In-vitro susceptibility and molecular characterisation of macrolide resistance mechanisms among Streptococcus pneumonia isolates in The Netherlands: the DUEL 2 study

In total, 881 presumptive clinical isolates of Streptococcus pneumoniae collected from throughout The Netherlands were analysed to determine their mechanisms of macrolide resistance. Isolates were identified initially by participating laboratories using their own standard identification technique, followed by determination of MICs with Etests. Only 797 isolates were confirmed as pneumococci following bile-solubility tests, lytA PCR and 16S rRNA sequencing. Of these confirmed pneumococci, 59 (7.4%) isolates were macrolide-resistant. Analysis by PCR indicated that 34 (57.6%) isolatesharboured only the erm(B) gene and 16 (27.1%) only the mef gene. Three (5.1%) isolates carried both erm(B) and mef, while six (10.2%) isolates were negative for both mechanisms. Of the six negative isolates, three had a mutation in the 23S rRNA gene, and three were negative for all mechanisms tested. No isolates with the erm(A) subclass erm(TR) gene were detected. Among the 19 mef-positive isolates, 14 (73.7%) carried the mef(A) gene, and only five (26.3%) carried the mef(E) gene. No linezolid cross-resistance or multiresistance (resistance to more than two classes of antibiotics) was observed.