Antimicrobial resistance: a class effect?

Transcription

1 Journal of Antimicrobial Chemotherapy (2002) 50, Suppl. S2, 7 12 DOI: /jac/dkf508 Antimicrobial resistance: a class effect? J. Prieto*, A. Calvo and M. L. Gómez-Lus Microbiology I Department, School of Medicine, Universidad Complutense, Avda Complutense s/n, Madrid, Spain Antibiotic use has led to increased resistance to certain group markers: penicillin, erythromycin and ciprofloxacin for β-lactams, macrolides and quinolones, respectively. The influence of resistance to markers in decreasing susceptibility to the drugs included (on the basis of chemical structure) in the corresponding antibiotic group can be defined as resistance class effect. In the case of macrolides, this effect is dependent on the prevalent resistant phenotype among the isolates of the target bacteria: the class effect exists completely if the mechanism of resistance is constitutive MLS B (all macrolides are affected by resistance to erythromycin), and only partially if the mechanism is the efflux M phenotype (all but 16-membered macrolides are affected). In Spain, the first case is exemplified by Streptococcus pneumoniae and the second by Streptococcus pyogenes. For β-lactams and quinolones, resistance to the group markers results in large decreases in the antimicrobial activity of the less potent members of the group, penicillin being a better driver of resistance for oral cephalosporins than for aminopenicillins, and ciprofloxacin being a better driver for older rather than for the newer quinolones, which have enhanced anti-pneumococcal activity. Empirical prescription guidelines based on the pharmacoepidemiology of resistance, recommending the use of potent drugs that are less influenced by resistance to the marker, may help to counter the spread of resistance in the community. Introduction Resistance is associated with the use of antimicrobial agents. 1,2 This is clearly evident in community-acquired respiratory tract infections (RTIs), where the development and dissemination of resistance have been related to consumption. 3 5 Since 90% of antibiotic use is in the community and 80% of this for the treatment of RTIs, 6 antibiotic treatment of community-acquired RTIs may be the leading cause of the resistance problem. Two of the most prevalent bacterial species isolated from RTIs present problems with respect to susceptibility: Streptococcus pneumoniae in relation to β-lactams, macrolides and quinolones; and Streptococcus pyogenes in relation to macrolides, to an increasing extent. This widely recognized problem 7 has posed difficulties, particularly with respect to the treatment of RTI, because of the need to select appropriate treatment empirically. Infecting organisms, if isolated, are categorized as susceptible or resistant up to several days after treatment has been initiated. So, despite the fact that prudent antimicrobial prescribing in the community may help to prevent the relentless increase in resistance, 7 the term prudent has to be used generally (from a population perspective) without the knowledge of the likely effectiveness of the antibiotic being used for a specific patient. In other words, empirical treatment is based mainly on epidemiological data. Prescribing in primary care is influenced by several factors influencing appropriate antibiotic use by general practitioners. 7 Guidelines based on treatment costs and clinical outcomes have probably had little effect on the emergence 8 and spread of resistance. Therefore, alternative guidelines could be based on the bacterial (resistance) and drug (antimicrobial effects against bacteria) factors that might counter resistance, since there is increasing evidence confirming that maximal reduction of the bacterial load (and in the end, bacterial eradication) should be the primary goal of antibiotic therapy. 8,9 Markers of resistance The antibiotics most frequently prescribed in communityacquired RTI are from three major groups: β-lactams, macrolides and quinolones. Among β-lactams, aminopenicillins are used more than oral cephalosporins 4,5 and recently long halflife macrolides have been used more frequently in Spain than those given more than twice a day. 3 5 The use of quinolones for the treatment of RTI is also increasing. 10,11... *Corresponding author. jprieto@med.ucm.es The British Society for Antimicrobial Chemotherapy

2 J. Prieto et al. Most studies of resistance use the term resistance with respect to the susceptibility to penicillin, erythromycin and ciprofloxacin. For this reason, it can be inferred that resistance to these antibiotics is generally considered as a marker for resistance to β-lactams, macrolides and quinolones, respectively. Because those antibiotics with lower intrinsic activity (oral cephalosporins versus aminopenicillins) or that reach lower concentrations in blood or tissue (longer selective windows of resistance) 12,13 have a greater responsibility in causing resistance to antibiotic markers (their consumption has greater influence on the emergence, maintenance and spread of resistance in the general population), 3 5 then the critical question is whether a decrease in susceptibility to the antibiotic marker implies a decrease in the antimicrobial activity of the drugs included in the corresponding group. The resistance class effect can be defined as an affirmative answer to this question, in that a decrease in susceptibility to the antibiotic marker affects equally all compounds in the antibiotic class. This concept is needed because, in terms of resistance, it does not seem logical to group drugs with different microbiological, pharmacokinetic and pharmacodynamic properties (and thus with different capabilities of resistance selection) only on the basis of their similar chemical structure. 14 Moreover, the problem should be analysed by combining bacterial species with specific antibiotics, since the class effect may occur for some antibiotic groups but not for others, or within an antibiotic group for some bacterial species but not for others. Resistance to β-lactams: a class effect? Does an increase in penicillin MIC affect to a similar extent the activity of all β-lactams against S. pneumoniae and S. pyogenes? Following NCCLS guidelines, 15 penicillin is a good marker of resistance with respect to the susceptibility of S. pyogenes to β-lactams: an isolate susceptible to penicillin should be considered susceptible to all β-lactams. In this case, the use of β-lactams has not resulted in the appearance of in vitro resistance in S. pyogenes, 16 probably due to the absence of selectable variants carrying mechanisms of resistance to β-lactams in this bacterial species. 3 One hundred per cent of S. pyogenes isolates remain susceptible to penicillin and thus to all β-lactams. With respect to S. pneumoniae, the latest surveillance carried out in Spain (1684 isolates) showed a prevalence of highly resistant strains of 5% for aminopenicillins, 7% for cefotaxime, 22% for penicillin, 31% for cefuroxime and 42% for cefaclor. 17 While resistance to aminopenicillins (intermediate or high) is only clustered in the 22% of strains showing high resistance to penicillin (resistance among strains susceptible or intermediately resistant to penicillin being practically absent), high rates (>35%) of high-level resistance were obtained for oral cephalosporins among penicillinintermediate and -resistant strains. Resistance to cefotaxime (mostly intermediate) was clustered at lower rates in strains showing intermediate resistance to penicillin but at high rates amongst the highly penicillin-resistant strains, although breakpoints to third generation cephalosporins (which have proved useful for penicillin-resistant strains in communityacquired pneumonia) 18 may be too low for the prediction of efficacy in RTI. 17 Differences in activity between β-lactams have also been found in other surveillance studies In terms of the differential activity of β-lactams for S. pneumoniae, several groups are proposed. The first group would include those compounds with higher activity than penicillin, such as amoxicillin, co-amoxiclav and cefotaxime. The second group would include those antibiotics with slightly higher activity than penicillin, such as cefuroxime. The third group would be those antibiotics with lower activity than penicillin, such as cefixime or cefaclor. Therefore, in relation to in vitro bacteriostatic activity, penicillin resistance is a good marker of resistance to oral cephalosporins, but not to aminopenicillins. This has an analogy in studies investigating the influence of the increase in penicillin MIC on the bactericidal activity of several β-lactams, where aminopenicillins were affected to a lesser extent than oral cephalosporins in in vitro experiments and to a similar extent to parenteral cephalosporins in ex vivo experiments. 25 Again, in terms of in vivo efficacy, the effect that the increase in the penicillin MIC had on the efficacy of β-lactams was not uniform, being lower for amoxicillin versus oral cephalosporins in sepsis 26,27 and otitis 28,29 animal models. Higher in vivo bactericidal activity of amoxicillin was demonstrated in an animal sepsis model where amoxicillin showed greater clearance of bacteraemia (and thus, an increase in survival rates) than cefotaxime against a serotype 6 penicillin-resistant strain (with MICs of 2 and 4 mg/l cefotaxime and amoxicillin, respectively). 30 Limited documentation on therapeutic failures in patients with adequate doses of β-lactams in pneumococcal RTI, 31 and the in vivo experience in animal models suggest that penicillin resistance is not a good marker of β-lactam resistance when prediction of therapeutic efficacy is needed. Macrolides: a class effect? Does the increase in erythromycin MIC affect to a similar extent the activity of all macrolides against S. pneumoniae and S. pyogenes? In this case, the response depends on the mechanism of resistance exhibited by the target bacteria. Two main mechanisms of resistance are exhibited by the two most prevalent Grampositive cocci in community-acquired RTI: S. pneumoniae and S. pyogenes. One mechanism depends on target modification and is the constitutive MLS B phenotype, which 8

3 Antimicrobial resistance: a class effect? determines high-level and cross-resistance to 14-, 15- and 16-membered macrolides, lincosamides, clindamycin and streptogramin B. 32 The other mechanism depends on the efflux of drugs and is the M phenotype, which confers lowlevel resistance to 14- and 15-membered macrolides, while not affecting 16-membered macrolides (espiramycin and josamycin), lincosamides, streptogramins or ketolides, which remain fully active. 32,33 Therefore, the problem should be analysed by geographical area, since erythromycin is a good marker of resistance to macrolides in areas where the prevalent mechanism of resistance is the constitutive MLS B phenotype, but not in areas where the prevalent mechanism of resistance is the M efflux phenotype, since strains carrying this type of resistance mechanism are susceptible to 16-membered macrolides. In Spain, the prevalence of resistance to erythromycin is 35% in S. pneumoniae and 20% in S. pyogenes. 17 While 90% of S. pneumoniae resistant to erythromycin exhibit the constitutive MLS B resistance phenotype, 90% of S. pyogenes resistant to erythromycin exhibit the M efflux phenotype. 17 Since both mechanisms of erythromycin resistance imply resistance to 14- and 15-membered macrolides, erythromycin is a good marker of resistance to clarithromycin and azithromycin in both species. Thus, when 16-membered macrolides are also considered, erythromycin is only a good marker of resistance for the whole antibiotic group in S. pneumoniae but not in S. pyogenes. The local prevalence of mechanisms of resistance is critical for macrolide prescribing since the very high MIC 90 ( 64 mg/l) 17 obtained with the constitutive MLS B mechanism demands strategies using larger doses that might exceed those concentrations. In any case, the MIC 90 (8 mg/l) 17 of erythromycin, clarithromycin and azithromycin for strains with the efflux M mechanism cannot be exceeded by standard oral doses of these antibiotics. The level of resistance is reflected therapeutically, and clinical failures have been described 34,35 with macrolides in bacteraemic respiratory infections caused by S. pneumoniae with the constitutive MLS B phenotype prevalent in Spain. Thus, in this country, erythromycin resistance is a marker of the therapeutic outcome of clarithromycin and azithromycin treatments for infections caused by S. pneumoniae. There appears little doubt that macrolide resistance correlates directly with clinical failure and that guidelines should exclude macrolides in areas of rising resistance prevalence. 8 Quinolones: a class effect? Does the increase in ciprofloxacin MIC affect to a similar extent the activity of all quinolones against S. pneumoniae? Resistance rates to quinolones in respiratory pathogens are relatively low, 10,36 but the current wide use of quinolones could cause future problems of resistance. The new fluoroquinolones show an activity greater than that of ciprofloxacin. 37,38 The use of quinolones has been related to ciprofloxacin resistance, 11 but logically it has to be attributed to older quinolones, since the new ones, with enhanced antipneumococcal activity, were not marketed. In Spain, the prevalence of ciprofloxacin resistance is 6% for high-level resistance (MIC 4 mg/l) and 22% for an MIC 2 mg/l. 10 While new quinolones exhibit high activity against S. pneumoniae 39 susceptible to ciprofloxacin (MIC 2 mg/l), an increase in the MICs of all quinolones occurs for ciprofloxacin-resistant strains (MIC 4 mg/l). Nevertheless, this increase is not equal for all drugs, being lower for gemifloxacin than for trovafloxacin, 40 clinafloxacin and moxifloxacin (MICs < 0.25 mg/l of all of them for ciprofloxacin-susceptible strains compared with MIC 90 of 0.5, 1, 4 and 8 mg/l of gemifloxacin, clinafloxacin, moxifloxacin and trovafloxacin, respectively, for strains with ciprofloxacin MIC 8 mg/l). 41 In terms of percentages of resistance to the new quinolones, the increase in ciprofloxacin MIC 41 did not affect gemifloxacin (since the MIC 90 remained constantly equal to or lower than the proposed 0.5 mg/l breakpoint 42 ) or clinafloxacin (MIC 90 remained <1 mg/l), while it affected moxifloxacin and trovafloxacin [MIC 90 higher than the NCCLS susceptibility breakpoints ( 1 mg/l) 15 ]. As expected, an increase in the MIC of ciprofloxacin influenced to a higher degree resistance to the older quinolones such as ofloxacin, grepafloxacin and sparfloxacin. 41 In a different study, gemifloxacin was also the quinolone least affected by both a decrease in in vitro susceptibility to ciprofloxacin and the presence of human serum. 43 These effects on in vitro bacteriostatic activity were also found when in vitro bactericidal activity was assessed in the presence of human serum in a study with concentrations similar to peak and trough serum concentrations in humans after standard doses of ciprofloxacin, gemifloxacin and trovafloxacin. 44 At peak concentrations, gemifloxacin decreased by 99.9% the initial inocula of four strains exhibiting ciprofloxacin MICs ranging from 0.5 to 16 mg/l, and showed significant bactericidal activity at trough concentrations, thus theoretically preventing regrowth over the dosing interval. 44 Ciprofloxacin and trovafloxacin did not obtain a reduction for any of the four strains tested. 44 This lesser effect on gemifloxacin was also corroborated in a Phase I study demonstrating that the ex vivo bactericidal activity of gemifloxacin was less influenced than that of trovafloxacin by the decrease in ciprofloxacin MIC exhibited by the strains tested. 45 In animal models, the gemifloxacin AUC:MIC ratios producing a 2 log kill over 24 h (a parameter related to survival) were similar against ciprofloxacin-susceptible and -resistant strains, 46 using an AUC similar to that obtained in humans. In the clinical setting, therapeutic failures have been described with older quinolones, 47 including levofloxacin, 48 9

4 J. Prieto et al. but the intrinsic activity of new fluoroquinolones may counter the risk derived from the increased prevalence of ciprofloxacin resistance. 14 In conclusion, the class effect in the quinolone group is similar to that of β-lactams: the group marker of resistance is a better determinant of resistance for the compounds with lower intrinsic activity than for the more potent ones, i.e. ciprofloxacin is a good predictor of resistance for older quinolones but not for the very new ones. Conclusions Resistance is a global problem among the most prevalent respiratory isolates. In areas with a high prevalence of resistance to macrolides in S. pyogenes, there is a high prevalence of resistance to macrolides in S. pneumoniae. 17 Areas with a high prevalence of resistance to macrolides in S. pneumoniae are also associated with resistance to penicillin. 17 In addition, ciprofloxacin resistance is clustered in strains with erythromycin resistance and with penicillin non-susceptibility. 10,17 From this situation, it can be concluded that resistance is a global problem with a geographical basis in RTIs. Because of this, empirical treatments aimed at decreasing the bacterial load in infected patients to avoid the spread of clonal resistant strains should be based on local data on prevalence of resistance and of resistance phenotypes. Due to the clustering of resistance to different markers in the same clones, success in the fight against penicillin resistance implies a decrease in erythromycin resistance and ciprofloxacin resistance in S. pneumoniae. In Spain, macrolides are not able to overcome the overall problem of resistance since resistance to erythromycin exerts a class effect over all macrolides in S. pneumoniae and a partial class effect (only to 14- and 15-membered macrolides) in S. pyogenes due to the prevalence of the respective resistance phenotypes. For this reason, the most active oral β-lactams (aminopenicillins) are, among the currently available antibiotics, a good option for empirical treatment of infants and adults in the community, and could also counter the spread of penicillin resistance. New quinolones are also an option provided they are not influenced in the future by the increase in resistance to the group marker. In conclusion, bacterial and antibiotic factors should be considered when selecting an empirical treatment for community-acquired RTI by using the most suitable antibiotic class (as defined by epidemiological data) and within it, the most potent antibiotic that is least affected by a decrease in susceptibility to the group marker. Acknowledgements We would like to thank M. J. Giménez and L. Aguilar for their critical review of the manuscript. References 1. Magee, J. T., Pritchard, E. L., Fitzgerald, K. A., Dunstan, F. D. & Howard, A. J. (1999). Antibiotic prescribing and antibiotic resistance in community practice: retrospective study British Medical Journal 319, Arason, V. A., Kristinsson, K. G., Sigurdsson, J. A., Stefansdottir, G., Molstad, S. & Gudmundsson, S. (1996). Do antimicrobials increase the carriage rate of penicillin resistant pneumococci in children? Cross sectional prevalence study. British Medical Journal 313, Granizo, J. J., Aguilar, L., Casal, J., Dal-Ré, R. & Baquero, F. (2000). Streptococcus pyogenes resistance to erythromycin in relation to macrolide consumption in Spain ( ). Journal of Antimicrobial Chemotherapy 46, Granizo, J. J., Aguilar, L., Casal, J., García-Rey, C., Dal-Ré, R. & Baquero, F. (2000). Streptococcus pneumoniae resistance to erythromycin and penicillin in relation to macrolide and β-lactam consumption in Spain ( ). Journal of Antimicrobial Chemotherapy 46, García-Rey, C., Aguilar, L., Baquero, F., Casal, J. & Dal-Ré, R. (2002). Importance of local variations in antibiotic consumption and geographical differences of erythromycin and penicillin resistance in Streptococcus pneumoniae. Journal of Clinical Microbiology 40, Huovinen, P. & Cars, O. (1998). Control of antimicrobial resistance: time for action. British Medical Journal 317, McNulty, C. A. M. (2001). Optimising antibiotic prescribing in primary care. International Journal of Antimicrobial Agents 18, Ball, P., Baquero, F., Cars, O., File, T., Garau, J., Klugman, K. et al. (2002). Antibiotic therapy of community respiratory tract infections: strategies for optimal outcomes and minimized resistance emergence. Journal of Antimicrobial Chemotherapy 49, Dagan, R., Klugman, K. P., Craig, W. A. & Baquero, F. (2001). Evidence to support the rationale that bacterial eradication in respiratory tract infection provides guidance for antimicrobial therapy. Journal of Antimicrobial Chemotherapy 47, García-Rey, C., Aguilar, L., Baquero, F. & The Spanish Surveillance Group for Respiratory Pathogens. (2000). Influences of different factors on prevalence of ciprofloxacin resistance in Streptococcus pneumoniae in Spain. Antimicrobial Agents and Chemotherapy 44, Chen, D. K., McGeer, A., De Azavedo, J. C. & Low, D. E. (1999). Decreased susceptibility of Streptococcus pneumoniae to fluoroquinolones in Canada. Canadian Bacterial Surveillance Network. New England Journal of Medicine 341, Baquero, F. (1999). Evolving resistance patterns of Streptococcus pneumoniae: a link with long-acting macrolide consumption? Journal of Chemotherapy 11, Suppl. 1, Drusano, G. L. & Craig, W. A. (1997). Relevance of pharmacokinetics and pharmacodynamics in the selection of antibiotics for respiratory tract infections. Journal of Chemotherapy 9, Suppl. 3, Aguilar, L., García-Rey, C. & Giménez, M. J. (2001). Presión antibiótica, desarrollo de resistencias en Streptococcus pneumo- 10

5 Antimicrobial resistance: a class effect? niae y fracaso clínico: un círculo no tan vicioso para algunos antibióticos. Revista Espanola de Quimioterapia 14, National Committee for Clinical Laboratory Standards. (2000). Methods for Dilution Antimicrobial Susceptibility Tests for Bacteria that Grow Aerobically Fifth Edition: Approved Standard M7-A5. NCCLS, Wayne, PA, USA. 16. Gerber, M. A. (1995). Antibiotic resistance in Group A streptococci. Pediatric Clinics of North America 42, Pérez-Trallero, E., Fernández-Mazarrasa, C., García-Rey, C., Bouza, E., Aguilar, L., García-de-Lomas, J. et al. (2001). Antimicrobial susceptibilities of 1,684 Streptococcus pneumoniae and 2,039 Streptococcus pyogenes isolates and their ecological relationships: Results of a 1-year ( ) multicenter surveillance study in Spain. Antimicrobial Agents and Chemotherapy 45, Roson, B., Carratalá, J., Tubau, F., Dorca, J., Liñares, J., Pallarés, R. et al. (2001). Usefulness of β-lactam therapy for community-acquired pneumonia in the era of drug-resistant Streptococcus pneumoniae: a randomized study of amoxicillin clavulanate and ceftriaxone. Microbial Drug Resistance 7, Sahm, D. F., Jones, M. E., Hickey, M. L., Diakun, D. R., Mani, S. V. & Thornsberry, C. (2000). Resistance surveillance of Streptococcus pneumoniae, Haemophilus influenzae and Moraxella catarrhalis isolated in Asia and Europe, Journal of Antimicrobial Chemotherapy 45, Felmingham, D., Grüneberg, R. N. & the Alexander Project Group. (2000). The Alexander Project : latest susceptibility data from this international study of bacterial pathogens from community-acquired lower respiratory tract infections. Journal of Antimicrobial Chemotherapy 45, Oteo, J., Alós, J. I. & Gómez-Garcés, J. L. (2001). Antimicrobial resistance of Streptococcus pneumoniae isolates in 1999 and 2000 in Madrid, Spain: a multicentre surveillance study. Journal of Antimicrobial Chemotherapy 47, Fenoll, A., Casal, J., Giménez, M. J., Jado, I. & Aguilar, L. (2000). Short-term bactericidal activity of amoxicillin and cefotaxime against penicillin susceptible and resistant pneumococcal strains: An in vitro pharmacodynamic simulation. Journal of Chemotherapy 12, Prieto, J., Giménez, M. J., Balcabao, I. P., Gómez-Lus, M. L. & Aguilar, L. (1999). Does the degree of penicillin susceptibility of Streptococcus pneumoniae affect the bactericidal activity of coamoxiclav versus oral cephalosporins at physiological concentrations? An in vitro pharmacodynamic simulation. Journal of Chemotherapy 11, Balcabao, I. P., Aguilar, L., Martín, M., García, Y., Dal-Ré, R. & Prieto, J. (1996). Activities against Streptococcus pneumoniae of amoxicillin and cefotaxime at physiological concentrations: in vitro pharmacodynamic simulation. Antimicrobial Agents and Chemotherapy 40, Aguilar, L., Rosendo, J., Balcabao, I. P., Martin, M., Gimenez, M. J., Frias, J. et al. (1997). Pharmacodynamic effects of amoxicillin versus cefotaxime against penicillin-susceptible and penicillinresistant pneumococcal strains: a phase I study. Antimicrobial Agents and Chemotherapy 41, Soriano, F., Ponte, C., Nieto, E. & Parra, A. (1996). Correlation of in-vitro activity and pharmacokinetic parameters with in-vivo effect of amoxycillin, co-amoxiclav and cefotaxime in a murine model of pneumococcal pneumonia. Journal of Antimicrobial Chemotherapy 38, Pérez-Trallero, E., Alkorta, M., Giménez, M. J., Vicente, D. & Aguilar, L. (2001). Prediction of in-vivo efficacy by in-vitro early bactericidal activity, with oral β-lactams, in a dose-ranging immunocompetent mouse sepsis model, using strains of Streptococcus pneumoniae with decreasing susceptibilities to penicillin. Journal of Chemotherapy 13, Cenjor, C., Ponte, C., Parra, A., Nieto, V., García-Calvo, G., Giménez, M. J. et al. (1998). In vivo efficacy of amoxicillin and cefuroxime against penicillin-resistant Streptococcus pneumoniae in a gerbil model of acute otitis media. Antimicrobial Agents and Chemotherapy 42, Soriano, F., Parra, A., Cenjor, C., Nieto, E., García-Calvo, G., Giménez, M. J. et al. (2000). Role of Streptococcus pneumoniae and Haemophilus influenzae in the development of acute otitis media and otitis media with effusion in a gerbil model. Journal of Infectious Diseases 181, Yuste, J., Jado, I., Fenoll, A., Aguilar, L., Giménez, M. J. & Casal, J. (2002). β-lactam modification of the bacteraemic profile and its relationship with mortality in a pneumococcal mouse sepsis model. Journal of Antimicrobial Chemotherapy 49, Soriano, F. (1999). Lectura farmacodinámica de la sensibilidad antibiótica a Streptococcus pneumoniae. Medicina Clínica 113, Mlynarczyk, G., Mlynarczyk, A. & Jeljaszewicz, J. (2001). Epidemiological aspects of antibiotic resistance in respiratory pathogens. International Journal of Antimicrobial Agents 18, Alós, J. I., Aracil, B., Oteo, J., Torres, C., Gómez-Garcés, J. L. & the Spanish Group for the Study of Infection in the Primary Health Care Setting. (2000). High prevalence of erythromycin-resistant, clindamycin/miocamycin-susceptible (M phenotype) Streptococcus pyogenes: results of a Spanish multicentre study in Journal of Antimicrobial Chemotherapy 45, Kelly, M. A., Weber, D. J., Gilligan, P. & Cohen, M. S. (2000). Breakthrough pneumococcal bacteremia in patients being treated with azithromycin and clarithromycin. Clinical Infectious Diseases 31, Garau, J., Lonks, J. R., Gómez, L., Xercavins, M. & Medeiros, A. (2000). Failure of macrolide therapy in patients with bacteremia due to macrolide resistant Streptococcus pneumoniae. In Program and Abstracts of the Fifth International Conference on Macrolides, Azalides, Streptogramins, Ketolides and Oxazolidinones, Seville, Abstract Buxbaum, A., Straschil, U., Moser, C., Graninger, W., Georgopoulos, A. & the Austrian Bacterial Surveillance Network. (1999). Comparative susceptibility to penicillin and quinolones of 1385 Streptococcus pneumoniae isolates. Journal of Antimicrobial Chemotherapy 43, Loza, E., Morosoni, M. I., Negri, M. C., Almaraz, F., Cantón, R., Baquero, F. et al. (2000). Estudio multicéntrico nacional de la actividad in vitro de moxifloxacino frente a patógenos respiratorios. Revista Espanola de Quimioterapia 13, Rittenhouse, S., McCloskey, L., Broskey, J., Niconovich, N., Jakielaszek, C., Poupard, J. et al. (2000). In vitro antibacterial activity of gemifloxacin and comparator compounds against common respiratory pathogens. Journal of Antimicrobial Chemotherapy 45, Suppl. 1,

6 J. Prieto et al. 39. García-Garrote, F., Cercenado, E., Martín-Pedroviejo, J. & Bouza, E. (1999). Comparative in vitro activity of gemifloxacin (SB ) with other fluoroquinolones against Streptococcus pneumoniae and Haemophilus influenzae. In Program and Abstracts of the Thirty-ninth Interscience Conference on Antimicrobial Agents and Chemotherapy, San Francisco, CA, USA, Abstract 2296, p American Society for Microbiology, Washington, DC, USA. 40. Fuentes, F., Giménez, M. J., Marco, F., Alou, L., Aguilar, L. & Prieto, J. (2000). In vitro susceptibility to gemifloxacin and trovafloxacin of Streptococcus pneumoniae strains exhibiting decreased susceptibility to ciprofloxacin. European Journal of Clinical Microbiology and Infectious Diseases 19, Pérez-Trallero, E., Larruskain, J., Marimon, J. M., Ayestaran, I. & García-Arenzana, J. M. (2001). In vitro activities of 10 fluoroquinolones against clinical isolates of Streptococcus pneumoniae during 1999 and 2000 in Gipuzkoa, Spain. In Program and Abstracts of the Forty-first Interscience Conference on Antimicrobial Agents and Chemotherapy, Chicago, IL, USA, Abstract E-727, p American Society for Microbiology, Washington, DC, USA. 42. Wise, R. & Andrews, J. M. (1999). The in-vitro activity and tentative breakpoint of gemifloxacin, a new fluoroquinolone. Journal of Antimicrobial Chemotherapy 44, Balcabao, I. P., Alou, L., Aguilar, L., Gomez-Lus, M. L., Giménez, M. J. & Prieto, J. (2001). Influence of the decrease in ciprofloxacin susceptibility and the presence of human serum on the in vitro susceptibility of Streptococcus pneumoniae to five new quinolones. Journal of Antimicrobial Chemotherapy 48, Joyanes, P., Pascual, A., Giménez, M. J., García, I., Aguilar, L. & Perea, E. (2001). Differences between two new quinolones (gemifloxacin and trovafloxacin) and ciprofloxacin in their concentrationdependent killing of Streptococcus pneumoniae. Chemotherapy 47, Prieto, J., Aguilar, L., Fuentes, F., Giménez, M. J., Alou, L., Dal-Ré, R. et al. (2001). Influence of diminished susceptibility of Streptococcus pneumoniae to ciprofloxacin on the serum bactericidal activity of gemifloxacin and trovafloxacin after a single dose in healthy volunteers. International Journal of Antimicrobial Agents 18, Woodnutt, G. (2000). Pharmacodynamics to combat resistance. Journal of Antimicrobial Chemotherapy 46, Topic T1, Pérez-Trallero, E., García-Arenzana, J. M., Jiménez, J. A. & Peris, A. (1990). Therapeutic failure and selection of resistance to quinolones in a case of pneumococcal pneumonia treated with ciprofloxacin. European Journal of Clinical Microbiology and Infectious Diseases 9, Empey, P. E., Jennings, H. R., Thornton, A. C., Rapp, R. P. & Evans, M. E. (2001). Levofloxacin failure in a patient with pneumococcal pneumonia. Annals of Pharmacotherapy 35,