Expressed and Unexpressed tet(M) Genes and the erm(B)-Carrying Tn1116 Transposon in Streptococcus pyogenes and Streptococcus pneumoniae

It was exciting to read, in an excellent countrywide survey of erythromycin-resistant Streptococcus pneumoniae in Spain just published by Calatayud et al. (3), that Tn1116—a novel erm(B)-carrying element that we have recently described in Italian isolates of Streptococcus pyogenes and published in a recent issue of this journal (2)—was detected in 16 out of their 125 test strains. However, more careful reading reveals that the element detected in the Spanish pneumococci is not Tn1116, but a new structure.

Tn1116 is a composite element of ca. 50 kb (2) resulting from the insertion of an erm(B)-containing DNA fragment into a defective Tn5397, a Tn916-related transposon originally found in Clostridium difficile (6) where the conventional int (integrase) and xis (excisase) genes (4) are replaced by the tndX (resolvase) gene (7). Most interestingly, in Tn1116 the erm(B)-containing DNA is inserted into the coding sequence of the tet(M) gene of the defective Tn5397, at base 15,019 of its published sequence (accession no. AF333235). Therefore, although detectable by PCR using suitable primers, the tet(M) gene in Tn1116 is incomplete and silent, due to the lack of the portion of the gene upstream of the insertion. [The nucleotide sequence of Tn1116 between erm(B) and tndX has been deposited under accession no. AM411377.] Accordingly, a distinguishing feature of Tn1116-carrying strains, among S. pyogenes isolates with erm(B)-mediated erythromycin resistance, is the association between a tet(M) genotype and a tetracycline-susceptible phenotype.

In contrast, all of the 16 Spanish pneumococci putatively carrying Tn1116 harbored the erm(B), tet(M), and tndX genes but were tetracycline resistant (3). This means that, unlike in Tn1116, tet(M) was complete and was regularly expressed. It should be noted that the tet(M)-specific primer pair (1) used by Calatayud et al. (3) would be unable to detect the incomplete tet(M) of Tn1116, because the forward primer targets the initial, lacking portion of the gene.

If this pneumococcal element—sharing with Tn1116 an erm(B), tet(M), and tndX genotype but associated with an erythromycin-resistant and tetracycline-resistant phenotype instead of an erythromycin-resistant and tetracycline-susceptible phenotype—is not Tn1116, it is nevertheless an interesting new element warranting further investigation. The crucial issue is the location of the erm(B) gene: the insertion of erm(B)-containing DNA into a Tn5397-related element outside the coding sequence of tet(M) may be a working hypothesis, but an actual linkage of erm(B) with tet(M) and tndX is still to be proved. On the other hand, PCR data suggest that erm(B) is neither on Tn916-like elements with conventional int and xis genes (4, 5, 7) nor on Tn917 (8). A completely new organization cannot be ruled out. Finally, it seems likely (though not expressly stated by the authors) (3) that the 16 isolates in question also share the iMLS_B phenotype of erythromycin resistance. This would entail that they represent a particular clone relatively widespread among erythromycin-resistant pneumococci in Spain but less prevalent in other countries, where iMLS_B pneumococci are very uncommon.