Prevalence of gyrA, gyrB, parC, and parE mutations in clinical isolates of Streptococcus pneumoniae with decreased susceptibilities to different fluoroquinolones and originating from Worldwide Surveillance Studies during the 1997-1998 respiratory season

Optimal use of antibiotic resistance surveillance systems

Increasing concern about the emergence of resistance in clinically important pathogens has led to the establishment of a number of surveillance programmes to monitor the true extent of resistance at the local, regional and national levels. Although some programmes have been operating for several years, their true usefulness is only now being realised. This review describes some of the major surveillance initiatives and the way in which the data have been used in a number of different settings. In the hospital, surveillance data have been used to monitor local antibiograms and determine infection control strategies and antibiotic usage policies. In the community, surveillance data have been used to monitor public health threats, such as infectious disease outbreaks involving resistant pathogens and the effects of bioterrorism countermeasures, by following the effects of prophylactic use of different antibiotics on resistance. Initially, the pharmaceutical industry sponsored surveillance programmes to monitor the susceptibility of clinical isolates to marketed products. However, in the era of burgeoning resistance, many developers of antimicrobial agents find surveillance data useful for defining new drug discovery and development strategies, in that they assist with the identification of new medical needs, allow modelling of future resistance trends, and identify high-profile isolates for screening the activity of new agents. Many companies now conduct pre-launch surveillance of new products to benchmark activity so that changes in resistance can be monitored following clinical use. Surveillance data also represent an integral component of regulatory submissions for new agents and, together with clinical trial data, are used to determine breakpoints. It is clear that antibiotic resistance surveillance systems will continue to provide valuable data to health care providers, university researchers, pharmaceutical companies, and government and regulatory agencies.

Antimicrobial activities of garenoxacin (BMS 284756) against Asia-Pacific region clinical isolates from the SENTRY program, 1999 to 2001

Between 1999 and 2001, 16,731 isolates from the Asia-Pacific Region were tested in the SENTRY Program for susceptibility to six fluoroquinolones including garenoxacin. Garenoxacin was four- to eightfold less active against Enterobacteriaceae than ciprofloxacin, although both drugs inhibited similar percentages at 1 microg/ml. Garenoxacin was more active against gram-positive species than all other fluoroquinolones except gemifloxacin. For Staphylococcus aureus, oxacillin resistance was high in many participating countries (Japan, 67%; Taiwan, 60%; Hong Kong, 55%; Singapore, 52%), with corresponding high levels of ciprofloxacin resistance (57 to 99%) in oxacillin-resistant S. aureus (ORSA). Of the ciprofloxacin-resistant ORSA isolates, the garenoxacin MIC was >4 microg/ml for only 9% of them. For Streptococcus pneumoniae, penicillin nonsusceptibility and macrolide resistance were high in many countries. No relationship was seen between penicillin and garenoxacin susceptibility, with all isolates being susceptible at <2 microg/ml. There was, however, a partial correlation between ciprofloxacin and garenoxacin MICs. For ciprofloxacin-resistant isolates for which garenoxacin MICs were 0.25 to 1 microg/liter, mutations in both the ParC and GyrA regions of the quinolone resistance-determining region could be demonstrated. No mutations conferring high-level resistance were detected. Garenoxacin shows useful activity against a wide range of organisms from the Asia-Pacific region. In particular, it has good activity against S. aureus and S. pneumoniae, although there is evidence that low-level resistance is present in those organisms with ciprofloxacin resistance.

Quinolone resistance mechanisms in pneumococci

Quinolones are widely used in the treatment of respiratory infections, in large part because of their activity against Streptococcus pneumoniae and other commonly encountered respiratory tract pathogens. Pneumococcal isolates that are resistant to these "respiratory quinolones" have now begun to emerge. Resistance is attributable to mutations affecting the intracellular targets of these drugs, topoisomerase IV and DNA gyrase; drug efflux contributes to quinolone resistance in some isolates. Most commonly, strains fully resistant to the newer quinolones have one or more mutations affecting DNA gyrase and topoisomerase IV. Although various agents of this class exhibit selectivity in primarily targeting one or the other of these enzymes, the passage of isolates in the presence of any agent can result in selection of mutations affecting both enzymes. Quinolone resistance in S. pneumoniae has arisen in heterogeneous genetic backgrounds but, ominously, has now appeared in strains that are well adapted for regional and global transmission.

Quinolone Resistance among Pneumococci: Therapeutic and Diagnostic Implications

Fluoroquinolones are widely recommended as empirical monotherapy for community-acquired pneumonia. Since 1999, case reports of failure of levofloxacin therapy due to levofloxacin-resistant strains of Streptococcus pneumoniae have started to appear. Most worrying is that, in some cases, levofloxacin resistance has been acquired by pneumococci within days of the initiation of therapy. Because use of current clinical antimicrobial resistance breakpoints fail to identify the majority of S. pneumoniae isolates with only first-step mutations, current treatment guidelines not only may have implications with regard to the ability of surveillance programs to detect emerging resistance but may have therapeutic implications as well.

In vitro activity of daptomycin against gram-positive European clinical isolates with defined resistance determinants

The in vitro activity of daptomycin against 337 gram-positive European clinical isolates with known resistance genes was determined. The MIC ranges for Staphylococcus aureus, enterococci, pneunococci, and streptococci were 0.03 to 1, 0.25 to 8, 0.12 to 1, and 0.06 to 8 micro g/ml, respectively. For only one streptococcus isolate and seven enterococcus isolates was the MIC 8 micro g/ml.

Fluoroquinolone Resistance in Clinical Isolates ofStreptococcus pneumoniaefrom Asian Countries: ANSORP Study

Seventeen clinical isolates of Streptococcus pneumoniae showing reduced susceptibility to ciprofloxacin (MIC >/= 4 micro g/ml) collected from eight different Asian countries were analyzed by antimicrobial susceptibility, serotyping, pulsed-field gel electrophoresis (PFGE), and DNA sequencing of the quinolone resistance-determining regions (QRDRs) in gyrA, gyrB, parC, and parE. All isolates but one showed more than one amino acid alteration in QRDRs of four responsible genes. Ile460 --> Val in parE was the most common mutation. Data suggest that Lys137 --> Asn in parC may be a primary step in the development of high-level and multiple FQ resistance. An additional mutation of Ser81 --> Phe in gyrA resulted in high-level resistance to ciprofloxacin, levofloxacin, and gatifloxacin, whereas Ser79 --> Phe in parC may exert an important role in the development of moxifloxacin resistance. Two novel amino acid changes in gyrB, Ala390 --> Val and Asn423 --> Thr, were found. Data from PFGE suggest an introduction and local spread of multiple resistant Spain(23F)-1 clone in Hong Kong, but isolates from other Asian countries were not related to this clone.

The activity of levofloxacin and other antimicrobials against clinical isolates of Streptococcus pneumoniae collected worldwide during 1999-2002

Streptococcus pneumoniae is the most important causative bacterial pathogen in respiratory infections. Globally, increasing levels of resistant strains highlight the need for continued surveillance programs to guide antibiotic choice. The current study compared susceptibility results of 4,788 strains of S. pneumoniae collected during 2001-2002 to susceptibility results from 3,884 strains collected from the same hospitals during 1999-2000. Participant centers were dispersed throughout five regions. By region, the prevalence of penicillin-resistant S. pneumoniae and percentage change from the previous 1999-2000 study was Mexico (26.0%, 12.5%), Brazil (7.9%; 5.5%), Asia (China, Hong Kong, South Korea, Thailand) (44.1%; 0.8%), Europe (France, Germany, Italy, Spain, UK) (11.1%; -0.6%) and South Africa (7.9; -1.8%). Multidrug-resistant (MDR) strains of S. pneumoniae were most frequently isolated from Asia (36.3%) compared with approximately 5% in the other four regions. Increases in the incidence of MDR isolates in Mexico (13.5%), Brazil (1.7%) and Asia (6.1%) were reported with no increases in MDR in South Africa and Europe. Levofloxacin resistance was rarely associated with MDR phenotypes. Levofloxacin maintained an MIC(90) of 1 microg/ml against the isolates collected from all five regions with no change during the study periods, despite differences in levofloxacin resistance rates between regions or nations (0%-3.2%). The prevalence of levofloxacin resistance (MIC > or =8 microg/ml) increased only slightly over the study period in Europe (0.3%-0.7%) and in Asia (3.0-3.2%), but little or no change was seen in Mexico (3.8%-0%) or Brazil or South Africa, where no levofloxacin resistant isolates were detected in either study period.

In vitro pharmacodynamic activity of gatifloxacin, gemifloxacin, moxifloxacin and levofloxacin against Streptococcus pneumoniae containing specific mutations in DNA gyrase and topoisomerase IV

An in vitro pharmacodynatnic modeling apparatus (PDMA) generated specific bacterial kill profiles for single-dose regimens of gatifloxacin (GT), gemifloxacin (GM), moxifloxacin (MX) and levofloxacin (LV) against isolates of Streptococcus pneumoniae with specific QRDR profiles: SP-WT (no modifications); SP-C (changes in parC); and SP-AC (changes in both parC and gyrA). No differences in 3-log reduction time or total log reduction were observed among the four agents for SP-WT; however, LV failed to achieve a 3-log reduction in SP-C and SP-AC, and total log reduction after 12 hrs was minimal compared to the other agents. GM and MX required less time for 3-log reduction of SP-AC compared to GT, but total log reductions in SP-AC were similar among the three newer quinolone agents (GM > MX > GT). The study isolates with QRDR modifications greatly reduced LV activity. GM and MX maintained the greatest degree of activity against all study isolates and their activity was not adversely influenced by the genetic modifications in SP-C and SP-AC. The dual targeting characteristic of GM was also assessed, but did not offer significant advantages relative to MX and GT.

In vitro selection and characterization of Bacillus anthracis mutants with high-level resistance to ciprofloxacin

Mutants of attenuated Bacillus anthracis with high-level ciprofloxacin resistance were isolated using a three-step in vitro selection. Ciprofloxacin MICs were 0.5 micro g/ml for first-step mutants, which had one of two gyrA quinolone resistance-determining region (QRDR) mutations. Ciprofloxacin MICs were 8 and 16 microg/ml for second-step mutants, which had one of three parC QRDR mutations. Ciprofloxacin MICs for third-step mutants were 32 and 64 microg/ml. Mutants for which MICs were 64 microg/ml had one of two additional mutations within the gyrA QRDR or one of two mutations within the gyrB QRDR. Mutants for which MICs were 32 microg/ml had no additional target modifications but showed evidence of enhanced ciprofloxacin efflux.

Antimicrobial susceptibility breakpoints and first-step parC mutations in Streptococcus pneumoniae: redefining fluoroquinolone resistance

Clinical antimicrobial susceptibility breakpoints are used to predict the clinical outcome of antimicrobial treatment. In contrast, microbiologic breakpoints are used to identify isolates that may be categorized as susceptible when applying clinical breakpoints but harbor resistance mechanisms that result in their reduced susceptibility to the agent being tested. Currently, the National Committee for Clinical Laboratory Standards (NCCLS) guidelines utilize clinical breakpoints to characterize the activity of the fluoroquinolones against Streptococcus pneumoniae. To determine whether levofloxacin breakpoints can identify isolates that harbor recognized resistance mechanisms, we examined 115 S. pneumoniae isolates with a levofloxacin MIC of >2 mg/mL for first-step parC mutations. A total of 48 (59%) of 82 isolates with a levofloxacin MIC of 2 mg/mL, a level considered susceptible by NCCLS criteria, had a first-step mutation in parC. Whether surveillance programs that use levofloxacin data can effectively detect emerging resistance and whether fluoroquinolones can effectively treat infections caused by such isolates should be evaluated.

Antimicrobial resistance in respiratory tract Streptococcus pneumoniae isolates: results of the Canadian Respiratory Organism Susceptibility Study, 1997 to 2002

A total of 6,991 unique patient isolates of Streptococcus pneumoniae were collected from October 1997 to June 2002 from 25 medical centers in 9 of the 10 Canadian provinces. Among these isolates, 20.2% were penicillin nonsusceptible, with 14.6% being penicillin intermediate (MIC, 0.12 to 1 microg/ml) and 5.6% being penicillin resistant (MIC, > or =2 microg/ml). The proportion of high-level penicillin-resistant S. pneumoniae isolates increased from 2.4 to 13.8% over the last 3 years of the study, and the proportion of multidrug-resistant S. pneumoniae isolates increased from 2.7 to 8.8% over the 5-year period. Resistant rates (intermediate and resistant) among non-beta-lactam agents were as follows: macrolides, 9.6 to 9.9%; clindamycin, 3.8%; doxycycline, 5.5%; chloramphenicol, 3.9%; and trimethoprim-sulfamethoxazole, 19.0%. Rates of resistance to non-beta-lactam agents were higher among penicillin-resistant strains than among penicillin-susceptible strains. No resistance to vancomycin or linezolid was observed; however, 0.1% intermediate resistance to quinupristin-dalfopristin was observed. The rate of macrolide resistance (intermediate and resistant) increased from 7.9 to 11.1% over the 5 years. For the fluoroquinolones, the order of activity based on the MICs at which 50% of isolates are inhibited (MIC(50)s) and the MIC(90)s was gemifloxacin > clinafloxacin > trovafloxacin > moxifloxacin > grepafloxacin > gatifloxacin > levofloxacin > ciprofloxacin. The investigational compounds ABT-773 (MIC(90), 0.008 microg/ml), ABT-492 (MIC(90), 0.015 microg/ml), GAR-936 (tigecycline; MIC(90), 0.06 microg/ml), and BMS284756 (garenoxacin; MIC(90), 0.06 micro g/ml) displayed excellent activities. Despite decreases in the rates of antibiotic consumption in Canada over the 5-year period, the rates of both high-level penicillin-resistant and multidrug-resistant S. pneumoniae isolates are increasing in Canada.

Genetic relatedness of levofloxacin-nonsusceptible Streptococcus pneumoniae isolates from North America

We characterized 32 levofloxacin-nonsusceptible Streptococcus pneumoniae (LNSP) isolates obtained from a broad geographic region of North America over a 5-year period by using capsular serotypes, antimicrobial susceptibility profiles, BOX-PCR, multilocus sequence typing (MLST), and pulsed-field gel electrophoresis (PFGE). Sixteen international clones identified by the Pneumococcal Molecular Epidemiology Network also were included for comparison. Fifteen serotypes were represented, with serogroups 6, 9, 14, 19, and 23 accounting for 63% of isolates. Among isolates whose quinolone resistance-determining regions were sequenced, all contained gyrA and parC point mutations. Sixty-three percent were penicillin susceptible, and 84% were erythromycin susceptible. BOX-PCR analysis identified 39 different band patterns among 32 LNSP and 16 international clones and grouped 16 isolates, including 2 international clones, into seven unrelated groups of 2 to 4 isolates each. PFGE analysis identified 35 different band patterns among 32 LNSP and 16 international clones and grouped 21 isolates, including 3 international clones, into eight unrelated groups of 2 to 6 isolates each. MLST performed on 10 isolates identified five allelic profiles and separated 9 isolates into four groups of 2 to 3 isolates each. Overall, each typing method indicated that the LNSP were heterogeneous and that resistance to fluoroquinolones was not closely associated with a particular serotype or with coresistance to other antimicrobial classes and suggests that LNSP have likely arisen through independent mutational events as a result of selective pressure. However, seven LNSP were found to be related to three international clones by PFGE.

Factors associated with relative rates of antimicrobial resistance among Streptococcus pneumoniae in the United States: results from the TRUST Surveillance Program (1998-2002)

To identify factors associated with antimicrobial resistance, data were analyzed from 27,828 isolates of Streptococcus pneumoniae submitted to the Tracking Resistance in the United States Today (TRUST) surveillance program during 4 consecutive respiratory seasons. From the 1998-1999 season to the 2001-2002 season, the prevalence of azithromycin resistance increased by 4.8% to 27.5%, the prevalence of penicillin resistance increased by 3.7% to 18.4%, the prevalence of ceftriaxone resistance increased by 0.5% to 1.7%, and the prevalence of levofloxacin resistance increased by 0.3% to 0.9%. Isolates recovered from patients <18 years of age and lower respiratory tract specimens had elevated rates of penicillin, azithromycin, and trimethoprim-sulfamethoxazole resistance (P<.00001); penicillin resistance correlated with coresistance to trimethoprim-sulfamethoxazole (87.3%), azithromycin (76.3%), ceftriaxone (9.1%), and levofloxacin (1.3%) (P<.00001). Only 62 (0.2%) of 27,828 isolates were concurrently resistant to penicillin and levofloxacin. Minimum inhibitory concentrations (MICs) of penicillin correlated strongly with MICs of ceftriaxone (R2=0.90), trimethoprim-sulfamethoxazole (R2=0.53), and azithromycin (R2=0.41). Patient age, specimen source, and penicillin resistance were factors associated with antimicrobial resistance, particularly for nonfluoroquinolone antimicrobial agents.

PCR-oligonucleotide ligation assay for detection of point mutations associated with quinolone resistance in Streptococcus pneumoniae

We have developed a PCR-oligonucleotide ligation assay to rapidly identify base substitutions in topoisomerase genes that are associated with quinolone resistance in clinical isolates of Streptococcus pneumoniae. Thirty-seven strains for which the ciprofloxacin MICs were >/=4 micro g/ml and 16 strains for which the MICs were </=2 micro g/ml were assayed. Compared with sequence data, the assay correctly identified the DNA bases that encoded amino acids at the four positions most commonly associated with quinolone resistance (Ser79 and Asp83 of ParC and Ser81 and Glu85 of GyrA). Therefore, this procedure can rapidly distinguish single base substitutions associated with quinolone-resistant topoisomerases in S. pneumoniae.

In vitro antibacterial activity and pharmacodynamics of new quinolones

This synopsis of published literature summarises data on the in vitro antibacterial activity and pharmacodynamics of fluoroquinolones. Data were compiled for ciprofloxacin, levofloxcin, moxifloxacin, gatifloxacin, grepafloxacin, gemifloxacin, trovafloxacin, sitafloxacin and garenoxacin. All of these quinolones are almost equipotent against gram-negative bacteria but demonstrate improved activity against gram-positive species. The new quinolones are uniformly active against gram-positive species except Streptococcus pneumoniae; against which gemifloxacin, sitafloxacin and garenoxacin are one to two dilution steps more active than moxifloxacin. All of the new quinolones except gemifloxacin demonstrate enhanced activity against anaerobes. Since all the new quinolones show similar activity against the major respiratory tract pathogens except Streptococcus pneumoniae and members of the family Enterobacteriaceae, their pharmacokinetics and pharmacodynamics will be clinically relevant differentiators and determinants of their overall activity and efficacy. In vitro simulations of serum concentrations revealed that (i). gemifloxacin and levofloxacin were significantly and gatifloxacin moderately less active than moxifloxacin against Streptococcus pneumoniae and Staphylococcus aureus, and (ii). resistant subpopulations emerged following exposure to levofloxacin and gatifloxacin (gemifloxacin not yet published) but not to moxifloxacin. The emergence of resistance is a function of drug concentrations achievable in vivo and the susceptibility pattern of the target organisms. Therefore, the use of less potent fluoroquinolones with borderline or even suboptimal pharmacokinetic/pharmacodynamic surrogate parameters will inadvertently foster the development of class resistance. Drugs with the most favourable pharmacokinetic/pharmacodynamic characteristics should be used as first-line agents in order to preserve the potential of this drug class and, most importantly, to provide the patient with an optimally effective regimen.

Phenotypic and genotypic analysis of levofloxacin-resistant clinical isolates of Streptococcus pneumoniae collected in 13 countries during 1999-2000

During 1999-2000, 5015 isolates were collected from 13 countries and tested against levofloxacin. Overall, levofloxacin resistance minimum inhibitory concentration (MIC>or =8 mg/l) was found in 40 isolates (0.8%). The highest resistance rates were in Hong Kong (8.0%), China (3.3%) and Spain (1.6%). Levofloxacin retained an MIC(90) of 1 mg/l in all countries. Pulsed-field gel electrophoresis analysis of resistant isolates demonstrated the presence of clones in countries where levofloxacin resistance exceeded 1%, suggesting that the elevated resistance rates could result from resistant clones within participating hospitals. DNA-sequence analysis of the quinolone-resistance-determining regions of gyrA, gyrB, parC and parE genes showed that the most common mutations were in GyrA (Ser81Phe), ParC (Ser79Phe, Lys137Asn) and ParE (Ile460Val), accounting for 40% of the isolates tested. Levofloxacin-resistant isolates were generally non-susceptible to other fluoroquinolones tested. Future studies to characterise resistant isolates by other molecular methods may ensure that the appropriate counter-measures can be taken to control the spread of resistant isolates.

Fluoroquinolone resistance in Streptococcus pyogenes

Streptococcus pyogenes isolated from blood and urine samples obtained from a 78-year-old woman was tested for susceptibility, and fluoroquinolone resistance (minimum inhibitory concentration of levofloxacin, 16 microg/mL) was found. DNA amplification and sequencing revealed a serine81-->tyrosine substitution in gyrA and 2 substitutions in parC: serine79-->phenylalanine and alanine121-->valine. This is the second report of a clinical isolate of S. pyogenes with high-level fluoroquinolone resistance.

Screening of large numbers of Streptococcus pneumoniae isolates for mutations associated with fluoroquinolone resistance using an oligonucleotide probe assay

Mutations at a relatively small number of sites in parC, parE and gyrA account for most of the fluoroquinolone resistance in Streptococcus pneumoniae clinical isolates. A high throughput oligonucleotide probe assay was developed to screen for mutations in the quinolone-resistance determining region (QRDR) of parC (Ser79), gyrA (Ser81) and parE (Asp435) of Streptococcus pneumoniae. Eight oligonucleotide probes (17mers) were used in the presence of tetramethyl ammonium chloride so that the melting temperature was dependent on length and not on base composition. Using this assay it was possible to accurately detect QRDR mutations from several hundred S. pneumoniae clinical isolates that were grown on nylon membranes.

AcrAB Efflux System: Expression and Contribution to Fluoroquinolone Resistance in Klebsiella spp

Seven Klebsiella pneumoniae and four Klebsiella oxytoca clinical isolates with different levels of resistance to ciprofloxacin were studied. Mutations in the topoisomerase genes were found in almost all strains, but the contribution of a multidrug efflux system homologous to AcrAB in Escherichia coli was also observed. Overexpression of this efflux system was demonstrated by immunoblotting with antibodies against E. coli AcrA.

In vitro activity of moxifloxacin against recent community-acquired respiratory tract pathogens isolated in France: a national survey

Between February and June 2000, 2345 consecutive strains of Streptococcus pneumoniae, Haemophilus influenzae, Moraxella catarrhalis, Staphylococcus aureus and Klebsiella pneumoniae were isolated from 2088 adult patients suffering from community-acquired respiratory tract infections, in 97 hospital laboratories. Of the 1037 S. pneumoniae isolates, 48.3% were intermediately or highly penicillin resistant. For invasive isolates, the MIC90s of penicillin G, amoxycillin, cefuroxime, ceftriaxone, erythromycin, ofloxacin, ciprofloxacin and moxifloxacin were 2, 2, 4, 0.5, 1024, 2, 2 and 0.25 mg/l, respectively. All but one invasive strain were susceptible to moxifloxacin whereas 97.5% were susceptible to levofloxacin. The MIC90s of clinical isolates with intermediate susceptibility or high resistance to penicillin G, were 2, 2, 4, 1, 1024, 2, 2 and 0.25 mg/l. About 98.1, 97.0, and 83.1% of strains were inhibited by concentrations < or = 1 mg/l of moxifloxacin, levofloxacin and ciprofloxacin, respectively (E-test). Eight of the 1037 S. pneumoniae strains were not susceptible to moxifloxacin and had mutations in gyrA (eight strains), parC (four strains) or parE (three strains). Against H. influenzae (32.7% were beta-lactamase producers) and M. catarrhalis (96.3% were beta-lactamase producers), the MIC90s of moxifloxacin, amoxycillin and co-amoxiclav were 0.094 and 0.125 mg/l, 64 and 8 mg/l, and 1.5 and 0.25 mg/l, respectively. Against oxacillin-susceptible S. aureus and K. pneumoniae, the MIC90s of moxifloxacin were 0.125 and 0.84 mg/l respectively. Moxifloxacin had the highest in vitro activity of all antibiotics tested.

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.

Topoisomerase mutations associated with in vitro selection of resistance to moxifloxacin in Streptococcus pneumoniae

We analyzed the frequencies of selection, the order of acquisition, and the mutations selected on moxifloxacin in two wild-type pneumococcal strains, R6 and 5714. The first selection step showed either a single GyrA mutation or no mutation in any of the quinolone resistance-determining regions. Second-step mutants selected had either a second mutation in ParC or in ParE. Moxifloxacin could belong to these fluoroquinolones, which preferentially target GyrA though probably acting equally through both gyrase and topoisomerase IV.

[Interpretative reading of the antibiogram in gram-positive cocci]

Resistance to methicillin in Staphylococcus is related to expression of the gene mecA, and implies resistance to all beta-lactams. Breakpoints for interpretation of this mechanism differ in S. aureus and in coagulase-negative species. In relation to macrolides-lincosamides-streptograminsB, the most frequent mechanism among resistant strains is expression of methylases (erm genes). Topoisomerase changes caused by point mutations and expression of the efflux pump NorA determine resistance to quinolones, but there are great differences on the activity of different compounds, which makes interpretative reading difficult. Strains of S. aureus with intermediate susceptibility to glycopeptides (GISA strains) have been recently described. In Spain, there is a high percentage of S. pneumoniae strains intermediate or resistant to penicillin, and a low percentage of strains intermediate or resistant to third generation cephalosporins, because of mutations in genes encoding penicillin-binding proteins. The most frequent phenotype of resistance to macrolides in this species is caused by methylase production. Resistance to quinolones is still uncommon, and is related to the mechanisms previously indicated for Staphylococcus, but clinical interpretation of the antibiograma for this organism is even more complex. No strains of S. pyogenes resistant to penicillin have yet been described. In Spain the most common phenotype of resistance to macrolides in S. pyogenes is determined by efflux pumps (mef genes), affecting 14- and 15-membered macrolides. E. faecalis is usually susceptible to ampicillin, in contrast to E. faecium. Enterococci show intrinsic resistance to aminoglycosides, but still remain susceptible to the combination of these antimicrobials and cell-wall active agents. Strains expressing different aminoglycoside-modifying enzymes became resistant to the combination. Glycopeptide-resistant strains of enterococci are uncommon in our country, but several genotypes, of which vanA is the most relevant from a clinical point of view, have been described in other regions.

Longitudinal assessment of antipneumococcal susceptibility in the United States

The prevalence of antimicrobial resistance among 4,940 U.S. pneumococcal isolates collected during 1999 was as follows: penicillin, 16.2%; amoxicillin-clavulanate, 12.2%; cefuroxime, 28.1%; ceftriaxone, 3.6%; trimethoprim-sulfamethoxazole, 30.3%; azithromycin, 21.4%; levofloxacin, 0.6%; and moxifloxacin, 0.1%. Compared to the previous 1997-1998 study (Jones et al., Antimicrob. Agents Chemother. 44:2645-2652, 2000), increases were noted for resistance to penicillin (3.7%; P < 0.001), amoxicillin-clavulanate (3.9%; P < 0.001), cefuroxime (5.7%; P < 0.001), azithromycin (2.4%; P = 0.014), trimethoprim-sulfamethoxazole (15.4%; P < 0.001), and levofloxacin (0.3%; P = 0.017). Resistance to ceftriaxone (0.1%; P = 0.809) and moxifloxacin (0.03%; P = 0.570) decreased. Concurrently, multidrug resistance increased (P < 0.001) from 6.3% to 11.3%.

Treatment of drug-resistant pneumococcal pneumonia

The increasing prevalence of resistance to penicillin and other drugs among pneumococci has considerably complicated the empirical treatment of community-acquired pneumonia. Penicillin resistance has become widespread and is a worldwide occurrence. Resistance to other classes of antibiotics traditionally used as alternatives in the treatment of pneumococcal infections has also increased markedly during recent years. In some areas of the USA, Europe, and east Asia a prevalence of macrolide resistance as high as 35% or more has been reported recently. From the clinical standpoint, a growing number of failures following the use of these agents has been described. Resistance to fluoroquinolones remains low but several failures have been reported in different parts of the world. Pharmacokinetic/pharmacodynamic parameters have become essential at the time of making a rational choice and calculation of dosage. Penicillin G remains the mainstay of therapy for the treatment of penicillin-susceptible pneumococcal pneumonia. Penicillin-resistant pneumococcal pneumonia (minimum inhibitory concentration <4 microg/mL) can be safely treated with adequate betalactams at the right dosage. The new fluoroquinolones are very active and effective in pneumococcal pneumonia. Caution should be exercised in the widespread prescription of these drugs if we are to limit the rate of resistance to these agents.

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.

A critical review of the fluoroquinolones: focus on respiratory infections

The new fluoroquinolones (clinafloxacin, gatifloxacin, gemifloxacin, grepafloxacin, levofloxacin, moxifloxacin, sitafloxacin, sparfloxacin and trovafloxacin) offer excellent activity against Gram-negative bacilli and improved Gram-positive activity (e.g. against Streptococcus pneumoniae and Staphylococcus aureus) over ciprofloxacin. Ciprofloxacin still maintains the best in vitro activity against Pseudomonas aeruginosa. Clinafloxacin, gatifloxacin, moxifloxacin, sitafloxacin, sparfloxacin and trovafloxacin display improved activity against anaerobes (e.g. Bacteroides fragilis) versus ciprofloxacin. All of the new fluoroquinolones display excellent bioavailability and have longer serum half-lives than ciprofloxacin allowing for once daily dose administration. Clinical trials comparing the new fluoroquinolones to each other or to standard therapy have demonstrated good efficacy in a variety of community-acquired respiratory infections (e.g. pneumonia, acute exacerbations of chronic bronchitis and acute sinusitis). Limited data suggest that the new fluoroquinolones as a class may lead to better outcomes in community-acquired pneumonia and acute exacerbations of chronic bronchitis versus comparators. Several of these agents have either been withdrawn from the market, had their use severely restricted because of adverse effects (clinafloxacin because of phototoxicity and hypoglycaemia; grepafloxacin because of prolongation of the QTc and resultant torsades de pointes; sparfloxacin because of phototoxicity; and trovafloxacin because of hepatotoxicity), or were discontinued during developmental phases. The remaining fluoroquinolones such as gatifloxacin, gemifloxacin, levofloxacin and moxifloxacin have adverse effect profiles similar to ciprofloxacin. Extensive post-marketing safety surveillance data (as are available with ciprofloxacin and levofloxacin) are required for all new fluoroquinolones before safety can be definitively established. Drug interactions are limited; however, all fluoroquinolones interact with metal ion containing drugs (eg. antacids). The new fluoroquinolones (gatifloxacin, gemifloxacin, levofloxacin and moxifloxacin) offer several advantages over ciprofloxacin and are emerging as important therapeutic agents in the treatment of community-acquired respiratory infections. Their broad spectrum of activity which includes respiratory pathogens such as penicillin and macrolide resistant S. pneumoniae, favourable pharmacokinetic parameters, good bacteriological and clinical efficacy will lead to growing use of these agents in the treatment of community-acquired pneumonia, acute exacerbations of chronic bronchitis and acute sinusitis. These agents may result in cost savings especially in situations where, because of their potent broad-spectrum activity and excellent bioavailability, they may be used orally in place of intravenous antibacterials. Prudent use of the new fluoroquinolones will be required to minimise the development of resistance to these agents.

Gatifloxacin: a review of its use in the management of bacterial infections

Gatifloxacin is an 8-methoxy fluoroquinolone antibacterial agent. The drug has a broader spectrum of antibacterial activity than the older fluoroquinolones (e.g. ciprofloxacin) and shows good activity against many Gram-positive and Gram-negative pathogens, atypical organisms and some anaerobes. Notably, gatifloxacin is highly active against both penicillin-susceptible and -resistant strains of Streptococcus pneumoniae, a common causative pathogen in community-acquired pneumonia (CAP), acute sinusitis and acute bacterial exacerbations of bronchitis. Gatifloxacin is absorbed well from the gastrointestinal tract (oral bioavailability is almost 100%). Therefore, patients can be switched from intravenous to oral therapy without an adjustment in dosage. High concentrations of gatifloxacin are achieved in plasma and target tissues/fluids. Gatifloxacin has a long plasma elimination half-life, thus allowing once-daily administration. Few clinically significant interactions between gatifloxacin and other drugs have been reported. In patients with CAP, clinical response rates in recipients of intravenous/oral gatifloxacin 400 mg/day ranged from 86.8 to 98.0% and rates of bacterial eradication ranged from 83.1 to 100% (up to 28 days post-treatment). Gatifloxacin showed efficacy similar to that of amoxicillin/clavulanic acid, ceftriaxone (with or without erythromycin) with or without stepdown to clarithromycin, levofloxacin or clarithromycin. Gatifloxacin was as effective as clarithromycin or amoxicillin/clavulanic acid, and was significantly more effective (in terms of clinical response; p < 0.035) than 7 to 10 days' treatment with cefuroxime axetil in the treatment of acute exacerbations of chronic bronchitis. In acute sinusitis, gatifloxacin showed clinical efficacy similar to that of clarithromycin, trovafloxacin or amoxicillin/clavulanic acid. Genitourinary infections were also successfully treated with gatifloxacin. Gatifloxacin is generally well tolerated. Its tolerability profile was broadly similar to those of comparator agents in comparative trials. The most common adverse events are gastrointestinal symptoms (oral formulation) and injection site reactions.

CONCLUSIONS

Gatifloxacin has an extended spectrum of antibacterial activity and provides better coverage of Gram-positive organisms (e.g. S. pneumoniae) than some older fluoroquinolones. The drug has favourable pharmacokinetic properties, is administered once daily and is at least as well tolerated as other fluoroquinolones. Gatifloxacin is a useful addition to the fluoroquinolones currently available for use in the clinical setting and has an important role in the management of adult patients with various bacterial infections. As with other fluoroquinolones, careful control of gatifloxacin usage in the community is important in order to prevent the emergence of bacterial resistance and thus preserve the clinical value of this agent.

Levofloxacin treatment failure in a patient with fluoroquinolone-resistant Streptococcus pneumoniae pneumonia

The frequency of fluoroquinolone-resistant Streptococcus pneumoniae has increased as fluoroquinolone administration for treatment of respiratory tract infections has increased. Levofloxacin treatment failed in a patient who had pneumococcal pneumonia and had received three previous courses of levofloxacin therapy. Susceptibility testing revealed high-level resistance to levofloxacin (minimum inhibitory concentration [MIC] > 32 microg/ml), and cross-resistance to moxifloxacin (MIC 4 microg/ml), trovafloxacin (6 microg/ml), and gatifloxacin (12 microg/ml). Sequencing of the quinolone-resistance determining region revealed a mutation of serine-81 to phenylalanine (Ser81-->Phe) in the gyrA region of DNA gyrase and a Ser79-->Phe mutation in the parC region of topoisomerase IV The patient was treated successfully with intravenous ceftriaxone followed by oral cefprozil. Clinicians must be aware of local resistance patterns and the potential for fluoroquinolone treatment failures in patients with infections caused by S. pneumoniae.

Topoisomerase II and IV quinolone resistance-determining regions in Stenotrophomonas maltophilia clinical isolates with different levels of quinolone susceptibility

The quinolone resistance-determining regions (QRDRs) of topoisomerase II and IV genes from Stenotrophomonas maltophilia ATCC 13637 were sequenced and compared with the corresponding regions of 32 unrelated S. maltophilia clinical strains for which ciprofloxacin MICs ranged from 0.1 to 64 microg/ml. GyrA (Leu-55 to Gln-155, Escherichia coli numbering), GyrB (Met-391 to Phe-513), ParC (Ile-34 to Arg-124), and ParE (Leu-396 to Leu-567) fragments from strain ATCC 13637 showed high degrees of identity to the corresponding regions from the phytopathogen Xylella fastidiosa, with the degrees of identity ranging from 85.0 to 93.5%. Lower degrees of identity to the corresponding regions from Pseudomonas aeruginosa (70.9 to 88.6%) and E. coli (73.0 to 88.6%) were observed. Amino acid changes were present in GyrA fragments from 9 of the 32 strains at positions 70, 85, 90, 103, 112, 113, 119, and 124; but there was no consistent relation to higher ciprofloxacin MICs. The absence of changes at positions 83 and 87, commonly involved in quinolone resistance in gram-negative bacteria, was unexpected. The GyrB sequences were identical in all strains, and only one strain (ciprofloxacin MIC, 16 microg/ml) showed a ParC amino acid change (Ser-80-->Arg). In contrast, a high frequency (16 of 32 strains) of amino acid replacements was present in ParE. The frequencies of alterations at positions 437, 465, 477, and 485 were higher (P < 0.05) in strains from cystic fibrosis patients, but these changes were not linked with high ciprofloxacin MICs. An efflux phenotype, screened by the detection of decreases of at least twofold doubling dilutions of the ciprofloxacin MIC in the presence of carbonyl cyanide m-chlorophenylhydrazone (0.5 microg/ml) or reserpine (10 microg/ml), was suspected in seven strains. These results suggest that topoisomerases II and IV may not be the primary targets involved in quinolone resistance in S. maltophilia.

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.

Expression of efflux pump gene pmrA in fluoroquinolone-resistant and -susceptible clinical isolates of Streptococcus pneumoniae

Thirty-four ciprofloxacin-resistant (MIC > or = 2 microg/ml) and 12 ciprofloxacin-susceptible clinical isolates of Streptococcus pneumoniae were divided into four groups based upon susceptibility to norfloxacin and the effect of reserpine (20 microg/ml). The quinolone-resistance-determining regions of parC, parE, gyrA, and gyrB of all ciprofloxacin-resistant clinical isolates were sequenced, and the activities of eight other fluoroquinolones, acriflavine, ethidium bromide, chloramphenicol, and tetracycline in the presence and absence of reserpine were determined. Despite a marked effect of reserpine upon the activity of norfloxacin, there were only a few isolates for which the activity of another fluoroquinolone was enhanced by reserpine. For most isolates the MICs of acriflavine and ethidium bromide were lowered in the presence of reserpine despite the lack of effect of this efflux pump inhibitor on fluoroquinolone activity. The strains that were most resistant to the fluoroquinolones were predominantly those with mutations in three genes. Expression of the gene encoding the efflux pump PmrA was examined by Northern blotting (quantified by quantitative competitive reverse transcriptase PCR) and compared with that of S. pneumoniae R6 and R6N. Within each group there were isolates that had high-, medium-, and low-level expression of this gene; however, increased expression was not exclusively associated with those isolates with a phenotype suggestive of an efflux mutant. These data suggest that there is another reserpine-sensitive efflux pump in S. pneumoniae that extrudes ethidium bromide and acriflavine but not fluoroquinolones.

Prevalence of single mutations in topoisomerase type II genes among levofloxacin-susceptible clinical strains of Streptococcus pneumoniae isolated in the United States in 1992 to 1996 and 1999 to 2000

Levofloxacin resistance in Streptococcus pneumoniae is rare, requiring at least two mutations in the quinolone resistance-determining region (QRDR) of topoisomerase IV and DNA gyrase. The prevalence of single QRDR mutations in these genes is unknown. Of 9,438 levofloxacin-susceptible pneumococci from the TRUST 4 surveillance study (1999-2000), 528 strains (MICs of 0.5 to 2.0 microg/ml) were selected for analysis. For comparison, 214 levofloxacin-susceptible strains (MICs of 0.5 to 1 microg/ml) isolated between 1992 and 1996 were analyzed. Oligonucleotide probe assay and DNA sequencing were used to detect QRDR mutations leading to changes at Ser79 and Asp83 in ParC, Ser81 in GyrA, and Asp435 in ParE, the most frequently found substitutions among levofloxacin-resistant strains. Among the 1992 to 1996 isolates only one strain (levofloxacin MIC, 1 microg/ml) had a mutation (Ser79 to Phe in ParC). No single mutations were found among 270 TRUST 4 strains with levofloxacin MICs of 0.5 microg/ml. Among 244 strains for which levofloxacin MICs were 1 microg/ml, 15 strains (6.1%) had a parC mutation and 3 strains (1.2%) had a parE mutation. Of 14 strains for which levofloxacin MICs were 2 microg/ml, 10 strains (71%) had a parC mutation; no parE mutations were found. No gyrA mutations were detected. It was estimated that 4.5% of the 9,438 levofloxacin-susceptible TRUST 4 isolates (MICs, < or =0.06 to 2 microg/ml) had a single parC or parE QRDR mutation. Although there has been an increase in the prevalence of single-step mutants, the increase may have been overestimated due in part to differences in geographical distribution for the two sets of isolates.

Genetic analyses of mutations contributing to fluoroquinolone resistance in clinical isolates of Streptococcus pneumoniae

Twenty-one clinical isolates of Streptococcus pneumoniae showing reduced susceptibility or resistance to fluoroquinolones were characterized by serotype, antimicrobial susceptibility, and genetic analyses of the quinolone resistance-determining regions (QRDRs) of gyrA, gyrB, parC, and parE. Five strains were resistant to three or more classes of antimicrobial agents. In susceptibility profiles for gatifloxacin, gemifloxacin, levofloxacin, moxifloxacin, ofloxacin, sparfloxacin, and trovafloxacin, 14 isolates had intermediate- or high-level resistance to all fluoroquinolones tested except gemifloxacin (no breakpoints assigned). Fluoroquinolone resistance was not associated with serotype or with resistance to other antimicrobial agents. Mutations in the QRDRs of these isolates were more heterogeneous than those previously reported for mutants selected in vitro. Eight isolates had amino acid changes at sites other than ParC/S79 and GyrA/S81; several strains contained mutations in gyrB, parE, or both loci. Contributions to fluoroquinolone resistance by individual amino acid changes, including GyrB/E474K, ParE/E474K, and ParC/A63T, were confirmed by genetic transformation of S. pneumoniae R6. Mutations in gyrB were important for resistance to gatifloxacin but not moxifloxacin, and mutation of gyrA was associated with resistance to moxifloxacin but not gatifloxacin, suggesting differences in the drug-target interactions of the two 8-methoxyquinolones. The positions of amino acid changes within the four genes affected resistance more than did the total number of QRDR mutations. However, the effect of a specific mutation varied significantly depending on the agent tested. These data suggest that the heterogeneity of mutations will likely increase as pneumococci are exposed to novel fluoroquinolone structures, complicating the prediction of cross-resistance within this class of antimicrobial agents.

Molecular surveillance of macrolide, tetracycline and quinolone resistance mechanisms in 1191 clinical European Streptococcus pneumoniae isolates

Streptococcus pneumoniae isolates (n=1191) were collected during a 1997-1999 European surveillance study. In addition to susceptibility data, a molecular epidemiological survey of their mechanisms of resistance to macrolides, tetracyclines, and quinolones was provided. Of the isolates tested, 72.6% were penicillin-susceptible, 19.9% penicillin-intermediate and 7.5% penicillin-resistant. There was an obvious relationship between resistance to penicillin and resistance to erythromycin (19% of all isolates), clindamycin (14%) and tetracycline (23%). Only one isolate was resistant to levofloxacin. Seventy-three percent of the European S. pneumoniae isolates resistant to erythromycin (n=229) carried the erm(B) gene, while the remaining 27% possessed the mef(A) gene. No mutations were detected in 23S rRNA or in ribosomal proteins L4 and L22. All tetracycline-resistant isolates (n=277) carried the tet(M) gene; none carried the tet(O) gene. Classical mutations in gyrA (Ser 81-Phe or Tyr) and parC (Ser 79-Phe and Asp 83-Asn) and efflux contributed to the decreased quinolone susceptibility. This study of recent European S. pneumoniae isolates can be used to recognize any changes in susceptibility patterns and resistance mechanisms that may occur in the future.

Molecular detection of antimicrobial resistance

The determination of antimicrobial susceptibility of a clinical isolate, especially with increasing resistance, is often crucial for the optimal antimicrobial therapy of infected patients. Nucleic acid-based assays for the detection of resistance may offer advantages over phenotypic assays. Examples are the detection of the methicillin resistance-encoding mecA gene in staphylococci, rifampin resistance in Mycobacterium tuberculosis, and the spread of resistance determinants across the globe. However, molecular assays for the detection of resistance have a number of limitations. New resistance mechanisms may be missed, and in some cases the number of different genes makes generating an assay too costly to compete with phenotypic assays. In addition, proper quality control for molecular assays poses a problem for many laboratories, and this results in questionable results at best. The development of new molecular techniques, e.g., PCR using molecular beacons and DNA chips, expands the possibilities for monitoring resistance. Although molecular techniques for the detection of antimicrobial resistance clearly are winning a place in routine diagnostics, phenotypic assays are still the method of choice for most resistance determinations. In this review, we describe the applications of molecular techniques for the detection of antimicrobial resistance and the current state of the art.

Mechanisms and frequency of resistance to gatifloxacin in comparison to AM-1121 and ciprofloxacin in Staphylococcus aureus

Gatifloxacin, an 8-methoxyfluoroquinolone, was found to be two- to fourfold more active against wild-type Staphylococcus aureus ISP794 than its desmethoxy derivative, AM-1121, and ciprofloxacin, another desmethoxy fluoroquinolone. Single grlBA mutations caused two- to fourfold increases in the MIC of gatifloxacin, and a single gyrase mutation was silent. Double mutations in gyrA and grlA or grlB caused a 32-fold increase in the MIC of gatifloxacin, in contrast to a 128-fold increase for ciprofloxacin and AM-1121. Overexpression of the NorA efflux pump had minimal effect on the MIC of gatifloxacin. The bactericidal activity of the three quinolones at four times the MIC differed only for a double mutant, with gatifloxacin exhibiting a killing pattern similar to that for ISP794, whereas ciprofloxacin and AM-1121 failed to show any killing. With gatifloxacin, selection of resistant mutants at twice the MIC was 100- to 1,000-fold less frequent than with the comparison quinolones, and mutants could rarely be selected at four times the MIC. The limit resistance in ISP74 was 512 times the MIC of gatifloxacin and 1,024 times the MICs of ciprofloxacin and AM-1121. Novel mutations in topoisomerase IV were selected in five of the six single-step mutants, three of which were shown to cause quinolone resistance by genetic studies. In conclusion, topoisomerase IV is the primary target of gatifloxacin. In contrast to comparison quinolones, mutations in both topoisomerase IV and gyrase are required for resistance to gatifloxacin by clinical breakpoints and do not abolish bactericidal effect, further supporting the benefit of the 8-methoxy substituent in gatifloxacin.

Clinical isolates of Streptococcus pneumoniae resistant to levofloxacin contain mutations in both gyrA and parC genes

Thirty Streptococcus pneumoniae clinical isolates resistant to levofloxacin were analyzed for the quinolone resistance-determining DNA sequences to identify point mutations and were tested for in vitro susceptibility to multiple drug classes. Of these isolates, 29 had mutations in both gyrA and parC genes of DNA gyrase and topoisomerase IV, respectively. In GyrA, an amino acid change from Ser-81-->Phe was detected in 27 isolates and a Glu-85-->Lys change was found in the remaining three. Of the 29 isolates for which ParC data were available, Ser-79-->Tyr or Phe were the predominant mutations observed. MICs for levofloxacin were 4-16 mg/l, whereas those for moxifloxacin were 1-2 mg/l. Twenty-four (80%) isolates were susceptible to erythromycin, 25 (83%) to azithromycin, 26 (87%) to clarithromycin, 27 (90%) to clindamycin, 20 (67%) to penicillin, 21 (70%) to ceftriaxone and 30 (100%) to amoxycillin/clavulanate. These results confirm the presence of double mutations among clinical isolates of S. pneumoniae from diverse geographical regions of North America and also suggest that quinolone resistance may develop independently of resistance to other drug classes.

Streptococcus pneumoniae isolates with reduced susceptibility to ciprofloxacin in Spain: clonal diversity and appearance of ciprofloxacin-resistant epidemic clones

Analysis of the pulsed-field gel electrophoretic profiles of 82 pneumococcal isolates with reduced susceptibility to ciprofloxacin (RSC) and of 90 co-occurring susceptible isolates indicates a considerable genetic diversity among isolates with RCS and points to a close relation between the two groups. This finding suggests that pneumococci with RCS emerge through independent mutational events.

Antimicrobial use and the emergence of antimicrobial resistance with Streptococcus pneumoniae in the United States

The rapid emergence of resistance to antimicrobial agents by Streptococcus pneumoniae in the United States has been influenced by various factors, including the clonal nature of most resistant strains and the fact that organisms with a multiresistant phenotype have become stably endemic. The ease with which transmission occurs and the fact that humans, especially children, are often colonized asymptomatically in the upper respiratory tract have contributed to the problem. Clearly, the most important factor in the emergence of antimicrobial resistance with S. pneumoniae, however, is the selective pressure of antimicrobial agents. Potency, defined as a product of both antibacterial effect and drug delivery, is a key factor. Generally speaking, the more potent an antimicrobial agent, the less likely it is to select for resistance. This is germane to comparisons of oral agents within specific antimicrobial classes (e.g., beta-lactams, macrolides, and fluoroquinolones). Within each class, potencies differ. In view of the existence of stably endemic multidrug-resistant S. pneumoniae, given comparable cost, side-effect profile, palatability, convenience of dosing, and accessibility, use of the most potent agent(s) within a particular class is advocated.

Contributions of the 8-methoxy group of gatifloxacin to resistance selectivity, target preference, and antibacterial activity against Streptococcus pneumoniae

Gatifloxacin (8-methoxy, 7-piperazinyl-3'-methyl) at the MIC selected mutant strains that possessed gyrA mutations at a low frequency (3.7 x 10(-9)) from wild-type strain Streptococcus pneumoniae IID553. AM-1147 (8-methoxy, 7-piperazinyl-3'-H) at the MIC or higher concentrations selected no mutant strains. On the other hand, the respective 8-H counterparts of these two compounds, AM-1121 (8-H, 7-piperazinyl-3'-methyl) and ciprofloxacin (8-H, 7-piperazinyl-3'-H), at one and two times the MIC selected mutant strains that possessed parC mutations at a high frequency (>2.4 x 10(-6)). The MIC of AM-1147 increased for the gyrA mutant strains but not for the parC mutant strains compared with that for the wild-type strain. These results suggest that fluoroquinolones that harbor 8-methoxy groups select mutant strains less frequently and prefer DNA gyrase, as distinct from their 8-H counterparts. The in vitro activities of gatifloxacin and AM-1147 are twofold higher against the wild-type strain, eight- and twofold higher against the first-step parC and gyrA mutant strains, respectively, and two- to eightfold higher against the second-step gyrA and parC double mutant strains than those of their 8-H counterparts. These results indicate that the 8-methoxy group contributes to enhancement of antibacterial activity against target-altered mutant strains as well as the wild-type strain. It is hypothesized that the 8-methoxy group of gatifloxacin increases the level of target inhibition, especially against DNA gyrase, so that it is nearly the same as that for topoisomerase IV inhibition in the bacterial cell, leading to potent antibacterial activity and a low level of resistance selectivity.

Interspecies recombination contributes minimally to fluoroquinolone resistance in Streptococcus pneumoniae

Analysis of 71 ciprofloxacin-resistant (MIC > or = 4 microg/ml) Streptococcus pneumoniae clinical isolates revealed only 1 for which the quinolone resistance-determining regions of the parC, parE, and gyrB genes were genetically related to those of viridans group streptococci. Our findings support the occurrence of interspecies recombination of type II topoisomerase genes; however, its contribution to the emergence of quinolone resistance among pneumococci appears to have been minimal.

Fluoroquinolone susceptibility, resistance, and pharmacodynamics versus clinical isolates of Streptococcus pneumoniae from Indiana

The in vitro activity and pharmacodynamics (AUC(0-24)/MIC) of levofloxacin, gatifloxacin, moxifloxacin, and gemifloxacin were evaluated against 307 clinical isolates of Streptococcus pneumoniae from Indianapolis, Indiana. Organisms were collected between January 1999 and April 2000, and MICs were determined by broth microdilution. Serum concentration-time profiles were simulated for the following oral regimens administered once daily: levofloxacin 500 mg and 750 mg; gatifloxacin 400 mg; moxifloxacin 400 mg; gemifloxacin 320 mg. Free 24 h area under the serum concentration-time curves (AUC(0-24)) were calculated, and the average AUC(0-24)/MIC was calculated for each regimen. Differences in AUC(0-24)/MIC among agents were determined by analysis of variance (Scheffe post-hoc test, p < 0.05). Overall, gemifloxacin was the most potent agent tested. Five (1.7%), 4 (1.3%), and 2 (0.7%) isolates were resistant to levofloxacin, gatifloxacin, and moxifloxacin, respectively. None of the isolates was resistant to gemifloxacin. Gemifloxacin AUC(0-24)/MIC was significantly greater than all other regimens (p < 0.0001), with the exception of moxifloxacin. However, the percent of isolates for which an AUC(0-24)/MIC >or= 30-50 can be achieved is similar for gemifloxacin, moxifloxacin, gatifloxacin, and levofloxacin 750 mg. Large comparative studies are needed to determine if the differences in AUC(0-24)/MIC among fluoroquinolones are clinically significant.

Mutation in the DNA gyrase A Gene of Escherichia coli that expands the quinolone resistance-determining region

In three Escherichia coli mutants, a change (Ala-51 to Val) in the gyrase A protein outside the standard quinolone resistance-determining region (QRDR) lowered the level of quinolone susceptibility more than changes at amino acids 67, 82, 84, and 106 did. Revision of the QRDR to include amino acid 51 is indicated.

Acute exacerbations of chronic bronchitis: what role for the new fluoroquinolones?

Acute exacerbations of chronic bronchitis (AECB) are a major cause of morbidity and mortality. Bacterial pathogens are implicated in about half the episodes of AECB. Empirical antibacterials have a significant benefit in AECB; however, several recent developments have considerably complicated antibacterial choice for this condition. New fluoroquinolone antibacterials introduced in the last decade are theoretically well suited for the treatment of AECB, as the in vitro antimicrobial spectrum of these drugs includes all the major pathogens involved. The pharmacokinetic and pharmacodynamic properties of the new fluoroquinolones are superior to many other antibacterials used to treat AECB. In trials, clinical success with the new fluoroquinolones was equivalent and bacteriological success was occasionally superior to nonfluoroquinolone comparators. However, these clinical trials did not assess several potentially important end-points for which the theoretical superiority of the fluoroquinolones may translate into differences in outcome. Rare but serious adverse effects with some of the new fluoroquinolones have shaken the confidence of prescribing physicians in this class of drugs. Emergence of the resistance of Streptococcus pneumoniae to fluoroquinolones has raised concerns about indiscriminate and widespread use of the new agents for trivial infections. Patients with AECB are a heterogeneous population who should be stratified in order to appropriately choose empirical antibacterial therapy. Highly efficacious antibacterial therapy, such as the new fluoroquinolones, is appropriate as a first-line choice for patients who have risk factors for a poor outcome or are in intensive care units. Such selected use of the new fluoroquinolones balances individual benefit with societal concerns of the use of these agents for AECB.

Worldwide prevalence of antimicrobial resistance in Streptococcus pneumoniae, Haemophilus influenzae, and Moraxella catarrhalis in the SENTRY Antimicrobial Surveillance Program, 1997-1999

The in vitro activities of numerous antimicrobials against clinical isolates of Streptococcus pneumoniae, Haemophilus influenzae, and Moraxella catarrhalis from patients with bloodstream and respiratory tract infections in the United States, Canada, Europe, Latin America, and the Asia-Pacific region were studied in the SENTRY Antimicrobial Surveillance Program. Penicillin resistance (minimum inhibitory concentration, > or =2 microg/mL) was noted in all 5 geographic regions, and a high and increasing rate of macrolide resistance among S. pneumoniae isolates was observed. Elevated rates of resistance to clindamycin, trimethoprim-sulfamethoxazole, chloramphenicol, and tetracycline were seen. beta-Lactamase-mediated resistance in H. influenzae to amoxicillin and variable trimethoprim-sulfamethoxazole resistance by region were documented. Resistance to several drugs continues to emerge among pneumococci worldwide, but more stable resistance patterns have been noted for H. influenzae and M. catarrhalis. Continued surveillance of this pathogen group appears to be prudent.