Comparison of bacteriological methods for the isolation of group of B Streptococcus from vaginal cultures

Three bacteriological techniques for the isolation of group B streptococci in vaginal cultures were compared. A selective broth medium (SBM) containing gentamicin and nalidixic acid was more sensitive for the detection of vaginal isolates (28/76, 36.8%) from 76 women enrolled in a venereal disease clinic than was an identical selective plate medium (SPM) (17/76, 25%). Similarly, SBM allowed identification of positive cultures from college women (82/459, 17.9%) significantly more often than direct inoculation of swabs onto nonselective blood agar medium (43/460, 9.4%; chi2 = 42.2, P = less than 0.001). Failure to isolate group B streptococci detected in SBM occurred in 32.1% cultures by SPM and 49.4% of cultures by nonselective agar medium. Multiple serotypes were detected in a single vaginal culture from approximately 5% of the patients studied. These data support the routine use of SBM for the most accurate identification of women vaginally colonized with group B Streptococcus.