Early Detection of Bacterial Growth, with Carbon-14-Labeled Glucose

A mmimum of one day is usually required to determine the presence of bacterial growth in blood cultures. We have developed a method for the detection of bacterial growth in blood cultures that is fast, objective, and sensitive. It is based on the incubation of the cultures in liquid media to which 14C-glucose has been added as the sole source of carbohydrate. The evolution of HC-dioxide is measured as an index of bacterial growth. Method 14C02 released by the bacterial metabolism of HC-labeled glucose is measured continuously with a commercially available monitor for radioactive gas. An air pump flushes the ionization chamber of this instrument at a rate of 9.5 liters per minute. The reaction flask is connected by means of Tygon tubing to form a closed system (Fig. 1). To record the radioactivity in the perfused gas, an analogue recorder is connected to the ionization chamber. The lower limit of sensitivity of this instrument is 0.0036 µCi of 14C (1). The culture medium was 20 ml of glucose-free thioglycholate broth (pH 7) and 0.5 µCi of 14C-dglucose (µ) (36 µg glucose). To this broth approximately approximately 10 organisms were added, and the flask was incubated at 37° C. The rate of 14C02 release was recorded continuously. Results More than 100 organisms of 15 species of bacteria from human infections were studied (TABLE I). All produced readily detectable quantities of 14dioxide. Usually there was little release of 14C during the first four to six hours of incubation, but after this time it evolved in large quantities. Figure 2 shows the rate of 14C02 released from a culture of Salmonella typhosa. The release of 14C02 was related to the pH of the media. In this example, the pH remained 7 for five hours. Between five and seven hours the pH decreased to 6 and between the seventh and eighth hours, to 5.5. The decrease in pH affects the release of CO2 from solution but has an adverse effect on bacterial growth. It was observed that slight agitation of the culture medium by air circulation in the system resulted in earlier release of 14C02. When the cultures were subjected to more intense agitation with continuous magnetic stirrers, the time required for 14C02 detection was reduced to one and a half to three and onehalf hours, depending on the organism (Fig. 2).