Macrolide Efflux Genes mef (A) and mef (E) Are Carried by Different Genetic Elements in Streptococcus pneumoniae

Abstract

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.Macrolide resistance in Streptococcus pneumoniae is typically due to acquisition of the erm(B) gene, which mediates ribosomal modification (10), or the mef gene, which encodes a drug efflux pump (28). Recently, mutations in the 23S rRNA or ribosomal proteins of S. pneumoniae have been found to confer erythromycin (ERY) resistance in some clinical isolates (30).The Mef pump confers a low to moderate level of resistance to 14-and 15-membered macrolides but not to lincosamide or streptogramin B antibiotics (M phenotype). Of the two variants of the mef gene, mef(A) was originally found in Streptococcus pyogenes (3) and mef(E) was originally found in S. pneumoniae (29). mef(A) and mef(E) are 90% identical at the nucleotide level and were assigned to the same class of macrolide resistance determinants (22). In most subsequent studies, mef was detected by a PCR assay that did not distinguish between the two variants (27). However, the two variants were considered species specific; therefore, if a mef gene was found in S. pneumoniae, it was generally assumed to be mef(E) (9, 16, 26). However, mef(A) was shown to be present in macrolideresistant Italian isolates of S. pneumoniae (18).Genetic elements carrying mef genes in S. pneumoniae were recently detected and characterized. The mef(A)-carrying element is a 7.2-kb defective transposon (Tn1207.1) that contains eight open reading fra...

Molecular Epidemiology of Pneumococcal Carriage among Children with Upper Respiratory Tract Infections in Hanoi, Vietnam

“…The presence of the mef gene was also detected by PCR (38). In order to discriminate between mef(A) and mef(E), PCR-RFLP analysis was performed as described by Del Grosso et al (9). The amplicon was restricted with BamHI and DraI.…”

Occurrence of Tylosin-Resistant Enterococci in Swine Manure and Tile Drainage Systems under No-Till Management

“…Acquired resistance to macrolides by active efflux has been detected in various bacteria including streptococci and staphylococci (Bean and Klena, 2002;Del Grosso et al, 2002;Descheemaeker et al, 2000;Luna et al, 1999 Wierzbowski et al, 2005).…”

“…The second mechanism is acquiring a mutation that modifies the target site of an antibiotic so that the target is no longer sensitive (Tenover, 2006). And the third mechanism involve in a change in the permeability of the cell membrane which either prevents the entry of the antibiotic into the cell or causes the antibiotic to be pumped out of the cell (Del Grosso et al, 2002).…”

Genetic Resistance Elements Carrying mef Subclasses Other than mef (A) in Streptococcus pyogenes

“…The mef(E) subclass is common in S. pneumoniae strains and in viridans group streptococci, where it is carried by the mega element of 5.5 kb (9,23). mega is frequently found associated with other resistance genes in composite transposons, the most common being the 23.5-kb element Tn2009, found in S. pneumoniae and in Gram-negative bacteria, where mega is associated with Tn916 carrying tet(M) (10,18).…”