resistance.

CONCLUSIONS

The mutant selection window hypothesis provides a framework for considering the contribution of dosing to resistance, and it offers ideas for restricting the enrichment of resistant mutants and antimicrobial resistance.

Mutant selection window in levofloxacin and moxifloxacin treatments of experimental pneumococcal pneumonia in a rabbit model of human therapy

For some pneumococci the fluoroquinolone MICs are low but the mutant prevention concentrations (MPCs) are high; this difference defines in vitro the mutant selection window (MSW). We investigated in vivo the bacterial reduction and the occurrence of resistant mutants with moxifloxacin (MFX; 400 mg once daily) or levofloxacin (LVX; 500 mg twice daily) in treatments similar to those in humans with experimental pneumonia due to pneumococci (expPP) exhibiting various MICs and MPCs. The MIC/MPC for MFX and LVX and genotypes were as follows: strain 16089, 0.125/0.125 and 0.5/0.5 (wild type); strain MS1A, 0.25/0.25 and 1/2 (efflux); strain MS2A, 0.25/4 and 1.75/28 (parC79); strain MR3B4, 0.25/4 and 2/32 (parC79); strain M16, 0.5/2 and 8/32 (parC83); strain Gyr-1207, 1.5/3 and 8/16 (gyrA); and strain MQ3A, 4/4 and 16/64 (parC and gyrA). Both drugs were efficient with wild type-expPP, but only MFX was efficient with efflux-expPP. No bacterial reduction was observed for parC-expPPs due to mutants observed in 18 to 100% of animals, depending on the strain and the drug tested. These mutants showed unbound area under the concentration-time curve and MICs of from 50 to 164 for MFX. The in vivo pharmacodynamic boundaries of the MSW were different for MFX and LVX. We conclude that, after LVX or MFX treatment, mutants occur in vivo if there is a preexisting parC mutation, since the drug concentrations fall below the MPCs of these strains. Since the MPC determination cannot be routinely determined, these phenotypes or genotypes should be detected by simple tests to guide the therapeutic options.

The era of antimicrobial resistance-implications for the clinical laboratory

The emergence of resistance to antimicrobial drugs in a range of bacterial pathogens of the respiratory tract creates a challenge for the clinical diagnostic laboratory. Resistance to macrolides among strains of Streptococcus pneumoniae has reached high levels in many countries. There are two main types of resistance mechanisms, one (MLSB phenotype) leading to high-level resistance and the other (M phenotype) resulting in a lower level of resistance. As there are indications that high-level resistance may have clinical relevance, it is important to be able to detect such strains. This is now possible with the erythromycin/clindamycin double disc diffusion test. Although resistance to beta-lactams has also increased, there is now evidence that some beta-lactams are still effective against isolates that have low-level resistance. In response to these observations, new breakpoints have now been introduced by the National Committee for Clinical Laboratory Standards (NCCLS) for amoxicillin, cefotaxime and ceftriaxone, which increase the percentage of pneumococcal isolates that can still be treated effectively with these agents. As a consequence of the increasing use of fluoroquinolones, resistance has now emerged to this group of compounds and this has been associated with clinical failures. Although standard minimum inhibitory concentration tests can detect strains with high levels of resistance (double step mutants) they are not reliable in detecting strains with a single mutation. This is important as there is increasing evidence that strains with a single mutation in the target topoisomerase are even more likely to develop a second mutation, leading to higher levels of resistance and thus probably, therapeutic failure. Because resistance to fluoroquinolones is currently low in the US, the NCCLS does not recommend that susceptibility testing with the newer respiratory fluoroquinolones be carried out routinely. However, since the respiratory fluoroquinolones have in fact become the first line of therapy for the treatment of community-acquired pneumonia, it may now be time to institute routine testing of clinical isolates of pneumococci to these agents. Simple techniques for the clinical diagnostic laboratory to enable these first-step mutants to be detected are urgently required.

Fluoroquinolone resistance among Gram-positive cocci

Resistance to fluoroquinolones among Gram-positive cocci has emerged as these antimicrobial agents have become extensively used in clinical medicine. Resistance is effected by changes in the bacterial target enzymes DNA gyrase and topoisomerase IV, which reduce drug binding, and by action of native bacterial membrane pumps that remove drug from the cell. In both cases, quinolone exposure selects for spontaneous mutants that are present in large bacterial populations, and which contain chromosomal mutations that alter the target protein or increase the level of pump expression. Resistance among clinical isolates has been greatest in Staphylococcus aureus and particularly among meticillin-resistant strains, in which both selection by quinolone exposure and transmission of clonal strains in health-care settings have contributed to high prevalence. Resistance in Streptococcus pneumoniae has also emerged in the community. Fluoroquinolone resistance has arisen in multidrug-resistant clones and its prevalence has been especially high in Hong Kong and Spain. Further spread and selection of such resistance could compromise the utility of a valuable class of antimicrobial agents, a point that emphasises the importance of the careful use of these agents in appropriate patients and doses, as well as careful infection-control practices.

In vitro activities of novel nonfluorinated quinolones PGE 9262932 and PGE 9509924 against clinical isolates of Staphylococcus aureus and Streptococcus pneumoniae with defined mutations in DNA gyrase and topoisomerase IV

Two 8-methoxy nonfluorinated quinolones (NFQs), PGE 9262932 and PGE 9509924, were tested against contemporary clinical isolates of Staphylococcus aureus (n = 122) and Streptococcus pneumoniae (n = 69) with genetically defined quinolone resistance-determining regions (QRDRs). For S. aureus isolates with wild-type (WT) sequences at the QRDRs, the NFQs demonstrated activities 4- to 32-fold more potent (MICs at which 90% of isolates are inhibited [MIC(90)s], 0.03 microg/ml) than those of moxifloxacin (MIC(90), 0.12 microg/ml), gatifloxacin (MIC(90), 0.25 microg/ml), levofloxacin (MIC(90), 0.25 microg/ml), and ciprofloxacin (MIC(90), 1 microg/ml). Against S. pneumoniae isolates with WT sequences at gyrA and parC, the NFQs PGE 9262932 (MIC(90), 0.03 microg/ml) and PGE 9509924 (MIC(90), 0.12 microg/ml) were 8- to 64-fold and 2- to 16-fold more potent, respectively, than moxifloxacin (MIC(90), 0.25 microg/ml), gatifloxacin (MIC(90), 0.5 microg/ml), levofloxacin (MIC(90), 2 microg/ml), and ciprofloxacin (MIC(90), 2 microg/ml). The MICs of all agents were elevated for S. aureus isolates with alterations in GyrA (Glu88Lys or Ser84Leu) and GrlA (Ser80Phe) and S. pneumoniae isolates with alterations in GyrA (Ser81Phe or Ser81Tyr) and ParC (Ser79Phe or Lys137Asn). Fluoroquinolone MICs for S. aureus strains with double alterations in GyrA combined with double alterations in GrlA were > or =32 microg/ml, whereas the MICs of the NFQs for strains with these double alterations were 4 to 8 microg/ml. The PGE 9262932 and PGE 9509924 MICs for the S. pneumoniae isolates did not exceed 0.5 and 1 microg/ml, respectively, even for isolates with GyrA (Ser81Phe) and ParC (Ser79Phe) alterations, for which levofloxacin MICs were > 16 microg/ml. No difference in the frequency of selection of mutations (< 10(-8) at four times the MIC) in wild-type or first-step mutant isolates of S. aureus or S. pneumoniae was detected for the two NFQs. On the basis of their in vitro activities, these NFQ agents show potential for the treatment of infections caused by isolates resistant to currently available fluoroquinolones.

Fluoroquinolone resistance in Streptococcus pneumoniae in United States since 1994-1995

The in vitro activities of ciprofloxacin, levofloxacin, gatifloxacin, and moxifloxacin against a large collection of clinical isolates of Streptococcus pneumoniae (n = 4,650) obtained over a 5-year period, 1994-1995 through 1999-2000, were assessed as part of a longitudinal multicenter U.S. surveillance study of antimicrobial resistance. Three sampling periods were used during this investigation, the winter seasons of 1994-1995, 1997-1998, and 1999-2000; and 1,523, 1,596 and 1,531 isolates were collected during these three periods, respectively. The overall rank order of activity of the four fluoroquinolones examined in this study was moxifloxacin > gatifloxacin > levofloxacin = ciprofloxacin, in which moxifloxacin (MIC at which 90% of isolates are inhibited [MIC(90)], 0.25 microg/ml; modal MIC, 0.12 microg/ml) was twofold more active than gatifloxacin (MIC(90), 0.5 microg/ml; modal MIC, 0.25 microg/ml), which in turn was fourfold more active than either levofloxacin (MIC(90), 1 microg/ml; modal MIC, 1 microg/ml) or ciprofloxacin (MIC(90), 2 microg/ml; modal MIC, 1 microg/ml). Changes in the in vitro activities of fluoroquinolones against S. pneumoniae strains in the United States over the 5-year period of the survey were assessed by comparing the MIC frequency distributions of the study drugs against the isolates obtained during the three sampling periods encompassing this investigation. These comparisons revealed no evidence of changes in the in vitro activities of the fluoroquinolones. In addition, the percentages of isolates in the three sampling periods for which MICs were above the resistance breakpoints were compared. Low percentages of resistant strains were detected, and there was no evidence of resistance rate changes over time. For example, by use of a ciprofloxacin MIC of > or = 4 microg/ml to define resistance, the proportions of isolates from the three sampling periods for which MICs were at or above this breakpoint were 1.2, 1.6, and 1.4%, respectively. A total of 164 unique isolates (n = 58 from 1994-1995, 65 from 1997-1998, and 42 from 1999-2000) were examined for evidence of mutations in the quinolone resistance-determining regions (QRDRs) of the parC and the gyrA genes. Forty-nine isolates harbored at least one mutation in the QRDRs of one or both genes (1994-1995, n = 15; 1997-1998, n = 19; 1999-2000, n = 15). Among the 4,650 isolates of S. pneumoniae examined in the study, we estimated that 0.3% had mutations in both the parC and gyrA loci. The majority of mutations (67.3% of the mutations in 49 isolates with mutations) were amino acid substitutions in the parC locus only. Four isolates had a mutation in the gyrA locus only, and 12 isolates had mutations in both genes (8.2 and 24.5% of isolates with mutations, respectively). There was no significant difference in the number of isolates with parC and/or gyrA mutations detected during each study period. Finally, because of the magnitude of the study, we had reasonably large numbers of pneumococcal isolates with genotypically defined mechanisms of fluoroquinolone resistance and were thus able to determine the effects of specific resistance mutations on the activities of different fluoroquinolones. In general, isolates with mutations in parC only were resistant to ciprofloxacin but remained susceptible to levofloxacin, gatifloxacin, and moxifloxacin, whereas isolates with mutations in gyrA only and isolates with mutations in both parC and gyrA were resistant to all four fluoroquinolones tested.