Modified Granada Agar Medium for the detection of group B streptococcus carriage in pregnant women

Objectives To improve the detection rate of group B streptococci (GBS) in pregnant women, aiming at the prevention of early-onset septicemia in the newborn. Methods The yield from culturing two sites, vaginal and anorectal, on a Modified Granada Medium (MGM) was compared with our standard approach of culturing a vaginal swab on blood agar (BA). Results Samples were processed from 430 consecutive pregnant women. GBS was isolated from the vagina in 11.6% with BA, and in 13.7% with MGM. In 17.0% of anorectal samples, GBS was identified with MGM. The combination of both sites and media had a yield of 20.0%. MGM identified all but six (2%) of 310 GBS strains after aerobic incubation, with use of a cover slide, and missed only three strains (1%) after anaerobic incubation. Conclusions Separate culture of vaginal and anorectal samples using the same MGM agar plate resulted in an increase in detection rate for GBS of 76% as compared to BA alone. The technique is simple and results are available after overnight incubation. MGM was confirmed as a specific medium for the identification of GBS, with a sensitivity of 98-99%.