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Radiometric Assay of Bacterial Growth: Analysis of Factors Determining System Performance and Optimization of Assay Technique

Department of Radiology & Radiation Research Laboratories, University of Iowa, Iowa City, Iowa

Correspondence: For reprints contact: Peter T. Kirchner, MD, Dept. of Radiology, University of Iowa Hospitals & Clinics, Iowa City, IA 52242.

ABSTRACT

A quantitative technique for the measurement of ¹⁴CO₂ released from a bacterial culture was evaluated. The technique uses liquid scintillation counting to record ¹⁴CO₂ accumulation on a fluor-impregnated filter paper within a double-chambered scintillation vial that also houses the bacterial growth medium. We have successfully identified and corrected the major causes for a variably low detection efficiency, and also established the optimum mixture of reagents for the detection system. Incorporation of Triton X-100 into the scintillation fluid used for the detector reduced the variability between identical assays in a single batch from 50% to 5%, and, in conjunction with an increase in the scintillator concentration, raised the counting efficiency from 30% to 70–88%. The response of the improved detector is linear over a wide range of count-rates. Another significant modification was the interchange of growth and detector chambers. Overall, a 40-fold increase in count-rate during the exponential phase of bacterial growth was obtained by improving ¹⁴CO₂ detection efficiency, increasing the rate of ¹⁴CO₂ transfer from liquid to gas phases and enlarging the growth supporting capacity of the detector system. The minimum detection time for bacterial growth was shortened and the exponential phase of bacterial proliferation was lengthened by at least 2 hr. High counting efficiency, precision, and linearity make the improved detector a sensitive and reliable tool for radiometry of bacterial growth and metabolism.

FOOTNOTES

^* Present address: Faculty of Medicine, Ramathibodi Hospital, Bangkok, Thailand.

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