Increased recovery of group B streptococcus by the inclusion of rectal culturing and enrichment

Detection of intrapartum carriage of group B streptococcus (GBS) and subsequent antibiotic prophylaxis may prevent GBS infections in neonates. Because the gastrointestinal tract is the primary source of this organism, detection of carrier status requires both rectal and vaginal swabs. Vaginal swabs from 651 obstetric outpatients were plated onto 5% sheep blood agar. A second vaginal and a rectal swab were collected and incubated overnight in an enrichment medium of Todd-Hewitt broth containing antibiotics. By at least one method, 110 (16.9%) patients were positive for GBS. Only 31.8% of these positive patients were detected by direct culture of vaginal swabs. The use of vaginal swabs directly plated onto blood agar identified only three carriers not detected by another method. Inoculation of an enrichment broth with the vaginal swab and subsequent subculture detected 70.9% of the total. The use of both vaginal and rectal swabs with enrichment detected 97.3% of total GBS carriers. A subset of enrichment broths inoculated with vaginal and rectal specimens from 279 patients was tested for GBS by direct latex agglutination (Streptex; Murex Diagnostics, Inc., Norcross, GA, USA). Of the 90 broths that grew GBS on subculture, only 59 (65.6%) were positive by the direct agglutination method. The use of this method, although reducing processing time by 1 day, gave false-negative results for one-third of the GBS-positive broths. An accurate detection of the GBS carrier state can only be achieved by a combination of vaginal and rectal swabs incubated in enrichment broth and subcultured on blood agar.