Abstract
A new approach to electronically counting microbial colonies is described. An experimental counter based on the theory, with no provision for calibration or setting of thresholds, returned counts comparing excellently with manual ones even in the presence of stained chicken, corn, tuna and luncheon meat debris, or highly coloured backgrounds of chocolate, paprika and turmeric. At the same time it accommodated illumination variations equivalent to at least four f-stops. In a few cases, where colonies were extremely difficult even for humans to distinguish, the counter returned poor counts; however, the new working principle permits it to alert the operator to the subject's dubious quality. The counter discriminated easily between two bacterial species on one plate, distinguishing and separately counting Aeromonas hydrophila (browny-yellow) and Citrobacter freundii (light yellow). It also identified and counted A. hydrophila in mixed culture by following the biochemical reactivity of colonies transferred across three substrates, counting only those which were positive on all three.
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