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.

FIG. 1.

Schematic representation of mef(A)-carrying element Tn1207.1 (22) and the mef(E)-carrying mega element (7). The hatched bar represents the fragment amplified and sequenced with primers MEF3 and OM4. Primers MS50 and OM18, used to amplify the chromosomal insertion of the mef(A)-carrying element, are indicated above Tn1207.1. The positions of the primers are indicated by arrows.

FIG. 2.

PFGE of SmaI-digested chromosomal DNA of S. pneumoniae. (A) mef(A)-carrying serotype 14 strains; (B) mef(E)-carrying strains. Lane 1, PNS06; lane 2, PNS07; lane 3, 1514; lane 4, 1711; lane 5, PN67; lane 6, PN151; lane 7, PN83; lane 8, PN98; lane 9, PN88; lane 10, PN137; lane 11, PN165; lane 12, PN17; lane 13, 1044; lane 14, PN138; lane 15, PN139; lane 16, PN92; lane 17, PNS11; lane 18, PN150; lane 19, PN34; lane 20, 713; lanes M, bacteriophage lambda ladder molecular mass marker. See Table 1 for details about the strains.