Comparison of Selective Broth Medium Plus Neomycin-Nalidixic Acid Agar and Selective Broth Medium Plus Columbia Colistin-Nalidixic Acid Agar for Detection of Group B Streptococcal Colonization in Women

Despite the availability of effective preventative antimicrobial therapy, group B streptococci (GBS) remain as the leading bacterial cause of significant neonatal morbidity and mortality in the United States (4, 7). The ability to intervene in the intrapartum transmission of GBS from mother to child is based either on the recognition of maternal risk factors at the time of delivery or on direct identification of maternal GBS carriers through prenatal screening of cultures obtained at 35 to 37 weeks of gestation (4). In either case, the institution of appropriate antimicrobial prophylaxis has been shown to reduce the incidence of early-onset GBS disease by 69 to 86% (3). The effectiveness of a prenatal screening culture procedure is dependent on a number of factors, including the timing of specimen collection, the source of the specimen(s), and the culture methodology used to isolate GBS from clinical sources. In that regard, a combination of vaginal and rectal specimens has been shown to provide superior detection of maternal GBS carriage as long as specimens are obtained as close to delivery as possible (6). In a recent report, Dunne and Holland-Staley (5) determined that the use of a neomycin and nalidixic acid (NNA) agar plate in combination with a selective broth medium (SBM) improved the sensitivity of genital and rectal cultures for the detection of GBS colonization in women by more than 14% over either medium alone and often provided results 24 h sooner than cultures obtained with SBM only. However, because of the cost of NNA agar ($0.53/plate), a study was initiated to determine whether Columbia colistin and nalidixic acid (CNA) at $0.22/plate would be an equally sensitive but less expensive alternative. Based on the average yearly volume of GBS screening cultures received in the Henry Ford Hospital microbiology laboratory, a savings of $0.31 per culture would translate into a reduction of nearly $3,000 in annual controllable direct expenses.

For this evaluation, 580 consecutive specimens submitted to the Henry Ford Hospital microbiology laboratory specifically for detection of GBS were included in the study. Specimens were received from 1 October 1998 through 31 December 1998 and included 536 vaginal and cervical, 20 rectal, and 24 vaginal and rectal swabs transported in Amies medium with charcoal (Copan Diagnostics, Inc., New York, N.Y.) or in Stuart's medium (Baxter Healthcare Corp., Deerfield, Ill.). All specimens were inoculated onto blood agar plates containing 30 mg of neomycin and 15 mg of nalidixic acid (NNA; Becton Dickinson Microbiology Systems, Cockeysville, Md.) per liter and onto CNA blood agar plates containing 10 mg of colistin and 10 mg of nalidixic acid (Becton Dickinson) per liter. If a single swab was received it was submersed in Todd-Hewitt broth supplemented with 8 mg of gentamicin per liter and 15 mg of nalidixic acid per liter (SMB; Becton Dickinson) as described by Baker et al. (1, 2). If two swabs were received, one was used for direct plating while the other was placed in SBM. The order in which the NNA and CNA plates were inoculated was randomized. All plated media were incubated overnight at 35°C with 5% CO₂. SBM were incubated at 35°C in an ambient atmosphere. On the following day, the NNA plates were examined by laboratory personnel according to our current standard protocol, while the CNA plates were evaluated by the investigator for colonies typical of GBS. Isolates were identified by Gram staining, by catalase reaction, and either by the CAMP test or by a particle agglutination assay (Streptex; Murex, Dartford, Kent, United Kingdom) depending on the number of isolated colonies present. Negative plates were reincubated and reexamined the following day before being discarded. If the NNA plate showed no evidence of GBS, the SBM component of each culture was subcultured by laboratory personnel onto a blood agar plate and incubated for 18 to 24 h at 35°C with 5% CO₂ prior to examination. As with the plated media, all SBM subcultures were examined for GBS and identified by the same methods. Negative SBM subcultures were reincubated and reexamined the following day. Laboratory personnel were blinded to the results of the CNA cultures, whereas the investigator was blinded to the results of the NNA and SBA cultures until the CNA cultures had been examined and the results had been recorded.

A total of 95 of 580 (16.4%) specimens were positive for GBS, including 91 of 536 (17%) vaginal and cervical, 2 of 20 (10%) rectal, and 2 of 13 (15.4%) vaginal and rectal specimens. Of the 95 positive specimens, GBS were recovered from 22 specimens on NNA but not on CNA; from 9 specimens on CNA but not on NNA; from 52 specimens on both NNA and CNA; and on subcultures of SBM from 12 specimens. Interestingly, 7 of the 9 isolates of GBS recovered from CNA only were not recovered from subcultures of SBM. Based on these findings, the overall sensitivity of CNA alone for the recovery of GBS from maternal screening cultures was 64.2%, compared to 77.9% for NNA alone (P < 0.05 by χ² analysis). The combination of NNA and SBM produced a sensitivity of 90.5% for the detection of GBS from screening cultures, whereas the tandem of CNA and SBM produced a sensitivity of 76.8% (P < 0.05)— slightly less than the sensitivity of NNA alone. Furthermore, the combination of NNA and SBM was statistically significantly more sensitive for the detection of GBS than NNA alone was (P < 0.005).

In a previous study (5), we determined that the combination of a direct plating method with selective broth amplification was beneficial for two reasons. First, direct plating allowed the identification of isolated GBS within 2 days of specimen receipt, whereas GBS recovered from SBM required a minimum of 2 days, and sometimes 3 days, before identification was complete. Second, direct plating reduced the potential of enterococci competitively overgrowing and masking the presence of GBS in the SBM, and this ultimately increased the sensitivity of the assay by nearly 15%. While the recovery rate of GBS in this study was not as high as in our previous report (16.4 versus 22.2%), the results clearly indicate that CNA is not equal to NNA for the isolation of GBS. The higher concentration of nalidixic acid in NNA or the combination of nalidixic acid and neomycin may better suppress genital or rectal flora than the colistin and nalidixic acid content of CNA and thus allow greater recovery of GBS. Regardless of the mechanism, I conclude that the more costly combination of NNA and SBM is best suited for the detection of GBS colonization in women.

• Copyright © 1999 American Society for Microbiology