Rapid, colorimetric method for the detection of microorganisms in blood culture

A semiautomated instrument (Septi-Screen) and method are described that semiquantify bacteria colorimetrically in blood-inoculated cultures in 2 to 8 h. If the blood culture is positive, concentrates (of viable bacteria) are obtained free of blood cells. In pediatric cases and in septicemia attributed to indwelling catheters where bacteremia is often manifested by high bacterial counts, the total time required to obtain bacterial concentrates is only 2 to 3 h. In brief, at various intervals after inoculation of 10 ml of blood into 100 ml of culture fluid, 25-ml blood culture samples are passed through the Septi-Filter. Erythrocytes and leukocytes pass into the filtrate and are discarded, but bacteria are electrostatically retained on the filter surface. The entrapped bacteria are backflushed and recovered as a concentrate that is processed through a second filter (Septi-Count) for colorimetric quantitation. The bacteria are stained, and the filter fibers are decolorized. The color intensity of the filter surface is proportional to the number of bacteria in the concentrate processed. The total Septi-Screen processing time is 3 to 5 min.