Detection of group B streptococci by agglutination testing from selective broth cultures

A rapid technique for the immunological identification of group B streptococci in vaginal swabs is reported. Vaginal swabs obtained from 567 pregnant women at term or during labor were incubated for eight hrs in Todd Hewitt broth containing 15 micrograms per ml nalidixic acid, one microgram per ml polymixin, and 0.1 microgram per ml crystal violet (NPC broth). After streaking the swabs on blood agar plates, both plain broth cultures and their nitrous acid extracted pellets were tested with a commercial latex agglutination reagent. Beta-hemolytic colonies grown on the blood agar plates after overnight incubation were grouped with commercial latex agglutination and coagglutination reagents for reference identification. Sensitivities for the broth culture and nitrous acid techniques were 86.8 percent and 94.7 percent, respectively; specificities were 97.4 percent and 98.7 percent respectively. Nitrous acid extraction of vaginal broth cultures followed by latex agglutination testing can significantly shorten the time needed to detect group B streptococci, resulting in the intrapartum detection of these organisms.