Saponin, an inhibitory agent of carbon dioxide production by white cells : its use in the microbiologic examination of blood components in an automated bacterial culture system

BACKGROUND: Blood components with a white cell count >100 x 10(9) per L may cause false-positive results when the BacT/Alert system is used for the microbiologic examination. The effects of different concentrations of saponin on bacterial growth and on carbon dioxide production by blood fractions with a high white cell count, in particular peripheral blood progenitor cells and buffy coats, were investigated. STUDY DESIGN AND METHODS: The effect of saponin on carbon dioxide production was studied by adding different fractions of white cell-rich material (buffy coat or leukapheresis material) to BacT/Alert culture bottles with or without saponin and incubating these bottles. Five bacterial strains were used to inoculate the culture bottles at four levels ranging from about 10(3) colony-forming unit per mt to about 103 colony-forming units per mt. Aerobic and anaerobic bottles with and without saponin were used. RESULTS: It was demonstrated that the addition of 0.5 percent saponin to BacT/Alert culture bottles effectively inhibited carbon dioxide production, without affecting bacterial growth. CONCLUSION: Saponin at a concentration of 0.5 percent is a valuable additive to BacT/Alert culture media because it prevents false-positive results in the examination of white cell-rich blood components.