Antimicrobial use and antimicrobial resistance: a population perspective

Unnoticed spread of integron-carrying Enterobacteriaceae in intensive care units

BACKGROUND

Integrons are strongly associated with multidrug resistance in Enterobacteriaceae. Little is known about the natural history of integron-associated resistance in hospitals during nonoutbreak periods. The prevalence of integrons and the incidence of cross-transmission and horizontal gene transfer in Enterobacteriaceae with reduced susceptibility to cephalosporins (ERSC) were determined for 2 intensive care units (ICUs).

METHODS

Microbiological surveillance using rectal swab samples obtained 2 times per week and genotyping using amplified fragment-length polymorphism (AFLP) were used to determine colonization with and genetic relatedness of ERSC. IntI1 integrase polymerase chain reaction (PCR), conserved-segment PCR, restriction fragment-length polymorphism, and DNA sequencing were used to determine the prevalence and contents of integrons.

RESULTS

Of 457 patients, 121 patients were colonized with ERSC, and 174 isolates underwent AFLP and PCR. In 34 isolates obtained from 31 patients, 11 different integrons were identified; these integrons encoded resistance to streptomycin/spectinomycin, gentamicin/tobramycin/kanamycin, chloramphenicol, and trimethoprim. Integrons could be divided into 7 clusters of > or =2 isolates each. Compared with isolates that were negative for integrons, isolates that were positive for integrons were associated with resistance to piperacillin, cephalosporins, aminoglycosides, and quinolones. Acquisition rates of integron-carrying ERSC were 10 cases per 1000 patient-days in the first ICU and 8 cases per 1000 patient-days in the second ICU, with most cases (26 of 34) being acquired during the ICU stay. Nineteen episodes resulted from cross-transmission. In addition, 2 cases of interspecies transfer and 1 case of intraspecies transfer of integrons were recorded. Younger age was independently associated with acquisition of integron-carrying ERSC (hazard ratio, 0.953; 95% confidence interval, 0.926-0.987).

CONCLUSION

Surveillance, genotyping, and integron analysis identified previously unnoticed outbreaks of integron-carrying ERSC. Cross-transmission appeared to be the dominant route of transmission. Therefore, barrier precautions are necessary to prevent further spread.

Finding a strategy: the case for co-operative research on resistance epidemiology

Progress on rational intervention to prevent increasing antibiotic resistance has been slow. We suggest that this is because the science of resistance epidemiology has received little attention, and that a systematic, co-operative investigation of this area might yield a relevant knowledge base, analogous to the basis for effective public health intervention in infectious disease given by infection epidemiology. The steps required to progress this approach in the UK are discussed, along with a summary of what is known and speculation on what might emerge.

Impact of amoxicillin on pneumococcal colonization compared with other therapies for acute otitis media

BACKGROUND

This study compared the effects of 4 outpatient antibiotic regimens on colonization by penicillin-susceptible and -nonsusceptible pneumococci to assess their relative potential to promote colonization with Streptococcus pneumoniae with reduced susceptibility to penicillin.

METHODS

Children presenting with acute otitis media were randomized to receive amoxicillin, cefprozil, ceftriaxone or azithromycin. Nasopharyngeal specimens were collected on days 0, 3-5, 10-14 and 28-30 and assessed for the presence of S. pneumoniae. At each visit, the proportions of penicillin-susceptible and -nonsusceptible pneumococci were compared among treatment groups.

RESULTS

Among 1009 enrollees, the prevalence of colonization by S. pneumoniae at baseline was 23.5%, of which 41.1% were penicillin-nonsusceptible. Colonization by nonsusceptible pneumococci was unaltered during the observation period in all treatment groups, with no detectable differences among groups at each visit. By contrast, there was a substantial reduction in the prevalence of colonization by penicillin-susceptible organisms, most notably in subjects treated with amoxicillin. This resulted in a proportional shift toward resistant organism colonization in all groups, with this shift being significantly more pronounced among amoxicillin recipients than in the other groups at 10-12 days (P < 0.02 for each comparison with amoxicillin).

CONCLUSIONS

Treatment with amoxicillin for acute otitis media resulted in a larger shift toward nonsusceptible organism colonization among those children still colonized postexposure than did treatment with 3 comparison agents. This phenomenon raises theoretical concerns that at the population level, amoxicillin produces conditions that promote the dissemination of the nonsusceptible phenotype more readily than other outpatient antibiotics. Confirmation of these results requires further study.

Dramatic increase in prevalence of fecal carriage of extended-spectrum beta-lactamase-producing Enterobacteriaceae during nonoutbreak situations in Spain

The occurrence of extended-spectrum beta-lactamase (ESBL)-producing isolates has increased worldwide. Fecal carriage of ESBL-producing isolates has mainly been detected in nosocomial outbreaks, and few studies have evaluated fecal carriage during nonoutbreak situations and among patients in the community. We have studied the prevalence of ESBLs in 1,239 fecal samples from 849 patients (64.1% of whom were ambulatory) in 1991 and have compared the prevalence data with those obtained in 2003 for 400 fecal samples from 386 patients (75.9% of whom were ambulatory) and 108 samples from independent healthy volunteers. Samples were diluted in saline and cultured in two MacConkey agar plates supplemented with ceftazidime (1 microg/ml) and cefotaxime (1 microg/ml), respectively. Colonies were screened (by the double-disk synergy test) for ESBL production. The clonal relatedness of all ESBL-producing isolates was determined by pulsed-field gel electrophoresis with XbaI digestion; and the ESBLs of all ESBL-producing isolates were characterized by isoelectric focusing, PCR, and sequencing. The rates of fecal carriage of ESBL-producing isolates increased significantly (P < 0.001) in both hospitalized patients and outpatients, from 0.3 and 0.7%, respectively, in 1991, to 11.8 and 5.5%, respectively, in 2003. The rate of occurrence of ESBL-producing isolates among healthy volunteers was 3.7%. All ESBL-producing isolates recovered in 2003 were nonepidemic clones of Escherichia coli. ESBL characterization revealed an increasing diversity of ESBL types: TEM-4 and CTX-M-10 were the only enzymes detected in 1991, whereas TEM-4, TEM-52, SHV-12, CTX-M-9, CTX-M-10, CTX-M-14, and a CTX-M-2-like enzyme were recovered in 2003. The ESBL-producing isolates recovered from outpatients in 2003 corresponded to a CTX-M-9-type cluster (62.5%) and SHV-12 (31.2%), whereas TEM-4 was detected only in hospitalized patients. The frequencies of coresistance in isolates recovered in 2003 were as follows: sulfonamide, 75%; tetracycline, 64.3%; streptomycin, 57.1%; quinolones, 53.5%; and trimethoprim, 50%. The increased prevalence of fecal carriage of ESBL-producing isolates during nonoutbreak situations in hospitalized patients and the establishment of these isolates in the community with coresistance to non-beta-lactam antibiotics, including quinolones, represent an opportunity for these isolates to become endemic.

Effect of hospitalization and antimicrobial therapy on antimicrobial resistance of colonizing Staphylococcus epidermidis

Endogenous infections with multi-resistant S. epidermidis are among the leading causes of nosocomial infections. The effect of hospitalization and antimicrobial therapy on antimicrobial resistance of colonizing staphylococci was determined from swabs of the nose, hand, axilla and groin from 157 patients on one day. Hospitalization for >72 hours, compared with <72 hours, was associated with a higher percentage of isolates resistant to oxacillin (56% versus 19%), gentamicin (40% versus 15%), trimethoprim (36% versus 17%), clindamycin (56% versus 17%), and fusidic acid (20% versus 4%; p < 0.01 for all), but not to rifampicin (6% versus 1%) or fosfomycin (43% versus 34%, p > 0.05 for both). Concurrent antimicrobial therapy resulted in increased resistance to oxacillin (61% versus 28%), gentamicin (43% versus 20%), and clindamycin (60% versus 26%; p < 0.01 for all), but not to trimethoprim (39% versus 23%), fusidic acid (19% versus 9%), rifampicin (6% versus 3%), or fosfomycin (46% versus 38%, p > 0.05 for all). The increase in resistant isolates was not independent, since hospitalization and antimicrobial therapy were correlated (p < 0.001). After adjustment for potential risk factors such as diabetes mellitus, central venous catheters, and hemodialysis, the odds ratio for oxacillin resistance was 2.8-3.6. None of the risk factors showed statistically significant results, except for the presence of neoplastic disease, which had a significant interaction (P=0.035). The within-subgroup odds ratios for patients with and without neoplasm were 4.2 (95% CI, 2.3-5.7) and 2.1 (95% CI, 0.78-3.12), respectively. These results show that hospitalization for more than three days, with or without antimicrobial therapy, and the presence of neoplastic disease are associated with increased antimicrobial resistance in colonizing S. epidermidis.

A view on antimicrobial resistance in developing countries and responsible risk factors

Antimicrobial resistance is one of the biggest challenges facing global public health. Although antimicrobial drugs have saved many lives and eased the suffering of many millions, poverty, ignorance, poor sanitation, hunger and malnutrition, inadequate access to drugs, poor and inadequate health care systems, civil conflicts and bad governance in developing countries have tremendously limited the benefits of these drugs in controlling infectious diseases. The development of resistance in the responsible pathogens has worsened the situation often with very little resource to investigate and provide reliable susceptibility data on which rational treatments can be based as well as means to optimise the use of antimicrobial agents. The emergence of multi-drug-resistant isolates in tuberculosis, acute respiratory infections and diarrhoea, often referred to as diseases of poverty, has had its greatest toll in developing countries. The epidemic of HIV/AIDS, with over 30 million cases in developing countries, has greatly enlarged the population of immunocompromised patients. The disease has left these patients at great risk of numerous infections and even greater risks of acquiring highly resistant organisms during long periods of hospitalisation. This review discusses antimicrobial resistance in developing countries and the risk factors responsible.

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.

The magnitude of the association between fluoroquinolone use and quinolone-resistant Escherichia coli and Klebsiella pneumoniae may be lower than previously reported

Case-control analyses of resistant versus susceptible isolates have implicated fluoroquinolone exposure as a strong risk factor for fluoroquinolone-resistant isolates of Enterobacteriaceae. We suspect that such methodology may overestimate this association. A total of 84 cases with fluoroquinolone-resistant isolates and 578 cases with fluoroquinolone-susceptible isolates of Escherichia coli or Klebsiella pneumoniae were compared with 608 hospitalized controls in parallel multivariable analyses. For comparison of previous estimates, the results of 10 published case-control studies of risk for fluoroquinolone resistance in isolates of Enterobacteriaceae were pooled by using a random-effects model. Exposure to fluoroquinolones was significantly positively associated with fluoroquinolone resistance (odds ratio [OR], 3.17) and negatively associated with fluoroquinolone susceptibility (OR, 0.18). Multivariable analyses yielded similar estimates (ORs, 2.04 and 0.10, respectively). As data on antibiotic exposure were limited to inpatient prescriptions, misclassification of fluoroquinolone exposure in persons who received fluoroquinolones as outpatients may have led to an underestimation of the true effect size. Pooling the results of previously published studies in which a direct comparison of fluoroquinolone-resistant and fluoroquinolone-susceptible cases was used resulted in a markedly higher effect estimate (OR, 18.7). Had we directly compared resistant and susceptible cases, our univariate OR for the association between fluoroquinolone use and the isolation of resistant Enterobacteriaceae would have been 19.3, and the multivariate OR would have been 16.5. Fluoroquinolone use is significantly associated with the isolation of fluoroquinolone-resistant Enterobacteriaceae; however, previous studies likely exaggerated the magnitude of this association.

Role of calf-adapted Escherichia coli in maintenance of antimicrobial drug resistance in dairy calves

The prevalence of antimicrobial drug-resistant bacteria is typically highest in younger animals, and prevalence is not necessarily related to recent use of antimicrobial drugs. In dairy cattle, we hypothesize that antimicrobial drug-resistant, neonate-adapted bacteria are responsible for the observed high frequencies of resistant Escherichia coli in calves. To explore this issue, we examined the age distribution of antimicrobial drug-resistant E. coli from Holstein cattle at a local dairy and conducted an experiment to determine if low doses of oxytetracycline affected the prevalence of antimicrobial drug-resistant E. coli. Isolates resistant to tetracycline (>4 microg/ml) were more prevalent in <3-month-old calves (79%) compared with lactating cows (14%). In an experimental trial where calves received diets supplemented with or without oxytetracycline, the prevalence of tetracycline-resistant E. coli was slightly higher for the latter group (P = 0.039), indicating that drug use was not required to maintain a high prevalence of resistant E. coli. The most common resistance pattern among calf E. coli isolates included resistance to streptomycin (>12 microg/ml), sulfadiazine (>512 microg/ml), and tetracycline (>4 microg/ml) (SSuT), and this resistance pattern was most prevalent during the period when calves were on milk diets. To determine if prevalence was a function of differential fitness, we orally inoculated animals with nalidixic acid-resistant strains of SSuT E. coli and susceptible E. coli. Shedding of SSuT E. coli was significantly greater than that of susceptible strains in neonatal calves (P < 0.001), whereas there was no difference in older animals (P = 0.5). These data support the hypothesis that active selection for traits linked to the SSuT phenotype are responsible for maintaining drug-resistant E. coli in this population of dairy calves.

An International Multicenter Study of Antimicrobial Consumption and Resistance in Staphylococcus aureus Isolates from 15 Hospitals in 14 Countries

Antibiotic consumption during 1996 was measured in 15 large hospitals from 14 countries and 3000 consecutive Staphylococcus aureus samples were collected, allowing calculation of local resistance rates and typing of isolates. Antibiotic consumption data were converted to defined daily doses (DDD), and similar antibiotics were grouped if they belonged to the same therapeutic subgroup. Variations in hospital size were corrected by using DDD per 1000 bed-days. The total antibiotic consumption in the 15 hospitals varied between 296 DDD/1000 bed-days and 1108 DDD/1000 bed-days. Differences in the usage of therapeutical subgroups of antimicrobials varied significantly between hospitals. A positive correlation was found between S. aureus resistance to methicillin (MRSA) and consumption of beta-lactam combinations, between resistance to quinolones and consumption of beta-lactam combinations and carbapenems and resistance to aminoglycosides and consumption of beta-lactam combinations. The consumption of beta-lactamase-sensitive antibiotics was negatively correlated to resistance to methicillin, quinolones, and aminoglycosides. Usage of the different antimicrobial therapeutical subgroups was also correlated. Consumption of beta-lactamase-sensitive antibiotics (penicillin) was positively correlated to consumption of beta-lactamase-resistant penicillins and negatively correlated to consumption of carbapenems, quinolones, and glycopeptides, whereas consumption of cephalosporins was positively correlated to consumption of aminoglycosides, quinolones, and glycopeptides. In this study of hospitals with MRSA prevalence of between 0% and 63%, significant correlations were found between resistance and consumption of antimicrobials. These findings support the importance of antimicrobial consumption on resistance. An accompanying paper addresses the issue of antibiotic resistance and clonality of isolates.

Antibiotic selection pressure and resistance in Streptococcus pneumoniae and Streptococcus pyogenes

We correlated outpatient antibiotic use with prevalence of penicillin-nonsusceptible Streptococcus pneumoniae (PNSP), macrolide-resistant S. pneumoniae (MRSP), and macrolide-resistant S. pyogenes (MRGAS) in 20 countries. Total antibiotic use was correlated with PNSP (r = 0.75; p < 0.001), as was macrolide use with MRSP (r = 0.88; p < 0.001) and MRGAS (r = 0.71; p = 0.004). Streptococcal resistance is directly associated with antibiotic selection pressure on a national level.

Report from the European Conference on the Role of Research in Combating Antibiotic Resistance, 2003

Europe has been at the forefront of efforts to control antibiotic resistance, and this globally important health care problem has prompted numerous recommendations for action at both the national and international levels. Starting in 2002, research on antimicrobial resistance has been considered to be one of the specific objectives of the Sixth Framework Programme (FP6) within the European Union. This report summarises the plenary presentations, as well as the findings of six Working Groups covering specific areas of antibiotic resistance, given at a conference in November 2003 entitled 'The Role of Research in Combating Antibiotic Resistance', co-organised by the European Union and the European Society for Clinical Microbiology and Infectious Diseases, and held in Rome under the patronage of the Italian government.

Antimicrobial therapy and resistance in dermatologic pathogens of the elderly

Dermatologic infections are treated to improve the elderly patient's quality of life and prevent disability and death. Pharmacokinetic changes, social issues, and the risk for adverse effects and drug interactions, however, increase the challenge to achieve successful treatment of those over age 65 as compared with younger patients. This challenge grows ever larger when coupled with the increasing trends of resistance among bacteria and fungi. The clinician can effectively face this challenge through careful consideration of diagnosis, patient risk factors, antimicrobial pharmacology, and resistance patterns.

Antimicrobial prescribing for otitis externa in children

We describe the patterns of antimicrobial prescribing and trends in disease occurrence among children with otitis externa using the National Ambulatory Medical Care Survey and the National Hospital Ambulatory Medical Care Survey from 1995 to 2000. Oral antimicrobial therapy for otitis externa is more likely to contribute to antimicrobial resistance than is topical antimicrobial therapy and is rarely indicated. Thirty-nine percent of visits resulted in a prescription for topical antibiotics, and 25% of visits resulted in a prescription for oral antibiotics. Inappropriate antimicrobial prescribing for otitis externa occurs frequently.

Acquisition of 16S rRNA methylase gene in Pseudomonas aeruginosa

BACKGROUND

Bacteria develop resistance to aminoglycosides by producing aminoglycoside-modifying enzymes such as acetyltransferase, phosphorylase, and adenyltransferase. These enzymes, however, cannot confer consistent resistance to various aminoglycosides because of their substrate specificity. Notwithstanding, a Pseudomonas aeruginosa strain AR-2 showing high-level resistance (minimum inhibitory concentration >1024 mg/L) to various aminoglycosides was isolated clinically. We aimed to clone and characterise the genetic determinant of this resistance.

METHODS

We used conventional methods for DNA manipulation, susceptibility testing, and gene analyses to clone and characterise the genetic determinant of the resistance seen. PCR detection of the gene was also done on a stock of P aeruginosa strains that were isolated clinically since 1997.

FINDINGS

An aminoglycoside-resistance gene, designated rmtA, was identified in P aeruginosa AR-2. The Escherichia coli transformant and transconjugant harbouring the rmtA gene showed very high-level resistance to various aminoglycosides, including amikacin, tobramycin, isepamicin, arbekacin, kanamycin, and gentamicin. The 756-bp nucleotide rmtA gene encoded a protein, RmtA. This protein showed considerable similarity to the 16S rRNA methylases of aminoglycoside-producing actinomycetes, which protect bacterial 16S rRNA from intrinsic aminoglycosides by methylation. Incorporation of radiolabelled methyl groups into the 30S ribosome was detected in the presence of RmtA. Of 1113 clinically isolated P aeruginosa strains, nine carried the rmtA gene, as shown by PCR analyses.

INTERPRETATION

Our findings strongly suggest intergeneric lateral gene transfer of 16S rRNA methylase gene from some aminoglycoside-producing microorganisms to P aeruginosa. Further dissemination of the rmtA gene in nosocomial bacteria could be a matter of concern in the future.

Prevalence and characterization of the mechanisms of macrolide, lincosamide and streptogramin resistance in viridans group streptococci

The presence of erm genes conferring constitutive and inducible resistance, as well as that of the mefA gene conferring only constitutive resistance, was investigated using PCR in 70 erythromycin resistant (MIC>or=1 mg/l) strains of viridans group streptococci (VGS) (18 Streptococcus mitis biotype 1, 16 S. mitis biotype 2, 15 S. oralis, 12 S. salivarius and nine S. sanguis) isolated from the oropharynx of healthy Greek children. All of the 56 isolates belonging to resistance phenotype M harbored the mefA gene. All of the 14 isolates constitutively resistant to macrolides and lincosamides (phenotype CR) harbored the ermB gene. Co-presence of both genes was not observed, whereas class A erm gene (previously known as ermTR) was not detected. Our results are consistent with a possible role of VGS as a reservoir of resistance genes now prevalent in pathogenic species of streptococci.

Optimizing Antibiotic Use in Hospitals: The Role of Population-Based Antibiotic Surveillance in Limiting Antibiotic Resistance

To minimize antibiotic resistance, pharmacists increasingly are becoming involved in antibiotic surveillance, formulation of antibiotic use policies, and day-to-day control of problematic antibiotic use. Population-based antibiotic surveillance has become common with the proliferation of electronic databases. The most widely applied measure of antibiotic consumption is the defined daily dose/1000 patient days. Most studies correlating antibiotic consumption with resistance have focused on antibiogram-related end points; antibiogram data generally reflect institutional nosocomial infection patterns. Most study designs have been derived from traditional epidemiology such as case-control with regression modeling or simple linear regression; however, these methods have limitations. Several experimental designs show promise. Many historical-control studies, including a multicentered study, suggest that population-based antibiotic surveillance and policy intervention can decrease antibiotic resistance in hospitals. Further research on the relationships among antibiotic surveillance, structured antibiotic policy interventions, and other microbiologic, patient-oriented, and economic end points is needed.

Antibiotics and resistance in dermatology: focus on treating the elderly

Although elderly patients are at an increased risk for skin infections, the chief culprits are no different than in younger patients. However, many of these organisms have developed resistance to antibiotics. Resistance increases the morbidity, mortality, and cost of treating infections. The mechanisms by which resistance occurs include efflux of antibiotic through a cellular pump, inactivation of the antibiotic by enzymes, or changes in the target affinity for the antibiotic. For dermatologic conditions in the elderly, documented resistance is seen in staphylococci, streptococci, and enterococci. Clinicians can reduce the development of resistance by following infection control and antibiotic use guidelines. To optimize the antibiotic effect and minimize adverse effects in the elderly, pharmacokinetic changes seen with aging should guide antibiotic choice and dosing.

Antimicrobial Use and Resistance

As antimicrobial use continues to rise, we are experiencing a concomitant rise in the prevalence of antimicrobial resistance. The precise relationship between use and resistance, however, has been challenging to define. Although the selection pressure exerted by antibiotic therapy appears to be the primary force promoting resistance, it is clear that the pathway to resistance is different for various organisms and antimicrobial agents. By understanding the mechanisms by which resistance emerges and spreads, it should be possible to design intervention strategies to slow or halt the process. This review summarizes some of our current understandings about the development and transmission of antibiotic-resistant bacteria, some of the control measures designed to interrupt the process, and how mathematical modeling can help us to better understand these complex pathways.

Microbial and viral drug resistance mechanisms

Microorganisms and viruses have developed numerous resistance mechanisms that enable them to evade the effect of antimicrobials and antivirals. As a result, many have become resistant to almost every available means of treatment. This problem, although not new, is becoming increasingly acute and it is now clear that a fundamental understanding of the mechanisms that microbes and viruses deploy in the development of resistance is essential if we are to gain new insights into ways to combat this problem.

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.