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Journal of Clinical Microbiology, August 1998, p. 2298-2300, Vol. 36, No. 8
Microbiology Division, Department of
Pathology, Henry Ford Health System, Detroit, Michigan
Received 9 March 1998/Returned for modification 9 April
1998/Accepted 27 April 1998
In 1996, the Centers for Disease Control and Prevention recommended
the use of a selective broth culture for the improved detection of
genital tract or anorectal carriage of group B streptococci (GBS) in
pregnant women. In order to verify this recommendation in our
laboratory, we compared the sensitivity of Todd-Hewitt medium with
gentamicin and nalidixic acid (SBM) with our current method of direct
plating on blood agar medium containing neomycin and nalidixic acid
(NNA). Five hundred consecutive cervicovaginal and anorectal specimens
submitted for GBS culture were included in the study. Swabs were plated
onto NNA and the swabs were immersed in SBM, followed by overnight
incubation at 35°C. On the following day, the NNA plates were
examined for colonies typical of GBS and the organisms were identified
by the CAMP test or by latex agglutination. SBM cultures were
subcultured onto blood agar and CNA agar plates, and the plates were
reincubated for 24 h. Negative specimens from either medium were
incubated for an additional 24 h and were examined again before
finalization of the results. GBS were recovered from 78 specimens by
both methods; from SBM only for 17 specimens (sensitivity, 86%) and
from NNA only for 16 specimens (sensitivity, 85%). A moderate to heavy
growth of Enterococcus faecalis was observed on plates
containing NNA-positive, SBM-negative specimens. Competitive growth
studies suggested that E. faecalis suppressed the growth
potential of GBS in SBM. Our study suggests that direct plating on NNA,
as a single method, is equivalent in sensitivity to SBM for the
recovery of GBS, and the results are often available 24 h sooner.
However, it appears that both direct plating and selective broth
amplification techniques are required for the maximum level of
identification of colonization with GBS in pregnant women.
0095-1137/98/$04.00+0
Copyright © 1998, American Society for Microbiology. All rights reserved.
Comparison of NNA Agar Culture and Selective Broth
Culture for Detection of Group B Streptococcal Colonization in
Women
*
Corresponding author. Mailing address: Microbiology
Division, Henry Ford Health System, 2799 W. Grand Blvd., Detroit, MI
40202. Phone: (313) 876-2341. Fax: (313) 556-8309. E-mail:
mdunne1{at}hfhs.org.
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