An instrument was developed to measure the concentration of bacterial suspensions by their electrical characteristics. It employed a square-wave signal generator, a tetra-polar electrode probe, and a voltage detector in the form of an oscilloscope. When electrical measurements were made on washed bacteria obtained from cultures or urine specimens, there was a direct relationship between the concentration of the cells and the electrical characteristics of the system as reflected by voltage changes. As little as 10(3) organisms per ml could be detected. The resolution between readings taken on samples containing 10(3) to 10(9) cells per ml was found to be a function of the input frequency. The maximal resolution between concentration readings was obtained at a input frequency of 10 Hz. Thus, with relatively simple instrumentation, bacterial concentrations could be determined within a few minutes. This technique, therefore, eliminates the more lengthy laboratory procedures as plate counts or the accumulation of measurable metabolic changes (such as the utilization of radioactive or other substrates). This method can efficiently monitor clinical urine specimens when a bacteriuria is suspected
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