Efficacy of STGG-transfer medium for Streptococcus pneumoniae: an experimental evaluation

  Objective  To evaluate the efficacy of skim milk-tryptone-glucose-glycerin (STGG) – a transport medium for Streptococcus pneumonia (Spn).

  Methods   Bacterial suspensions of Spn at three dilution levels were prepared with STGG and Todd-Hewitt yeast broth containing 20% glycerol (THY) cryopreservation solution separately. All the suspensions were stored at 4 °C for 48 hours and one week, at – 80 °C for one month and two months. Plate culture was adopted to assess the difference in colony numbers of preserved Spn with the two medium after repeated freezing and thawing; the variation in colony numbers was also observed for the preserved Spn with STGG stored at – 20 °C and – 80 °C for different period.

  Results  The number of Spn colonies changed only by 1.9% after storage with THY cryopreservation medium and STGG at 4 °C for 48 hours compared to that before the storage; while after the storage of one week, the Spn colony number dropped by 70.9% for the preservation with THY cryopreservation medium and 48.3% for with STGG. No significant difference was observed in survival rate of Spn preserved with STGG medium at different dilution levels stored at – 20 °C and – 80 °C for one month (P > 0.05), but the survival rate was significantly higher for the Spn stored at – 80 °C than that at – 20 °C for 2 months (P < 0.05). For the Spn preservation stored at – 80 °C, STGG freezing and thawing 5 times resulted in a change from 95% – 102% in the number of colonies.

  Conclusion  Compared with THY cryopreservation solution, STGG medium results in a better efficacy for Streptococcus pneumoniae preservation, especially when stored at a temperature of – 80 °C.