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

OBJECTIVES

Erythromycin resistance in Streptococcus pneumoniae is still increasing worldwide. All 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 Spain(9V)-3, Spain(6B)-2 and Denmark(14)-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 structure might provide a suitable background for further evolution of antibiotic resistance.