Phenotypic and genotypic characterization of group B streptococcal isolates in southern Brazil

Abstract

One-hundred sixty-eight group B streptococcal (GBS) isolates from a Brazilian hospital were phenotypically and genotypically characterized. Isolates were recovered from human sources from April 2006 to May 2008 and classified as either invasive, noninvasive, or colonizing isolates. Classical methods for serotyping and antibiotic resistance profiling were employed. Clonal groups were also defined by pulsed-field gel electrophoresis (PFGE). Results showed that susceptibility to beta-lactam antimicrobials was predominant among the isolates. Only 4.7% were resistant to erythromycin and clindamycin. The erm(B) gene was widely detected in our GBS isolates, according to our phenotypic results (constitutive macrolide-lincosamide-streptogramin B [cMLSB] resistance phenotype), and the erm(A) gene was also detected in some isolates. MLSB resistance was restricted to strains isolated from patients with noninvasive infections and carriers. Serotype Ia was predominant (38.1%), serotype IV isolates were found at a high frequency (13.1%), and few isolates of serotype III were identified (3%). Pulsed-field gel electrophoresis results revealed a variety of types, reflecting the substantial genetic diversity among GBS strains, although a great number of isolates could be clustered into two major groups with a high degree of genetic relatedness. Three main PFGE clonal groups were found, and isolates sharing the same PFGE type were grouped into different serotypes. Furthermore, in a few cases, isolates from the same patients and possessing the same PFGE type were of different serotypes. These findings could be related to the occurrence of capsular switching by horizontal transfer of capsular genes.

FIG. 1.

Dendrogram of 168 GBS isolates constructed on the basis of PFGE patterns. A Dice coefficient similarity within at least 80% included 14 PFGE groups numbered from 1 to 14. Isolate identifications (isolate ID) are indicated in the dendrogram. An expanded view of groups 2 and 14, which contain multiple numbers of strains with degrees of similarity higher than 80%, is presented on the right in gray. The site of isolation, serotype, and PFGE grouping of those isolates are shown. Isolates 155, 184, 132, 149, 152, 179, 80, and 94 are MLS_B resistant.