Factors in the preparation of biological indicators that affect the decimal reduction time

The ISO 11138 series on biological indicators (BIs) lacks concrete specifications of the suspension, carrier material, and culture medium optimal for the preparation of the microorganisms. The suspension and culture medium used are thought to influence the accuracy of decimal reduction time (D) values. Their combined effects have not been elucidated, but some individual effects of particular suspensions, carrier materials, and culture media have been reported. Contrary to these reports, however, the author did not find such individual effects in a study of D values obtained using distilled water or saline as the suspension solution, paper or cotton yarn as the carrier material, and soybean casein digest (SCD) broth or agar as the culture medium. The study demonstrated that the combined effect of salt in the suspension during drying and the entrapment of microorganisms in salt crystal by pores in the cotton yarn carrier material increased microorganism resistance. D values also differed depending on the culture medium. Further investigation is needed to establish the optimal culture medium for BI microorganisms.