High clonal diversity in erythromycin-resistant Streptococcus pneumoniae invasive isolates in Madrid, Spain (2000–07)

78 erythromycin-resistant S. pneumoniae isolates collected from blood cultures in our hospital (2000– 07) were studied and the population structure was analysed by using different mathematical diversity indexes. Methods: Erythromycin resistance determinants were screened by PCR. The population structure, including multilocus sequence typing, was analysed by using quantitative clonal diversity (diversity ratio, Simpson, Selander–Levin and Shannon mathematical indexes). Results: The leading resistance gene was erm(B) (74.3 % of the isolates), followed by the erm(B) plus mef(A) combination (17.9%) and mef(A) alone (7.7%). The most frequent serotypes were 14 (18%), 19A (15.4%) and 6B (11.5%). A polyclonal structure was detected in resistant strains, including the Spain9V-3, Spain6B-2 and Denmark14-32 international clones. Both genetic diversity and genetic distribution were high, particularly among clones containing erm(B) and erm(B) plus mef(A) determinants. Conclusions: The resistance determinants erm(B) and the combination of erm(B) plus mef(A) were observed within multiple S. pneumoniae bacteraemic clones. The preservation of a polyclonal struc-ture might provide a suitable background for further evolution of antibiotic resistance