Abstract
We evaluated the performance of a commercially prepared selective medium, Vi GBS agar, for the screening of Streptococcus agalactiae from vaginal and anorectal specimens from pregnant women. Ninety-seven out of 98 clinical isolates of S. agalactiae grew well on Vi GBS agar and exhibited characteristic red-purple colonies within 24 h of incubation. Few microorganisms that comprise the normal flora of the vagina and rectum grew on Vi GBS agar. Some strains of Enterococcus spp., Streptococcus mitis, Pseudomonas aeruginosa, and Candida spp. grew on Vi GBS agar; however, these colonies were nonpigmented or very small, and could be clearly distinguished from those of S. agalactiae. A total of 287 swab specimens from the vagina and anorectum were directly plated onto Vi GBS subsequent to growth on sheep blood agar/BTB lactose agar biplate media (BA/BTB), and then incubated overnight at 35ºC. Of these, 98.6% (283/287) of the isolates were S. agalactiae and matched those of the conventional culture. Out of the four discrepant results, two false negatives were from specimens with low bacterial counts, and the remaining two were successfully isolated on Vi GBS agar, but not on BA/BTB. Vi GBS agar recovered S. agalactiae from six stool specimens that were artificially mixed, without the inhibition of its growth by Enterococcus from the stool. Our study suggests that direct plating onto Vi GBS agar shows high performance for the screening of S. agalactiae colonization in pregnant women.
