Culture-based methods for detection and identification of Streptococcus agalactiae in pregnant women--what are we missing?

We aimed to compare BD group B Streptococcus differential agar (GBSDA) with inoculation into LIM broth and subculture onto blood agar plates (LIM-BA) as a method for GBS screening in pregnant women. First, we compared the detection threshold and the ease of use of GBSDA with LIM-BA by inoculating known numbers of GBS mixed with approximately 10(5) cfu of other bacteria. Second, we tested the production of carotenoid pigment in 155 GBS blood culture isolates. Finally, we compared GBSDA, LIM broth with direct GBS antigen detection (LIM-AG) and LIM-BA as methods for GBS screening in pregnant women. GBS colonies were easily detected on GBSDA at a threshold of 20 cfu. In contrast, GBS was not detected in a mixed culture from LIM broth with initial inocula of up to 100 cfu. Orange pigment was produced in 146/155 GBS blood culture isolates. Pigment was not produced in eight non-hemolytic and two hemolytic strains. GBS was detected by GBSDA in 58 out of 297 parturient women (19.5%) but in only 50 (16.8%, P > 0.05) and 46 (15.4%, P = 0.079) women by LIM-BA and LIM-AG, respectively. GBSDA is a satisfactory medium for the rapid detection of most GBS isolates, but other methods may be needed if detection of all non-pigmented strains is required.