Evaluation of culture methods for isolation of group B streptococci

Attempts were made to isolate group B streptococci (GBS) from 382 urethral and 140 rectal specimens by overnight incubation in three variants of Todd Hewitt broth, followed by subculture on human blood agar plates. All three broths prepared contained 5% sheep blood, but one contained no antibiotics (THB), while the other two contained gentamicin and nalidixic acid at concentrations of 0.8 microgram/ml and 1.5 micrograms/ml (THBL), and 8 micrograms/ml and 15 micrograms/ml (THBH), respectively. THBL gave the highest isolation frequency both for urethral and rectal specimens. While THBH was superior to THB for isolating GBS in rectal specimens, for urethral samples they were equally effective. Direct plating of 154 specimens on blood agar was less effective than was enrichment in any of the three broths followed by plating on blood agar.