Emergence of Group A Streptococcus Strains with Different Mechanisms of Macrolide Resistance

Abstract

The mechanisms of resistance to macrolides in seven group A streptococcal (Streptococcus pyogenes) isolates that were the cause of pharyngitis in children who were unsuccessfully treated with azithromycin (10 mg/kg of body weight/day for 3 days) were evaluated. All posttreatment strains were found to be genetically related to the pretreatment isolates by random amplified polymorphism DNA analysis and pulsed-field gel electrophoresis. Two isolates had acquired either a mef(A) or an erm(B) gene, responsible for macrolide efflux and ribosomal modification, respectively. Three isolates displayed mutations in the gene encoding the L4 ribosomal protein that is part of the exit tunnel within the 50S subunit of the bacterial ribosome. In the two remaining posttreatment strains, the mechanisms of macrolide resistance could not be elucidated.The recommended treatment for group A Streptococcus (GAS) pharyngitis, a 10-day course of penicillin, was established 50 years ago and remains the "gold standard."However, for patients hypersensitive to ␤-lactam antibiotics and in whom therapy with these drugs fails, macrolides are often the recommended substitute. Azithromycin is an azalide antibiotic chemically related to erythromycin and has a long half-life and excellent tissue penetration. Short-course azithromycin therapy has been reported to be effective for the eradication of oropharyngeal GAS (14).In a prospective, comparative, randomized, multicenter trial that was conducted between November 1997 and July 1998 and that involved 350 children with GAS pharyngitis, we observed better eradication rates on days 14 and 30 after a 3-day course of therapy with azithromycin at 20 mg/kg of body weight/day than after a 3-day course of therapy with azithromycin at 10 mg/kg/day. In the per protocol analysis, the failure rates on day 14 were 57 of 135 (42.2%) in the 10-mg/kg treatment arm and 8 of 139 (5.8%) in the 20-mg/kg treatment arm (8). Furthermore, analysis of bacterial isolates by random amplified polymorphic DNA (RAPD) analysis revealed seven cases of bacteriological treatment failure: azithromycin MICs for genetically related pre-and posttreatment GAS strains increased after treatment with this antimicrobial at 10 mg/kg/day for 3 days. In contrast, azithromycin MICs were not increased for any of the strains from any of the patients with treatment failure in the group treated with azithromycin at 20 mg/kg/day (8). The known mechanisms of macrolide resistance in streptococci are modification of the ribosomal target by a methylase encoded by erm genes (13, 20) and a macrolide-specific efflux mechanism encoded by the mef(A) gene (7). Mutation of the ribosomal target of macrolides is a rare resistance mechanism in streptococci and has been reported in only a few clinical pneumococcal isolates (21).In the study described here we investigated the mechanisms of macrolide resistance among the GAS strains that were the cause of bacteriological treatment failures in the group treated with azithromycin at 10 mg/kg/day. MATERIALS AND METHODS...

M.SpyI, a DNA Methyltransferase Encoded on a mefA Chimeric Element, Modifies the Genome of Streptococcus pyogenes

“…The numbering of the other lines corresponds to the positions of the nucleotides from the start of the bi-F primer in the sense strand amplicon of the bisulfite reaction ( (39,41,54,55). However, numerous studies have reported that the DNA from some macrolide-resistant isolates is refractory to SmaI digestion (7,22,35,36,44), an observation recently associated with the "M phenotype" of erythromycin resistance, which is imparted by Tn1207.3 or the ⌽10394.4 chimeric element (22,44). The results of our study agree with these reports.…”

“…One of the most common methods used to examine the clonality of such macrolide-resistant and -sensitive streptococcal isolates involves restriction of genomic DNA with the SmaI endonuclease, followed by pulsed field gel electrophoresis (PFGE) analysis (6,44,55). Recently, however, a number of epidemiological studies have reported that the DNA from a diverse group of erythromycin-resistant streptococci could not be digested with SmaI (7,22,35,36,44). This finding prompted the use of alternative restriction enzymes, such as ApaI, EagI, and SfiI, in the analysis, making it difficult subsequently to directly compare the clonalities of strains that differ in macrolide susceptibility and to relate the results of different epidemiological studies (7,35,36).…”

Clonal Spread of emm Type 28 Isolates of Streptococcus pyogenes That Are Multiresistant to Antibiotics

“…Our results confirm the importance of using an alternative to bacitracin susceptibility testing. The emergence of strains of S. pyogenes resistant to macrolides has been observed, but they have not been multiresistant to antibiotics (2,9,10,15). In this study, 53 pharyngitis-related and invasive isolates of S. pyogenes resistant to bacitracin were shown to also be resistant to streptomycin, kanamycin, macrolides, lincosamides, and streptogramin B.…”

Six-Month Multicenter Study on Invasive Infections Due to Streptococcus pyogenes and Streptococcus dysgalactiae subsp. equisimilis in Argentina

“…Mortality due to group A streptococci was 27.3% (6) in adults and 12.1% (4) in children. STSS occurred also more frequently in adults (31.8%) (7) than in children (12.1%) (4).…”

“…In gram-positive organisms, erythromycin (ERY) resistance is mediated by either target modification or active efflux (17,29). Target modification may be produced by mutation or by posttranscriptional methylation of adenine molecules in 23S rRNA (4,29). In streptococci, TET resistance is due to active efflux or ribosomal protection (22), while CMP resistance is mediated by target modification, active efflux, or antibiotic inactivation (23).…”