Typing of group B streptococci: comparison of pulsed-field gel electrophoresis and conventional electrophoresis

The SmaI restriction endonuclease digestion patterns of chromosomal DNAs from 35 group B streptococci were analyzed by pulsed-field gel electrophoresis (PFGE). Nineteen different patterns and four possible variants were identified. Twenty-four isolates were previously analyzed by conventional electrophoresis of HindIII-digested and/or BglII plus EcoRI double-digested chromosomal DNA. Although interpretations by both methods were essentially the same, PFGE identified as variants two isolates that were previously classified as the same isolate. More importantly, PFGE of the chromosomal DNA of group B streptococci digested with SmaI generated more easily defined patterns, since fewer and better separated bands were obtained, whereas digestion with HindIII or EcoRI plus BglII typically generated 100 or more bands. SalI digestion also yielded easily evaluable results, although the SalI fragments were somewhat smaller than those generated by SmaI. In our hands, PFGE patterns were more easily discerned and interpreted than were patterns previously generated by conventional electrophoresis.