Comparison of different culture media and storage temperatures for the long-term preservation of Streptococcus pneumoniae in the tropics

Objective The preservation of Streptococcus pneumoniae by standard freezing methods for subsequent tests such as serotyping and antibiotic susceptibil ity - is not possible or is difficult in many developing countries because of the high cost of equipment, inadequate equipment maintenance, and irregu lar power supply. We evaluated alternative low-cost methods, by comparing d ifferent culture media and storage temperatures. Methods Clinical isolates of five capsular types (1, 5, 7, 19, and 23) of 5 pneumoniae were preserved in rabbit blood, sheep blood, skimmed milk, or g lycerol-chocolate broth, and stored at -20 degreesC or -70 degreesC. The cu ltures were also preserved by lyophilization or sand desiccation, followed by storage at room temperature and 4 degreesC. The viability of the preserv ed cultures was determined by making serial colony counts on day 0 and afte r 1 week, 4 weeks, 4 months and 16 months. The viability of cultures preser ved by sand desiccation and storage at 4 degreesC was also determined every 6 months for up to 68 months. Findings Irrespective of the media used, cultures maintained at -20 degrees C became nonviable by the fourth month, while those maintained at -70 degre esC were still viable at 16 months. Cultures preserved by lyophilization or sand desiccation lost their viability by the fourth month when maintained at local room temperature (30-42 degreesC), but remained viable when stored at 4 degreesC for up to 68 months. Conclusions Our results confirm that freezing at -70 degreesC, or lyophiliz ation and storage at 4 degreesC are the ideal methods for the preservation of 5 pneumoniae. In laboratories where lyophilization is not feasible, sand desiccation and storage at 4 degreesC offers an alternative low-cost metho d for the long-term preservation of S. pneumoniae.