No abstract

A total of 387 clinical strains of erythromycin-resistant (MIC, ≥1 μg/ml) Streptococcus pyogenes, all isolated in Italian laboratories from 1995 to 1998, were examined. By the erythromycin-clindamycin double-disk test, 203 (52.5%) strains were assigned to the recently described M phenotype, 120 (31.0%) were assigned to the inducible macrolide, lincosamide, and streptogramin B resistance (iMLS) phenotype, and 64 (16.5%) were assigned to the constitutive MLS resistance (cMLS) phenotype. The inducible character of the resistance of the iMLS strains was confirmed by comparing the clindamycin MICs determined under normal testing conditions and those determined after induction by pregrowth in 0.05 μg of erythromycin per ml. The MICs of erythromycin, clarithromycin, azithromycin, josamycin, spiramycin, and the ketolide HMR3004 were then determined and compared. Homogeneous susceptibility patterns were observed for the isolates of the cMLS phenotype (for all but one of the strains, HMR3004 MICs were 0.5 to 8 μg/ml and the MICs of the other drugs were >128 μg/ml) and those of the M phenotype (resistance only to the 14- and 15-membered macrolides was recorded, with MICs of 2 to 32 μg/ml). Conversely, heterogeneous susceptibility patterns were observed in the isolates of the iMLS phenotype, which were subdivided into three distinct subtypes designated iMLS-A, iMLS-B, and iMLS-C. The iMLS-A strains (n = 84) were highly resistant to the 14-, 15-, and 16-membered macrolides and demonstrated reduced susceptibility to low-level resistance to HMR3004. The iMLS-B strains (n = 12) were highly resistant to the 14- and 15-membered macrolides, susceptible to the 16-membered macrolides (but highly resistant to josamycin after induction), and susceptible to HMR3004 (but intermediate or resistant after induction). The iMLS-C strains (n = 24) had lower levels of resistance to the 14- and 15-membered macrolides (with erythromycin MICs increasing two to four times after induction), were susceptible to the 16-membered macrolides (but resistant to josamycin after induction), and remained susceptible to HMR3004, also after induction. The erythromycin resistance genes in 100 isolates of the different groups were investigated by PCR. All cMLS and iMLS-A isolates tested had theermAM (ermB) gene, whereas all iMLS-B and iMLS-C isolates had the ermTR gene (neither methylase gene was found in isolates of other groups). The M isolates had only the macrolide efflux (mefA) gene, which was also found in variable proportions of cMLS, iMLS-A, iMLS-B, and iMLS-C isolates. The three iMLS subtypes were easily differentiated by a triple-disk test set up by adding a josamycin disk to the erythromycin and clindamycin disks of the conventional double-disk test. Tetracycline resistance was not detected in any isolate of the iMLS-A subtype, whereas it was observed in over 90% of both iMLS-B and iMLS-C isolates.

In tetracycline-susceptible pneumococci with erm(B)-mediated erythromycin resistance, the erm(B) gene is carried on a variety of Tn916-related genetic elements either lacking tet(M) or, more often, carrying an unexpressed tet(M) gene.

A posttranscriptional methylase-mediated modification of the 23S rRNA encoded by the erm(B) gene, which can be expressed either constitutively or inducibly, is the most common mechanism of resistance to macrolide, lincosamide, and streptogramin B antibiotics in Streptococcus pneumoniae (21). In Italy, this mechanism accounts for approximately two-thirds of the erythromycin-resistant pneumococci (24, 25), approximately 90% of which are cross-resistant to tetracycline (24). Different erm(B)-carrying Tn916-related elements (Tn1545, Tn3872, Tn6002, and Tn6003) have been detected in S. pneumoniae isolates with erm(B)-mediated erythromycin resistance.Tn1545, the first of these elements described, originally detected in multiply resistant S. pneumoniae strain BM4200, and reported to have a size of 25.3 kb (12, 13), is a transposon carrying the resistance genes tet(M) (tetracycline), erm(B) (macrolide, lincosamide, and streptogramin B antibiotics), and aphA-3 (kanamycin and structurally related aminoglycosides). Its genetic organization has been analyzed essentially by restriction mapping (3); only specific portions have been sequenced (4,5,22,28,34).Tn3872 (reported size, 23.3 kb) is a composite element resulting from the insertion of the erm(B)-containing Tn917 transposon (33) into Tn916 (23).Tn6002 is a 20.9-kb element resulting from the insertion of a ca. 2.8-kb erm(B)-containing DNA fragment [erm(B) element] into Tn916 (2, 10). Tn6002 was originally detected in Streptococcus cristatus and designated Tn916Erm, but it has not been published even though its complete sequence has been available in GenBank since 2005 (accession no. AY898750). In S. pneumoniae isolates, Tn6002 may also be part of composite transposon Tn2010 (26.3 kb) (14). Tn6003 (25.1 kb)-unlike Tn3872 and Tn6002 and similar to Tn1545-also carries, besides the tetracycline [tet(M)] and erythromycin [erm(B)] resistance determinants, the kanamycin resistance gene aphA-3. In Tn6003, aphA-3 has been shown to be contained in a 4.2-kb insertion called the MAS (macrolideaminoglycoside-streptothricin) element, also containing a second erm(B) gene (10).All of these erm(B)-carrying elements are derivatives of Tn916, a ubiquitous 18.0-kb transposon originally detected in Enterococcus faecalis (17), that confers tetracycline resistance via the tet(M) gene on a wide range of clinically important (especially gram-positive) bacteria (8, 29). As we recently showed, a minority of both S. pneumoniae (10) and Streptococcus pyogenes (2) isolates with erm(B)-mediated erythromycin resistance are tetracycline susceptible because the tet(M) gene is silent.The present study, by comparatively analyzing the genetic organization of erm(B)-carrying transposons in S. pneumoniae reference strains and investigating their distribution in tetracycline-resistant clinical isolates, shows a substantial correspondence between Tn1545 and Tn6003, while Tn6002 appears to be the most common Tn916-related transposon in pneumococci.

No abstract