Patterns of macrolide resistance determinants among community-acquired Streptococcus pneumoniae isolates over a 5-year period of decreased macrolide susceptibility rates

Distribution of Medically Relevant Antibiotic Resistance Genes and Mobile Genetic Elements in Soils of Temperate Forests and Grasslands Varying in Land Use

Antibiotic-resistant pathogens claim the lives of thousands of people each year and are currently considered as one of the most serious threats to public health. Apart from clinical environments, soil ecosystems also represent a major source of antibiotic resistance determinants, which can potentially disseminate across distinct microbial habitats and be acquired by human pathogens via horizontal gene transfer. Therefore, it is of global importance to retrieve comprehensive information on environmental factors, contributing to an accumulation of antibiotic resistance genes and mobile genetic elements in these ecosystems. Here, medically relevant antibiotic resistance genes, class 1 integrons and IncP-1 plasmids were quantified via real time quantitative PCR in soils derived from temperate grasslands and forests, varying in land use over a large spatial scale. The generated dataset allowed an analysis, decoupled from regional influences, and enabled the identification of land use practices and soil characteristics elevating the abundance of antibiotic resistance genes and mobile genetic elements. In grassland soils, the abundance of the macrolide resistance gene mefA as well as the sulfonamide resistance gene sul2 was positively correlated with organic fertilization and the abundance of aac(6')-lb, conferring resistance to different aminoglycosides, increased with mowing frequency. With respect to forest soils, the beta-lactam resistance gene blaIMP-12 was significantly correlated with fungal diversity which might be due to the fact that different fungal species can produce beta-lactams. Furthermore, except blaIMP-5 and blaIMP-12, the analyzed antibiotic resistance genes as well as IncP-1 plasmids and class-1 integrons were detected less frequently in forest soils than in soils derived from grassland that are commonly in closer proximity to human activities.

High prevalence of group B streptococcus ST17 hypervirulent clone among non-pregnant patients from a Hungarian venereology clinic

Background

Although Streptococcus agalactiae is the leading causative agent of neonatal sepsis and meningitis, recently it is increasingly isolated from non-pregnant adults. The relation between its presence in the genitourinary tract and manifested clinical symptoms of STD patients remains an open question. In this study, a complex epidemiological investigation of GBS isolates from a venerology clinic was performed.

Methods

Ninety-six GBS isolates were serotyped and their genetic relatedness determined by PFGE. MLST was also performed for a subset of 20 isolates. The antibiotic susceptibility was tested with agar dilution. Surface proteins and the ST-17 hypervirulent clone was detected by PCR.

Results

The serotype prevalence was the following: V (29.2%), III (27.1%), Ia (22.9%), IV (10.4%), II (5.2%) and Ib (4.2%). A strong association was demonstrated between surface protein genes and serotypes. All isolates were fully susceptible to penicillin, but erythromycin and clindamycin resistance was high (41.7 and 35.4%, respectively), and 8 phenotypically macrolide sensitive isolates carried the ermB gene.

21.9% of all strains belonged to the hypervirulent ST17 clone, most being of serotype III and all were rib +. We found a few serotype IV isolates belonging to several STs and one serotype V/ST110 strain, containing a 44-bp deletion in the atr allele.

Conclusions

The presence of silent ermB genes is of worry, as their expression upon macrolide exposure could lead to unforeseen therapeutic failure, while clindamycin is used for intrapartum antibiotic prophylaxis, in case of penicillin allergy. The other alarming result is the high prevalence of ST17 among these strains from STD patients, who could be sources of further infections.

This is the first report from Hungary providing both serotyping and genotyping data of GBS isolates. These results could be helpful for vaccine production as the major vaccine candidates are capsular antigens or surface proteins.

Patterns of Macrolide Resistance Determinants among S. pyogenes and S. pneumoniae Isolates in Saudi Arabia

In the study we characterized the macrolide sensitivity of recent clinical isolates of Streptococcus pyogenes and S. pneumoniae collected from major Saudi Arabian hospitals. Susceptibility testing was performed using standard National Committee for Clinical Laboratory Standards methodology on 335 S. pyogenes and 350 S. pneumoniae isolates. Macrolide resistance mechanism phenotypes were identified using double-disk diffusion. All S. pyogenes were penicillin sensitive, while 6.3% were macrolide resistant, the main mechanism of which was of M phenotype (96%). Approximately 51% of S. pneumoniae were penicillin non-susceptible. Macrolide resistance in S. pneumoniae accounted for 18.8%, the majority of which were M phenotype (91%). Low-level resistance mediated by mef-bearing strains predominated. Newer macrolides, including azithromycin, are still considered drugs of choice for empirical treatment of respiratory infection in such circumstances.

Epidemiological analysis of pneumococcal serotype 19A in healthy children following PCV7 vaccination

After the introduction of conjugate vaccines, a strong rearrangement of pneumococcal serotypes was observed globally. Probably most concerning was the emergence of serotype 19A, which has not only high invasive disease potential, but also high antibiotic resistance. In the current study we focused on the increased prevalence of serotype 19A after the PCV vaccination rate became widely used in Hungary. A total of 2262 children aged 3-6 years were screened for pneumococcus carriage using nasal swabs. Children were divided into two groups according to the vaccination rates, low level (group 1) vs. high level (group 2). While the carriage rate did not change over time (average 32·9%), the serotype distribution differed greatly in the two groups. The prevalence of serotype 19A increased >eightfold. Almost all 19A isolates had high-level macrolide resistance and elevated penicillin minimum inhibitory concentrations. Genotyping methods revealed that these new 19A isolates are different from the previously frequent Hungary19A-6 PMEN clone. Both the carriage rate and the overall penicillin and macrolide resistance remained stable over time, but while several serotypes were represented in group 1, serotype 19A alone was clearly dominant in group 2.

Radical serotype rearrangement of carried pneumococci in the first 3 years after intensive vaccination started in Hungary

Streptococcus pneumoniae is responsible for a significant amount of morbidity and mortality worldwide. Healthy carriers, mainly young children, are the most important sources of infections. In the current study, we aimed to determine the changes that have occurred since the introduction of PCV-7 in Hungary. Nasal specimens were collected from 1,022 healthy children aged 3-6 years attending day-care centres. After thorough identification, pneumococcal isolates were serotyped, and their antibiotic sensitivity was determined. The carriage rate was found to be 34.9%. A huge serotype rearrangement was detected compared to earlier results, with the previously leading serotype 14 having completely disappeared. Serotypes 11A, 35F, 19A, 6B, 15B, 3 and 38 were most prevalent, and 29 different types were identified in total. The PCV-7 types were responsible for 16.5% of all serotypes, and 36.0% are not covered by any pneumococcal vaccines. The isolates were sensitive to most tested antibiotics, except erythromycin (resistance was 21.6%). Only one penicillin-resistant strain was found. The newly and rapidly emerging non-vaccine serotypes are much more sensitive, except serotype 19A.

CONCLUSION

Due to PCV vaccination, a complete serotype arrangement occurred also in Hungary. The old "paediatric" serotypes were replaced by serotypes 11A, 35F, 19A, 6B, 15B, 3 and 38.

High prevalence of fluoroquinolone-resistant group B streptococci among clinical isolates in China and predominance of sequence type 19 with serotype III

A total of 146 group B streptococcus isolates from 8 cities across China belonged to four serotypes. Serotype Ia was more common in children. A high prevalence of resistance was observed for levofloxacin (37.7%), erythromycin (71.2%), clindamycin, (53.4%), and tetracycline (81.5%). The levofloxacin and clindamycin resistances among the 4 serotypes differed significantly. Eighty percent of fluoroquinolone-resistant isolates belonged to the sequence type 19 (ST19)/serotype III clone, with GyrA-ParC-ParE triple substitutions. This clone carried the erm(B), mef(E), and tet(M) genes.

Macrolide resistance determinants among Streptococcus pneumoniae isolates from carriers in Central Greece

Background

We sought to characterize the temporal trends in nasopharyngeal carriage of macrolide-resistant pneumococci during a period with increased heptavalent pneumococcal conjugate vaccine (PCV7) coverage in Central Greece.

Methods

Streptococcus pneumoniae isolates were recovered from 2649 nasopharyngeal samples obtained from day-care center attendees in Central Greece during 2005–2009. A phenotypic and genotypic analysis of the isolates was performed, including the identification of macrolide resistance genes mef(A), subclasses mef(A) and mef(E), as well as erm(B).

Results

Of the 1105 typeable S. pneumoniae isolates, 265 (24%) were macrolide-resistant; 22% in 2005, 33.3% in 2006, 23.7% in 2007, and 20.5% in 2009 (P=0.398). Among these macrolide-resistant pneumococci, 28.5% possessed erm(B), 24.3% erm(B)+mef(E), 41.8% mef(E), and 5.3% mef(A). A mef gene as the sole resistance determinant was carried by 31% of macrolide-resistant isolates belonging to PCV7 serotypes and 75.8% of the non-PCV7 serotypes. Across the 4 annual surveillances, pneumococci carrying mef(A) gradually disappeared, whereas serotype 19F isolates carrying both erm(B) and mef(E) persisted without significant yearly fluctuations. Among isolates belonging to non-PCV7 serotypes, macrolide-resistance was observed in those of serotypes 6A, 19A, 10A, 15A, 15B/C, 35F, 35A, and 24F. In 2009, ie 5 years after the introduction of PCV7 in our country, 59% of macrolide-resistant pneumococci belonged to non-PCV7 serotypes.

Conclusions

Across the study period, the annual frequency of macrolide-resistant isolates did not change significantly, but in 2009 a marked shift to non-PCV7 serotypes occurred. Overall, more than half of the macrolide-resistant isolates possessed erm(B) either alone or in combination with mef(E). erm(B) dominated among isolates belonging to PCV7 serotypes, but not among those of non-PCV7 serotypes.

Emergence of erythromycin- and clindamycin-resistant Streptococcus pyogenes emm 90 strains in Hawaii

We identified 12 erythromycin- and clindamycin-resistant emm 90 group A streptococcus (GAS) isolates during a retrospective invasive disease survey in Hawaii. A comparison with 20 type-matched isolates showed all resistant isolates to be emm 90.4b with the constitutive or inducible macrolide-lincosamide-streptogramin B resistance phenotype (cMLS(B) or iMLS(B)). All isolates had the same pulsed-field gel electrophoresis (PFGE) pattern, suggesting clonal spread.

Phenotypic and genotypic characterization of macrolide resistant Streptococcus pneumoniae in Tunisia

One hundred of non duplicate Streptococcus pneumoniae resistant to erythromycin collected from three teaching hospitals in Tunisia from January 1998 to December 2004 were investigated to evaluate determine their resistance level to different macrolides and the mechanisms involved. Most erythromycin resistant S. pneumoniae were isolated from respiratory tract (34%). Eighty-three percent showed constitutive MLS(B) phenotype with high MICs of macrolides and lincosamides (MIC90 >256 microg/ml), 12% M phenotype with moderately increased MICs of macrolides (MIC90: 12 microg/ml) and low MICs of lincosamides (MIC90=0.75 microg/ml) and 5% inducible MLS(B) with high MICs of macrolides (MIC90 >256 microg/ml) and moderately increased MICs of lincosamides (MIC90=8 microg/ml). All strains were susceptible to quinupristun-dafopristin association and linezolid (MIC90=1 microg/ml). Strains belonging to MLS(B) phenotype were PCR positive for the erm B gene (88%). Twelve percent categorized as M phenotype carried the mef A gene. The rates of associated resistance were 68% to penicillin G, 53% to tetracyclines, 61% to cotrimoxazole, 21% to chloramphenicol and 13% to ciprofloxacin. MLS(B) constitutive phenotype conferring cross resistance to macrolides, lincosamides and streptogramins B with high level of resistance was the most prevalent. Thus, quinupristin-dalfopristin association and linezolid remain the most active molecules.

Treatment of community-acquired pneumonia, with special emphasis on gemifloxacin

Community-acquired pneumonia (CAP) is the cause of substantial morbidity, mortality, and resource utilization worldwide. When choosing an antimicrobial, effective treatment depends on proper patient evaluation and the identification of numerous risk factors, such as recent antibiotic exposure or the presence of comorbidity. Patients without any risk factor should be treated effectively with a narrow spectrum beta-lactam agent, like amoxicillin, or a macrolide. If a risk factor is present, agents with a broader spectrum of activity should be selected for the empirical therapy. The newer-generation quinolones are suitable agents with their excellent in vitro activity and pharmacodynamic-pharmacokinetic properties. They are not only active against susceptible CAP pathogens, but also against the resistant strains. Among the quinolones, gemifloxacin has the best in vitro activity. Its improved bioavailability, pharmacokinetic-pharmacodynamic properties, and safety profile make this agent an excellent option for the treatment of CAP.

The use of macrolides in treatment of upper respiratory tract infections

Antimicrobial resistance is a growing problem among upper respiratory tract pathogens. Resistance to beta-lactam drugs among Streptococcus pneumoniae, Haemophilus influenzae, and Streptococcus pyogenes is increasing. As safe and well-tolerated antibiotics, macrolides play a key role in the treatment of community-acquired upper respiratory tract infections (RTIs). Their broad spectrum of activity against gram-positive cocci, such as S. pneumoniae and S. pyogenes, atypical pathogens, H. influenzae (azithromycin and clarithromycin), and Moraxella catarrhalis, has led to the widespread use of macrolides for empiric treatment of upper RTIs and as alternatives for patients allergic to beta-lactams. Macrolide resistance is increasing among pneumococci and recently among S. pyogenes, and is associated with increasing use of the newer macrolides, such as azithromycin. Ribosomal target modification mediated by erm(A) and erm(B) genes and active efflux due to mef(A) and mef(E) are the principal mechanisms of resistance in both S. pneumoniae and S. pyogenes. Recently, ribosomal protein and RNA mutations have been found to be responsible for acquired resistance to macrolides in S. pneumoniae, S. pyogenes, and H. influenzae. Although macrolides are only weakly active against macrolide-resistant streptococci species, producing an efflux pump (mef), and are inactive against pathogens with ribosomal target modification (erm), treatment failures are uncommon. Therefore, macrolide therapy, for now, remains a good alternative for treatment of upper RTIs; however, continuous monitoring of the local resistance patterns is essential.

Rise of Streptococcus pneumoniae isolates containing both erm(B) and mef(E) genes from an adult tertiary care community hospital system

The emergence of macrolide- and lincosamide-resistant Streptococcus pneumoniae is a worldwide concern. Of particular interest is the increasing prevalence of erythromycin and clindamycin-resistant isolates containing both erm(B) and mef genes. This study determined the prevalence of erythromycin and clindamycin resistance in 596 clinical S. pneumoniae isolates from 2 adult tertiary care hospitals over a 4-year period (2001-2004). Erythromycin resistance increased from 24% to 34%, but S. pneumoniae isolates resistant to clindamycin as well as to erythromycin increased from 3% in 2001 to 15.5% in 2004 (5-fold increase). Among erythromycin-resistant isolates, those also resistant to clindamycin (MLS(B) phenotype) increased 3-fold (12.8-45%). Of forty-one erythromycin/clindamycin-resistant S. pneumoniae isolates tested, 29 (71%) contained both erm(B) and mef(E) genes. Pulsed-field gel electrophoresis performed on 28 erm(B) + mef(E) positive isolates identified 2 predominant and possibly related clones, which made up 64% of the isolates.

Therapeutic options for pneumococcal pneumonia in Turkey

BACKGROUND

Community-acquired pneumonia (CAP) is a leading cause of morbidity and mortality worldwide. Streptococcus pneumoniae continues to be the most important causative agent in CAP.

OBJECTIVE

This article reviews options for the empiric treatment of pneumococcal pneumonia in Turkey based on local epidemiologic data.

METHODS

This was a retrospective review of studies evaluating antimicrobial susceptibility patterns among clinical isolates of S pneumoniae in Turkey from 2000 onward. Relevant studies were identified through literature searches of both Turkish (Ulakbim and Pleksus) and international (MEDLINE) databases using the search terms S pneumoniae and Turkey. Only antibiotics likely to be used in pneumococcal pneumonia were evaluated. The minimum concentration required to inhibit 90% of isolates (MIC(90)) for each antibiotic was obtained by averaging all reported values to arrive at a single value for the entire country.

RESULTS

The MIC(90) for penicillin was 1 g/mL; among all isolates of S pneumoniae, 6.4% were penicillin resistant and 30.9% showed intermediate susceptibility. The MIC(90)s and overall rates of resistance (combined intermediate susceptibility and resistance) for the other antibiotics studied were as follows: cefaclor, 4 microg/mL (26.3%); cefuroxime, 2 microg/mL (15.4%); ceftriaxone, 0.25 microg/mL (0.75%); imipenem, 0.06 microg/mL (0%); erythromycin, 2 microg/mL (13.9%); clarithromycin, 2 microg/mL (13.7%); azithromycin, 2 microg/mL (13.8%); telithromycin, 0.06 microg/mL (no published breakpoints); trimethoprim-sulfamethoxazole, 4 microg/mL (63.8%); tetracycline, 16 microg/mL (24.6%); ciprofloxacin, 2 microg/mL (no published breakpoints); ofloxacin, 2 microg/mL (4%); levofloxacin, 1 microg/mL (0%); gemifloxacin, 0.06 microg/mL (no published breakpoints); and moxifloxacin, 0.06 microg/mL (0%). Penicillin G, at standard parenteral doses, has been shown to achieve concentrations above the MIC for >40% to 100% of the dosing interval, depending on the MIC of the isolate. Based on pharmacodynamic studies, the MIC(90) for penicillin in Turkey should easily be exceeded with the use of penicillin G 3 mU QID. In vitro, susceptibility is generally greater to amoxicillin than to penicillin, with average amoxicillin MIC values approximately 1 dilution lower than those for penicillin. Amoxicillin's better pharmacodynamic/pharmacokinetic properties relative to penicillin make it a reasonable option for the treatment of CAP. In pharmacodynamic studies, amoxicillin 1 g TID achieved and maintained serum concentrations of 2 to 4 microg/mL for at least 40% of the dosing interval. A new formulation of amoxicillin/clavulanate given 2000/125 mg BID is expected to eradicate isolates of S pneumoniae at an amoxicillin MIC < or = 4 microg/mL.

CONCLUSIONS

Based on data from Turkish surveillance studies performed from 2000 onward, high-dose parenteral penicillin G and parenteral/oral amoxicillin may be initial choices for the empiric treatment of uncomplicated pneumococcal pneumonia in Turkey. If these agents cannot be used for any reason, other options include parenteral cefuroxime, ceftriaxone, cefotaxime, newer quinolones, macrolides, and telithromycin. Due to elevated rates of resistance in Turkey, trimethoprim-sulfamethoxazole and tetracyclines are not recommended for empiric use in these infections.

Prevalence and molecular genetics of macrolide resistance among Streptococcus pneumoniae isolates collected in Finland in 2002

The prevalence and mechanisms of macrolide resistance among 1,007 clinical pneumococcal isolates collected in Finland were investigated. Of these, 217 (21.5%) were resistant to erythromycin and 11% to clindamycin. Among the erythromycin-resistant isolates, mef(E) was present in 95 isolates (44%), mef(A) was present in 12 isolates (6%), and erm(B) was present in 90 isolates (41%). A double mechanism, mef(E) and erm(B), was detected in five isolates (2%). Ribosomal mutation was detected in 14 (6%) macrolide-resistant isolates in which no other determinant was found. Based on the telithromycin MICs, two groups of isolates were formed: 83.3% of the isolates belonged to a major group for which the telithromycin MIC range was < or =0.008 to 0.063 microg/ml, and 16.7% belonged to a minor group for which the telithromycin MIC range was 0.125 to 8 microg/ml. All except three isolates in the minor population carried a macrolide resistance gene.

Molecular characterisation of Hungarian macrolide-resistant Streptococcus pneumoniae isolates, including three highly resistant strains with the mef gene

The macrolide resistance of 304 Hungarian Streptococcus pneumoniae isolates was investigated. Antibiotic sensitivity testing was performed in air and in 5% CO(2). More erythromycin resistance was noted when growth was in CO(2). A resistance determinant was found in almost all isolates: erm(B) gene (87.4%), mef genes (9.2%) and one strain with the erm(TR) gene. This indicates that screening for carriage of resistance determinants should always be done in the presence of 5% CO(2). We found three isolates with mef(E), which were highly resistant to erythromycin. These contained multiple and some novel, ribosomal mutations. The most prevalent serogroups were 6, 19 and 14. Based on the PFGE pattern, we found identity between the Hungarian isolates and two PMEN clones.

Antibiotic susceptibility and serotype distribution in Streptococcus pneumoniae circulating in Italy: results of the SEMPRE surveillance study (2000-2002)

During 2000-2002, 20 clinical microbiology centres collected 1623 Streptococcus pneumoniae isolates. Susceptibility to penicillin, amoxicillin, amoxicillin/clavulanic acid, cefaclor, cefuroxime, cefotaxime, clarithromycin, ciprofloxacin, levofloxacin, rifampicin and teicoplanin was determined locally by the Etest and/or by the microdilution method by three co-ordinating centres. Total resistance to penicillin increased from 15.2% to 16.1% and macrolide resistance increased from 37.9% to 43.7%. Overall, the most effective drugs (>99% susceptible strains) were amoxicillin, amoxicillin/clavulanic acid, levofloxacin and rifampicin. The most frequent serotypes were: 23F (15.8%), 3 (10.8%) 14 (9.1%), 19F (9.1%), 6B (7.2%), 19A (6.9%) and 6A (4.8%). In conclusion, penicillin and macrolide resistance is increasing in Italy, whilst fluoroquinolone currently remains active. The most common serotypes circulating are included in the heptavalent conjugate vaccine, with the exception of type 3.

The relationship between serotypes and PFGE genotypes in isolates of Streptococcus pneumoniae from Hungary

The relatedness of 112 penicillin-non-susceptible isolates of Streptococcus pneumoniae from Hungary was determined by pulsed-field gel electrophoresis (PFGE), serotyping and antibiotic susceptibility tests. The differences in PFGE patterns closely mirrored the changes in resistance. Some genotypes comprised multiple serotypes, and the genetic diversity among certain serotypes was considerable. Generally, serotyping alone was insufficient for epidemiological mapping of pneumococcal isolates. There was considerable serotype diversity, but the five most frequent international serotypes (6, 9, 14, 23, 19) were the most prevalent. In addition, the presence of some well-defined resistant international pneumococcal clones in the Hungarian population was identified.

Mechanisms of macrolide resistance in clinical pneumococcal isolates in a university hospital, Ankara, Turkey

Macrolide resistance in Streptococcus pneumoniae is usually caused by the presence of the erm(B) or mef(A) resistance determinants. The aim of the present study was to identify the predominant macrolide resistance mechanisms among erythromycin-resistant S. pneumoniae isolated in a university hospital, Ankara, Turkey. A total of 669 S. pneumoniae strains were isolated from clinical specimens of patients admitted to the hospital between 1994--2002. The minimum inhibitory concentrations (MICs) of penicillin G, erythromycin A and clindamycin were determined by the agar dilution method according to NCCLS guidelines. Ninety-one (13.6%) isolates were resistant to erythromycin. Erythromycin-resistant isolates were examined for their macrolide resistance phenotypes by a triple disc diffusion assay. It assigned 57 (62.6%) of the 91 erythromycin-resistant pneumococci to cMLS(B) phenotype, 19 (20.9%) to iMLS(B) phenotype and 15 (16.5%) to M phenotype. All erythromycin-resistant isolates were analyzed by PCR for the presence of erm(B) and mef(A) determinants. The isolates were characterized for the underlying resistance genotype, with 83.5% having erm(B), 16.5% having the mef(A) genotypes. This study provides further evidence of the dissemination of macrolide-resistant mutants in pneumococci as the use of new, long-acting macrolides increases. This is the first article about MLS(B) resistance phenotypes and genotypes of S. pneumoniae from Turkey and it emphasizes the need for future epidemiological monitoring of macrolide-resistant pneumococci.

Erythromycin-resistant group a streptococcal isolates collected between 2000 and 2005 in Oahu, Hawaii, and their emm types

We examined erythromycin and clindamycin susceptibilities with Etest methodology among 546 group A streptococcal isolates collected in Hawaii between February 2000 and November 2004. Erythromycin resistance was low (3.1%). No isolate was clindamycin resistant. The prevalence of erythromycin resistance in group A streptococci remains low in Hawaii.

The use of macrolides in treatment of upper respiratory tract infections

Antimicrobial resistance is a growing problem among upper respiratory tract pathogens. Resistance to beta-lactam drugs among Streptococcus pneumoniae, Haemophilus influenzae, and Streptococcus pyogenes is increasing. As safe and well-tolerated antibiotics, macrolides play a key role in the treatment of community-acquired upper respiratory tract infections (RTIs). Their broad spectrum of activity against gram-positive cocci, such as S. pneumoniae and S. pyogenes, atypical pathogens, H. influenzae (azithromycin and clarithromycin), and Moraxella catarrhalis, has led to the widespread use of macrolides for empiric treatment of upper RTIs and as alternatives for patients allergic to b-lactams. Macrolide resistance is increasing among pneumococci and recently among S. pyogenes, and is associated with increasing use of the newer macrolides, such as azithromycin. Ribosomal target modification mediated by erm(A) and erm(B) genes and active efflux due to mef(A) and mef(E) are the principal mechanisms of resistance in S. pneumoniae and S. pyogenes. Recently, ribosomal protein and RNA mutations have been found responsible for acquired resistance to macrolides in S. pneumoniae, S. pyogenes, and H. influenzae. Although macrolides are only weakly active against macrolide-resistant streptococci species producing an efflux pump (mef) and are inactive against pathogens with ribosomal target modification (erm), treatment failures are uncommon. Therefore, macrolide therapy, for now, remains a good alternative for treatment of upper RTIs; however, continuous monitoring of the local resistance patterns is essential.

Molecular epidemiology and prevalence of macrolide efflux genes mef(A) and mef(E) in Streptococcus pneumoniae obtained in Canada from 1997 to 2002

One hundred forty M phenotype Streptococcus pneumoniae isolates were evaluated by PCR-restriction fragment length polymorphism, serotyping, and pulsed-field gel electrophoresis. Molecular genotyping revealed that the predominant macrolide resistance mechanism in S. pneumoniae in Canada is mef(E) and resistance dissemination is due to both spread of the genetic element MEGA as well as clonal dissemination of penicillin- and/or macrolide-resistant strains.

Clonal dissemination of macrolide-resistant and penicillin-susceptible serotype 3 and penicillin-resistant Taiwan 19F-14 and 23F-15 Streptococcus pneumoniae isolates in Japan: a pilot surveillance study

Large-scale surveillance studies using molecular techniques such as pulsed-field gel electrophoresis (PFGE) have revealed that the spread of antibiotic-resistant pneumococci is due to clonal spread. However, in Japan, surveillance studies using such molecular techniques have never been done. Therefore, we conducted a pilot surveillance study to elucidate the present situation in Japan. Among the 145 isolates examined, the most prevalent serotype was type 19F (20%), for which most isolates were not susceptible to penicillin (86.2%) but were positive for the mef(A)/mef(E) gene (89.7%). The secondmost prevalent was serotype 3 (16.6%), for which most isolates were susceptible to penicillin (87.5%) and positive for the erm(B) gene (91.7%). PFGE analysis showed that both serotypes consisted mainly of clonally identical or related isolates and, in particular, 38% of the type 19F isolates were indistinguishable from or closely related to the Taiwan 19F-14 clone. In addition, some of the Japanese type 23F isolates with the erm(B) gene were indistinguishable from or related to the Taiwan 23F-15 clone as analyzed by PFGE. Based on the results of our pilot study performed in a single institution, it is likely that international antibiotic-resistant clones have already spread in Japan; therefore, a nationwide surveillance study should be urgently conducted.

Phenotypic and genotypic characterization of macrolide resistant Streptococcus pneumoniae recovered from adult patients with community-acquired pneumonia in an Argentinian teaching hospital

Streptococcus pneumoniae isolates (n = 262) were recovered from adult patients with community-acquired pneumonia. Erythromycin-resistance levels increased from 9% (1997-1998) to 16% (2000-2002). Sampling for resistance mechanisms prevalent within 19 erythromycin-resistant S. pneumoniae showed mef(E) in 13/19 isolates while 4/19 carried the erm(B) gene (3/19 cMLS(B) and 1/19 iMLS(B) phenotype). MIC ranges for erythromycin and clindamycin were 0.5-16 mg/l and <0.008-0.063 mg/l for the M phenotype, 128-512 mg/l and 128-256 mg/l for the cMLS(B) phenotype, and 4 and <0.008 mg/l for the iMLS(B) phenotype. This is the first report studying the prevalence of macrolide resistance determinants in S. pneumoniae in our country.

Comparative in vitro activity of antiribosomal agents on penicillin-susceptible and -resistant Streptococcus pneumoniae in relation to their resistance genotypes

A collection of 326 strains of Streptococcus pneumoniae isolated from blood, cerebrospinal fluid, bronchoalveolar lavage, transtracheal aspirate or sputum from January 1996-June 2002 were included in this study. The activity of clarithromycin, clindamycin, telithromycin, linezolid and quinupristin/dalfopristin against penicillin G and erythromycin A susceptible and resistant pneumococci were determined; the erythromycin A resistance phenotypes and genotypes were identified and susceptibilities of these agents were assessed according to the resistance genotypes. MICs were determined for all strains of pneumococci using an agar dilution method. MLS(B) resistance phenotypes were determined by the double disk (erythromycin A and clindamycin) diffusion method. Genetic determinants for macrolide resistance were identified by PCR using primers specific for erm(B) and mef(A). Erythromycin A resistance was detected in 13.8% of the strains. MLS(B) resistance phenotype was observed in 82% of these (60% being cMLS(B) and 40% being iMLS(B)), and M type resistance in about 18%. All the MLS(B) phenotype strains except four, revealed the presence of erm(B) gene and all except one M phenotype strains revealed the mef(A) gene. Of the erythromycin A resistant pneumococci about 49% were also resistant to clindamycin. No strains were resistant to telithromycin, quinupristin/dalfopristin and linezolid. Telithromycin had the lowest MIC values for both erythromycin A resistant and susceptible strains of all the antiribosomal agents tested. The most prevalent mechanism of macrolide resistance was mediated by the erm(B) gene leading to the expression of MLS(B) phenotype. Telithromycin was the most active antiribosomal agent, regardless of the macrolide resistance genotype of the pneumococci tested.

Trachoma: new assault on an ancient disease

Trachoma is the leading infectious cause of blindness worldwide. The World Health Organization (WHO) estimated that approximately 5.9 million persons are blind or have severe vision-loss as a result of trachoma, and another 10 million are at high risk. Trachoma preferentially affects the most deprived communities, and within these communities, women and children bear the brunt of the burden. In recent years, there has been a renewed focus on research and heightened enthusiasm for strengthening trachoma control programs in afflicted countries. WHO has convened an alliance of member countries, non-governmental organizations, and other partners for the Global Elimination of Blinding Trachoma by the year 2020, and endorsed the multi-faceted SAFE strategy for trachoma control. SAFE-Surgery, Antibiotics, Face-washing, and Environmental improvement-has incorporated sound research on elements likely to reduce trachoma, and trachomatis blindness, in endemic communities. This review summarizes current knowledge about trachoma and its causative agent, Chlamydia trachomatis, the epidemiology and risk factors for trachoma as a prelude to reviewing the SAFE strategy. While ongoing research to support the knowledge base for SAFE must continue to be a priority, the full implementation of SAFE is the best hope for countries to reduce the global burden of blindness from this preventable cause.

The epidemiology of antibiotic resistance in Streptococcus pneumoniae and the clinical relevance of resistance to cephalosporins, macrolides and quinolones

Invasive non-meningeal pneumococcal infections remain a major cause of morbidity and mortality worldwide. The factors affecting the epidemiology and mortality of pneumococcal infections are discussed. The increase and spread of resistance to antimicrobial agents among pneumococci is a cause of concern to the clinician. There are links between the usage of antibacterial agents and the development of resistance. Resistance to penicillin and other beta-lactams has become widespread but this does not appear to have decreased the efficacy of some of these agents against non-meningeal infections. There is evidence that the good pharmacokinetic and pharmacodynamic features of the third generation cephalosporins (cefotaxime and ceftriaxone) contribute to their efficacy in vivo. New breakpoints for cefotaxime and ceftriaxone against non-meningeal pneumococcal isolates were proposed by the National Committee for Clinical Laboratory Standard (NCCLS, US), based on the clinical evidence of the efficacy of these drugs. In contrast there is increasing evidence that resistance to macrolides can lead to a poor clinical response. Fluoroquinolones have been widely used to treat respiratory tract infections among others, and pneumococcal resistance to these agents in vitro, although currently low, is increasing. There are reports that resistance to fluoroquinolones can develop during treatment and may be reflected in a lack of clinical response. Several clinical and epidemiological variables (e.g. prior antibiotic use) can be useful to identify patients at risk from infections with antibiotic-resistant pneumococci. These patients would be those who would benefit the most from a pneumococcal vaccination programme.

Antimicrobial susceptibility and macrolide resistance inducibility of Streptococcus pneumoniae carrying erm(A), erm(B), or mef(A)

Erythromycin-resistant Streptococcus pneumoniae isolates from young carriers were tested for their antimicrobial susceptibility; additionally, inducibility of macrolide and clindamycin resistance was investigated in pneumococci carrying erm(A), erm(B), or mef(A). Of 125 strains tested, 101 (81%) were multidrug resistant. Different levels of induction were observed with erythromycin, miocamycin, and clindamycin in erm(B) strains; however, in erm(A) strains only erythromycin was an inducer. Induction did not affect macrolide MICs in mef(A) strains.

Macrolide resistance among middle ear isolates of Streptococcus pneumoniae observed at eight United States pediatric centers: prevalence of M and MLSB phenotypes

BACKGROUND

Increasing macrolide resistance among middle ear isolates complicates treatment of otitis media in children. When macrolide resistance is mediated via an efflux pump (M phenotype), the MICs of erythromycin, clarithromycin and azithromycin are usually below 32 microg/ml, and the pneumococcus remains susceptible to clindamycin. The association of prior specific macrolide therapy with the isolation of a macrolide resistant strain has not been reported.

OBJECTIVES

To determine the mechanism of macrolide resistance in Streptococcus pneumoniae recovered from the middle ears of children with otitis media and their association, if any, with ethnicity, age, serogroup/serotype and prior antibiotic therapy.

METHODS

Middle ear isolates collected by members of the United States Pediatric Multicenter Pneumococcal Surveillance Group during a 6-year period from September 1994 through August 2000 were studied. Antibiotic susceptibility to penicillin and ceftriaxone was determined by microbroth dilution. Disc diffusion susceptibility to erythromycin and clindamycin was performed to categorize macrolide resistance mechanisms. The medical record was reviewed to determine demographics and history of previous antibiotic therapy. Isolates were serogrouped or serotyped by the capsular swelling method.

RESULTS

Of the 1088 isolates available for testing, 51% were nonsusceptible to penicillin and 37% were nonsusceptible to erythromycin. Erythromycin resistance increased form 15% in 1994 through 1995 to 56% in 1999 through 2000. Seventy-five percent of macrolide-resistant strains were M phenotype. Macrolide resistance was less likely in isolates recovered from African-Americans and more likely in isolates obtained from children <3 years of age and from isolates obtained at time of tympanostomy tube placement. Neither erythromycin, nor clarithromycin nor azithromycin prescribed in the 30 days before infection was more likely than another to be associated with increased macrolide resistance. However, any macrolide alone or in combination with another antimicrobial taken before infection was associated with increased macrolide resistance among the S. pneumoniae organisms isolated from the middle ear.

CONCLUSIONS

Macrolide resistance among middle ear isolates of S. pneumoniae increased during the 6-year study. The proportion of M phenotype remained constant at 75%, meaning that these isolates remain susceptible to clindamycin. Continued surveillance to document potential changes is essential.

Macrolide resistance phenotypes in Streptococcus pneumoniae in Santiago, Chile

The mechanism of resistance was investigated in 39 macrolide-resistant clinical isolates of Streptococcus pneumoniae isolated from January 1997 to July 1999 in Santiago, Chile. Our results showed that 22 (56.5%) were macrolide-resistant, clindamycin-susceptible isolates (M phenotype) and 17 (43.5%) were macrolide and clindamycin resistant (MLS(B) phenotype). mefE gene was detected in all M phenotype, while ermB gene was detected in all MLS(B)-phenotype strains. Serotype 14 was the most frequent serotype among M-phenotype strains, and serotypes 19 and 23F were the most frequent serotypes in MLS(B) strains. These results demonstrate that both phenotypes of macrolide-resistant S. pneumoniae are found in Santiago, Chile, with the M phenotype predominating.

The era of antimicrobial resistance-implications for the clinical laboratory

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

Community treatment with azithromycin for trachoma is not associated with antibiotic resistance in Streptococcus pneumoniae at 1 year

AIMS

To determine if macrolide resistant Streptococcus pneumoniae will be a major concern in areas that receive annual mass azithromycin distributions for trachoma.

METHODS

A cross sectional survey was conducted of nasopharyngeal S pneumoniae isolates for susceptibility to azithromycin 1 year after administering a single dose of azithromycin to treat trachoma in a village in Nepal.

RESULTS

S pneumoniae was isolated from 50 (86%) of 57 nasopharyngeal cultures and no resistance to azithromycin was detected.

CONCLUSION

The authors were unable to demonstrate that mass azithromycin therapy for trachoma produced macrolide resistant S pneumoniae that persists until the next scheduled annual treatment.

A critical assessment of published guidelines and other decision-support systems for the antibiotic treatment of community-acquired respiratory tract infections

Guidelines are an important means by which professional associations and governments have sought to improve the quality and cost-effectiveness of disease management for infectious diseases. Prescribing of initial antibiotic therapy for community-acquired respiratory tract infections (RTIs) is primarily empiric and physicians may often have a limited appreciation of bacterial resistance. Recent guidelines for managing RTIs have adopted a more evidence-based approach. This process has highlighted important gaps in the existing knowledge base, e.g. concerning the impact of resistance on the effectiveness of oral antibiotics for outpatient community-acquired pneumonia and the level of resistance that should prompt a change in empiric prescribing. In upper RTIs, the challenge is to identify patients in whom antibiotic therapy is warranted. Concentrated, sustained efforts are needed to secure physicians' use of guidelines. The information should be distilled into a simple format available at the point of prescribing and supported by other behavioral change techniques (e.g. educational outreach visits). Advances in information technology offer the promise of more dynamic, computer-assisted forms of guidance. Thus, RTI prescribing guidelines and other prescribing support systems should help control bacterial resistance in the community. However, their effect on resistance patterns is largely unknown and there is an urgent need for collaborative research in this area. Rapid, cost-effective diagnostic techniques are also required and new antibiotics will continue to have a role in disease management.

Surveillance of resistance in bacteria causing community-acquired respiratory tract infections

Bacterial resistance to antibiotics in community-acquired respiratory tract infections is a serious problem and is increasing in prevalence world-wide at an alarming rate. Streptococcus pneumoniae, one of the main organisms implicated in respiratory tract infections, has developed multiple resistance mechanisms to combat the effects of most commonly used classes of antibiotics, particularly the beta-lactams (penicillin, aminopenicillins and cephalosporins) and macrolides. Furthermore, multidrug-resistant strains of S. pneumoniae have spread to all regions of the world, often via resistant genetic clones. A similar spread of resistance has been reported for other major respiratory tract pathogens, including Haemophilus influenzae, Moraxella catarrhalis and Streptococcus pyogenes. To develop and support resistance control strategies it is imperative to obtain accurate data on the prevalence, geographic distribution and antibiotic susceptibility of respiratory tract pathogens and how this relates to antibiotic prescribing patterns. In recent years, significant progress has been made in developing longitudinal national and international surveillance programs to monitor antibiotic resistance, such that the prevalence of resistance and underlying trends over time are now well documented for most parts of Europe, and many parts of Asia and the Americas. However, resistance surveillance data from parts of the developing world (regions of Central America, Africa, Asia and Central/Eastern Europe) remain poor. The quantity and quality of surveillance data is very heterogeneous; thus there is a clear need to standardize or validate the data collection, analysis and interpretative criteria used across studies. If disseminated effectively these data can be used to guide empiric antibiotic therapy, and to support-and monitor the impact of-interventions on antibiotic resistance.

Molecular epidemiology of erythromycin resistance in Streptococcus pneumoniae isolates from blood and noninvasive sites

Erythromycin-resistant isolates of Streptococcus pneumoniae from blood cultures and noninvasive sites were studied over a 3-year period. The prevalence of erythromycin resistance was 11.9% (19 of 160) in blood culture isolates but 4.2% (60 of 1,435) in noninvasive-site isolates. Sixty-two of the 79 resistant isolates were available for study. The M phenotype was responsible for 76% (47 of 62) of resistance, largely due to a serotype 14 clone, characterized by multilocus sequence typing as ST9, which accounted for 79% (37 of 47) of M phenotype resistance. The ST9 clone was 4.8 times more common in blood than in noninvasive sites. All M phenotype isolates were PCR positive for mef(A), but sequencing revealed that the ST9 clone possessed the mef(A) sequence commonly associated with Streptococcus pyogenes. All M phenotype isolates with this mef(A) sequence also had sequences consistent with the presence of the Tn1207.1 genetic element inserted in the celB gene. In contrast, isolates with the mef(E) sequence normally associated with S. pneumoniae contained sequences consistent with the presence of the mega insertion element. All MLS(B) isolates carried erm(B), and two isolates carried both erm(B) and mef(E). Fourteen of the 15 MLS(B) isolates were tetracycline resistant and contained tet(M). However, six M phenotype isolates of serotypes 19 (two isolates) and 23 (four isolates) were also tetracycline resistant and contained tet(M). MICs for isolates with the mef(A) sequence were significantly higher than MICs for isolates with the mef(E) sequence (P < 0.001). Thus, the ST9 clone of S. pneumoniae is a significant cause of invasive pneumococcal disease in northeast Scotland and is the single most important contributor to M phenotype erythromycin resistance.

Increased prevalence of erythromycin resistance in streptococci: substantial upsurge in erythromycin-resistant M phenotype in Streptococcus pyogenes (1979-1998) but not in Streptococcus pneumoniae (1985-1999) in Taiwan

A total of 394 nonduplicate isolates of Streptococcus pyogenes collected from 1979 to 1998 and 267 nonduplicate isolates of Streptococcus pneumoniae collected from October, 1998, to May, 1999, in Taiwan were evaluated. Among the 220 erythromycin-resistant (MIC, > or =1 microg/ml) S. pyogenes isolates, 35% had an M phenotype and 65% had an ML phenotype (inducible resistance [iML], 0.5%, and constitutive resistance [cML], 64.5%). Among the 243 erythromycin-resistant S. pneumoniae isolates, the majority (65.4%) had an ML phenotype (iML, 0.4%, and cML, 65%) and 34.6% had an M phenotype. A substantial upsurge in the incidence of M-phenotype erythromycin-resistant isolates was found with time for S. pyogenes (0% in 1979-1984 and 100% in 1997-1998), and an increasing incidence of M-phenotype among erythromycin-resistant S. pneumoniae was also noted (<20% before 1994 and 45.4% in 1999). All S. pyogenes and all but four S. pneumoniae isolates exhibiting a cML or iML phenotype had harbored the ermAM gene. The presence of the mefA gene was demonstrated in all isolates of S. pyogenes and the mefE gene in all but four S. pneumoniae isolates exhibiting the M phenotype. Due to the increasing susceptibility of S. pyogenes and S. pneumoniae isolates to clindamycin, susceptibility tests of these two organisms to macrolides and clindamycin should be performed simultaneously in the clinical microbiology laboratory, particularly in areas with high rates of macrolide resistance.

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.

Resistance determinants and clonal diversity in group A streptococci collected during a period of increasing macrolide resistance

Susceptibility to macrolides and lincosamides was investigated with 299 consecutive nonduplicate Streptococcus pyogenes clinical isolates collected over a 6-year period (1992 to 1997) from an area of central Italy. During this period, macrolide resistance rates steadily increased (from 9% in 1992 to 53% in 1997; P < 0.001). The increase was caused by isolates with a macrolide-lincosamide-streptogramin B resistance phenotype, carrying mostly erm(B) but also erm(TR) genes, that were not detected in the first 2 years and were detected with increasing prevalence (8, 5, 26, and 37%, respectively) during the following 4 years. During the same period, the prevalence of isolates with a macrolide resistance phenotype, carrying mef(A) determinants, did not vary significantly; on average it was 13%, with modest rate fluctuations in different years and no definite trend. Molecular typing revealed a remarkable clonal diversity among susceptible and resistant isolates and a notable heterogeneity of the genetic environment of the resistance genes. The analysis of clonal diversity in relation with resistance phenotypes and genotypes revealed that increased macrolide resistance rates were due to a complex interplay of different mechanisms, with a relevant contribution played by an "epidemic" spread of genetic elements carrying the erm(B) gene among the circulating streptococcal population.

Prevalence of antimicrobial resistance in Streptococcus pneumoniae circulating in Italy: results of the Italian Epidemiological Observatory Survey (1997-1999)

The Italian Epidemiological Observatory (IEO), a surveillance program supported by the SmithKline Foundation, analyzed the susceptibility of 2,664 community-acquired respiratory Streptococcus pneumoniae derived from over 50 clinical microbiology laboratories during 1997-1999, against 21 antibiotics adopting a quantitative methodology. Throughout these years, total penicillin resistance varied from 14.3% to 10.2%. High-level resistance has remained stable, ranging from 3.8% to 4.1%, while a decrease in low-level resistance (from 10.3% to 6.1%) has been recorded. Lack of susceptibility to macrolides ranged from 29.1% in 1997 to 25.5% in 1999. Similar figures have also been observed with tetracycline and co-trimoxazole (rates of resistance around 30%). As expected, large geographical variations in resistance rates were found for all drugs. Amoxicillin and amoxicillin-clavulanate were 100% active on penicillin-intermediate isolates. Injectable third-generation cephalosporins and carbapenems were also capable of inhibiting a large proportion of these microorganisms. Rifampin was the most potent non-beta-lactam compound tested. In contrast to the situation prevailing elsewhere, in Italian children (aged 0-5 years) presenting with respiratory conditions, the total rate of penicillin resistance (3%) was lower than that shown by the adult population (10.9%). However, lack of susceptibility to macrolides, tetracycline, and cotrimoxazole (35%, 41%, 44%) was more incident in pediatric than in adult patients (25%, 26%, 28% respectively). Strains recovered from blood in 1999 (67) were much more susceptible to penicillin (98.5%) than respiratory pneumococci (89.8%), whereas macrolides, tetracycline, and cotrimoxazole were consistently less active (75%, 67%, 64%).

Macrolides: structures and microbial targets

The macrolide class of antibiotics is well established and often recommended for use in the treatment of community-acquired respiratory tract infections. A number of agents with varying antimicrobial activity have been developed via chemical modification of the core macrolide structure, a macrocyclic lactam ring. Although structurally diverse, the macrolides share a common ability to bind to the bacterial 50S ribosome subunit and inhibit protein synthesis, thereby preventing bacterial multiplication. Resistance in the clinic is due to modification of the 50S subunit in the area of the peptidyl transferase center or to an efflux pump. The newer macrolides, and in particular azithromycin, with their broad-spectrum microbiological profile have extended the therapeutic uses of this class of antibiotics and ensured that they remain an integral part of the clinician's armamentarium.

Rokitamycin: bacterial resistance to a 16-membered ring macrolide differs from that to 14- and 15-membered ring macrolides

Rokitamycin is the latest semi-synthetic 16-membered ring macrolide introduced into clinical practice. It is characterized by greater hydrophobicity, better bacterial uptake and a slower release, more cohesive ribosomal binding, and a longer post-antibiotic-effect (PAE) than can be observed with other available 14-, 15- and 16-membered ring macrolides. Rokitamycin exerts its activity on strains that harbor inducible erm genes or the efflux gene, mef(A). It has also been reported to be more active in vitro than other 16-membered ring macrolides. However, these recognized features are not fully exploited yet because current automated test procedures used in many microbiological laboratories determine susceptibility only to erythromycin or clarithromycin. Resistance to 16-membered ring macrolides cannot be predicted solely on the basis of known resistance to erythromycin or clarithromycin as revealed by an automated susceptibility assay. At least equally important is the knowledge of the bacterial resistance phenotype. This is underlined by the existence of Gram-positive coccal strains resistant to erythromycin and other 14-,15-membered ring macrolides but susceptible to 16-membered ring macrolides. Since the local prevalence of erythromycin phenotypes is generally unknown but might determine the outcome of treatment, the procedure for identifying the phenotypes in erythromycin-resistant strains (which can be easily and cheaply performed using the two- or three-disk assay) should become routine, at least in the countries in which 16-membered ring macrolides are used. This approach should help to optimize the use of macrolides, improve our knowledge of the local prevalence of phenotypes resistant to erythromycin, and offer the possibility of treating infections caused by certain types of erythromycin-resistant pathogens.

Clinical relevance of macrolide-resistant Streptococcus pneumoniae for community-acquired pneumonia

Macrolides are often the first choice for empirical treatment of community-acquired pneumonia. However, macrolide resistance among Streptococcus pneumoniae has escalated at alarming rates in North America and worldwide. Macrolide resistance among pneumococci is primarily due to genetic mutations affecting the ribosomal target site (ermAM) or active drug efflux (mefE). Prior antibiotic exposure is the major risk factor for amplification and perpetuation of resistance. Clonal spread facilitates dissemination of drug-resistant strains. Data assessing the impact of macrolide resistance on clinical outcomes are spare. Many experts believe that the clinical impact is limited. Ribosomal mutations confer high-grade resistance, whereas efflux mutations can likely be overridden in vivo. Favorable pharmacokinetics and pharmacodynamics, high concentrations at sites of infections, and additional properties of macrolides may enhance their efficacy. In this article, we discuss the prevalence of macrolide resistance among S. pneumoniae, risk factors and mechanisms responsible for resistance, therapeutic strategies, and implications for the future.

Macrolide efflux genes mef(A) and mef(E) are carried by different genetic elements in Streptococcus pneumoniae

Susceptibilities to macrolides were evaluated in 267 Streptococcus pneumoniae isolates, of which 182 were from patients with invasive diseases and 85 were from healthy carriers. Of the 98 resistant isolates, 20 strains showed an M phenotype and carried mef. Strains that carried both mef(A) and mef(E) were found: 17 strains carried mef(A) and 3 carried mef(E). The characteristics of the strains carrying the mef genes and the properties of the mef-containing elements were studied. Strains carrying mef(A) belonged to serotype 14, were susceptible to all the antibiotics tested except erythromycin, and appeared to be clonally related by pulsed-field gel electrophoresis (PFGE). The three mef(E) strains belonged to different serotypes, showed different susceptibility profiles, and did not appear to be related by PFGE. The sequences of a fragment of the mef-containing element, which encompassed mef and the msr(A) homolog, were identical among the three mef(E)-positive strains and among the three mef(A)-positive strains, although there were differences between the sequences for the two variants at 168 positions. In all mef(A)-positive strains, the mef element was inserted in celB, which led to impairment of the competence of the strains. In line with insertion of the mef(E) element at a different site, the competence of the mef(E)-positive strains was maintained. Transfer of erythromycin resistance by conjugation was obtained from two of three mef(A) strains but from none of three mef(E) strains. Due to the important different characteristics of the strains carrying mef(A) or mef(E), we suggest that the distinction between the two genes be maintained.

Streptococcus pneumoniae in community-acquired pneumonia. How important is drug resistance?

Patients hospitalized with community-acquired pneumonia caused by S. pneumoniae strains with intermediate susceptibility to penicillin according to the conventional definition respond well to treatment with adequate doses of beta-lactam antibiotics. All studies currently available comparing mortality between patients with pneumonia caused by nonsusceptible and susceptible pneumococci agree that resistance of MIC 2 mg/L is not associated independently with an increased mortality. Most but not all studies could not prove an effect of microbial resistance on morbidity. There are data suggesting, however, that pneumococcal pneumonia caused by highly resistant strains (MIC > or = 4 mg/L) does affect the outcome. Pneumococcal resistance remains a matter of concern. Most reports show an increase not only of resistance rates, but also of the proportion of highly resistant strains. The selection of initial empirical antimicrobial treatment of patients with community-acquired pneumonia should be performed judiciously. Because the serum and pulmonary levels achieved with penicillin or related drugs are several times higher than the MICs of most strains, pneumonias caused by S. pneumoniae currently defined as not susceptible to penicillin should respond well to treatment with a beta-lactam antibiotic, used in optimal doses. Consequently, there is no reason fundamentally to change the current approach to initial empiric antimicrobial treatment of patients with community-acquired pneumonia. Nevertheless, increases in resistance to macrolides may prompt a limited use of these drugs in the outpatient setting. In any case, treatment failures may occur at higher levels of resistance, and a change in the definition of susceptibility categories toward higher cutoffs for S. pneumoniae seems to be reasonable.