Distribution of PFGE types of invasive Norwegian group B streptococci in relation to serotypes.

BACKGROUND & OBJECTIVES: Pulsed-field gel electrophoresis (PFGE) has been used for characterisation of group B streptococci, non-typeable by serotyping. We wanted to compare PFGE with serotyping in order to see the how well the method discriminates between epidemiological unrelated strains.

METHODS

A total of 78 epidemiological unrelated invasive GBS strains were examined by PFGE using SmaI digested chromosomal DNA. Of these, 11 were nontypeable (NT) with regard to capsular polysaccharide (CPS) serotype. PFGE patterns were analyzed and classified in a dendrogram.

RESULTS

75 strains were typeable by PFGE, and a total of 62 restriction profiles were identified. At an 85 per cent similarity level, 53 different PFGE patterns were identified. Within each serotype, PFGE patterns differed considerably, the largest degree of heterogeneity observed among type IV, Ia, and II strains. Serotype Ib, III, and V strains were more homogeneous. Strains with identical macrorestriction profiles belonged to the same CPS type, but varied with regard to serosubtypes. Any strain among the ones investigated showing a greater than 88 per cent similarity to a restriction profile in the database, could correctly be ascribed to a particular CPS type. Of the 11 NT strains 10 restriction profiles were found, two of which were identical to the PFGE profile for a cluster of type V strains, and one profile were identical to the profile showed by a cluster of 5 type Ib strains.

INTERPRETATION & CONCLUSION: PFGE is a useful technique for classifying strains that are non-typeable by conventional serotyping.