[Detection of colonization by Streptococcus agalactiae: prospective study comparing real-time gene amplification with...

We carried out a prospective study including 554 vaginal swabs simultaneously tested for antenatal screening of Group B Streptococcus (GBS or Streptococcus agalactiae) using culture on the chromogenic medium Strepto B ID (Biomérieux, Marcy l'Etoile, France) and real time gene amplification on LightCycler (Roche Applied Science). We centrifuge the swabs with "SETS" device and separate centrifugates in 2 parts: one for the culture and the other one for molecular biology. First half of the centrifugate is inoculated onto Todd-Hewitt broth enriched with antibiotics. This broth is incubated to 35 degrees C during 24 hours and then subcultured on a Strepto B ID medium. This last one is incubated during 24 hours to 35 degrees C in capnophilic conditions before interpretation. DNA extraction for molecular biology is simply obtained by heating the microtubes to 95 degrees C in a water bath. The cfb gene is amplified, allowing a specific gene amplification of GBS even within a polymorphic flora. The concordance between both methods is 94.8%. The sensitivity and negative predictive values obtained are respectively 88.0 and 97.4% for real time PCR and 83.0 and 96.4% for culture on Strepto B ID. Both methods are thus concordant, with equal sensitivity and valid for detection of GBS colonization in pregnant women. However real time gene amplification allows reducing turn around time since molecular biology process (extraction+amplification) does not exceed 1 hour.