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The colonial growth of Escherichia coli and Staphylococcus aureus on agar solidified medium (a food model system) was estimated by using a microbial calorimeter. Changes in the metabolic heat of the colonial growth over time (the growth thermogram) were obtained with good reproducibility. The actual heat evolution curves [f(t) curves] obtained from the thermograms of growing colonies gave good agreement with the changes in viable cell numbers in colonies under the same conditions. The f(t) curve was also compared with the process of colony formation. During the early period of the f(t) curve (the period of 0 10h after inoculation), colonies could not be observed, but at 15h after inoculation, some small colonies were visible. During the period between 15h and 25h, the numbers and size of colonies increased. During the later period of the f(t) curve (the period of 25 34h), although the size of colonies still increased, the colonial profiles on LBA medium were almost similar in spite of the continued evolution of growth heat, indicating that the direction of colony growth shifted from being predominantly radial to predominantly .vertical with regard to the agar surface. From the logarithmic f(t) curve, the exponential growth phase was estimated to be the period between 8.5 h and 13.5 h after inoculation. The growth rate constants (ƒÊ) for colonial growth obtained from the f(t) curves were 0.643 •} 0.037 h-1 for E. coli and 0.597 •} 0.05811-1 for S. aureus. The suppressive activities of salts and kumazasa (Sasa albo-marginata Makino et Shibata) extracts on colonial growth were evaluated in terms of ƒÊi' ƒÊm' derived from the f(t) curves. The MIC (minimum ihhibitory concentration) values for NaCI were estimated to be 9.7 •} 1.5%, equivalent to a water activity (Aw) of around 0.906, against E. coli colonial growth and 12.8 •}1.4%, equivalent to an Aw of around 0.882, against S. aureus. The MIC value for kumazasa extracts was estimated to be 4.6 •}0.6% (1.79 mg of non-volatile components per 1 ml) against S. aureus colonial growth. These values were in good agreement with those obtained by the agar plate dilution method. The study of bacterial growth on solid medium, as opposed to traditional liquid culture, should provide a model system more representative of the food environment. Microbial calorimetry is a method suitable for non-destructive growth measurements of microbial colonies on solid medium.

Evaluation of the Growth Activity of Escherichia coli and Staphylococcus aureus Colonies on Solid Medium Using Microbial Calorimetry