ROUTINE microbiology has not kept pace with the advances towards rapid and automated methods that have occurred in clinical chemistry and in haematology. The long incubation periods necessary for the growth of bacteria both in isolation procedures and in specific secondary tests, such as antibioticsensitivity and biochemical tests, make the time saved by automation insignificant. Automation and speeding up of microbiological tests would therefore benefit from sensitive methods based on features observable at very low concentrations of organisms. Ur (19704 suggested that changes in the electrical impedance of a growing bacterial culture might be monitored to detect activity of organisms, the probable advantages of the approach being thehigh sensitivity offered by detection of minute impedance changes and the relative simplicity of the apparatus needed. We report the development of such a system (see also Ur and Brown, 1973) and some of its potential applications in microbiology. The method is based on the continuous comparison of changes in the electrical impedance of a growing culture with changes in the impedance of a control sample of sterile broth. The method, previously used for detection of other reactions, such as blood coagulation (Ur, 1970a and b), isolates the signal caused by the activity of the micro-organisms by cancelling a major part of the " noise " and " drift " from other processes, such as temperature fluctuation, evaporation of water, enzymatic reactions in the medium and accidental contamination, which are common to test and control sample. The apparatus may provide the basis for rapid automated systems in routine microbiology as well as providing results of theoretical interest. MATERIALS AND METHODSBacterial strains. These were strains of Staphylococcus aureus (including the " Oxford H " strain no. NCTC657 l), Escherichia coli, Klebsiella aerogenes, Pseudomonas aeruginosa, and Streptococcus faecalis. Inocula were prepared by emulsifying several colonies from an overnight blood-agar culture in the experimental medium and diluting the suspension in the same medium to provide the inoculum required. Viable counts of bacteria were made by the technique of Miles, Misra and Irwin (1938).Media. Peptone Water (Oxoid CM 9), Tryptose-phosphate Broth (Difco no. 0060), Nutrient Broth (Oxoid CM l), and PPLO Broth (Difco no. 0554) were prepared as directed by the makers. Glucose broth was prepared by adding sterile glucose solution to nutrient broth to a final concentration of 1 %. Supplemented PPLO broth was prepared by adding 10 ml of 25% yeast extract, 20 ml of horse serum, 2 ml of 0.1 % phenol red, and 1 ml of 10% arginine to 70 ml of PPLO broth. The pH was then adjusted to 7.0 with N HC1. All the media except the PPLO broths are commonly used in bacteriology.
Sizing normal RBC by an electronic method showed that with certain aperture currents a skewed bimodal size‐distribution curve is obtainable. In these studies some possible causes for such distributions were investigated. The experiments show that an electrical field can cause change in shape, size, and staining properties of RBC as well as lysis. Extrapolation of the results of these experiments to the conditions of the RBC sizing method indicate that these changes are not responsible for the skewed bimodal distribution. We believe, therefore, that normal RBC vary among themselves in some structural or physiologic way, not necessarily size, that was measured by this system.The use of aperture currents within the range that displays the bimodal distribution is helpful in diagnostic practice, because the bimodal size distribution enhances the discrimination of the system for detection of simultaneously existing different red cell populations.
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