SUMMARY: Growth of colonies of on trypticase soy agar (TSA) and glucose-salts agar (GSA) is described in terms of diameter, height and viable count and of calculated volume and viable cell density. All increased with time except viable cell density, which was constant for colonies at 3·11 x 10 cell/mm on TSA and 10·08 x 10 cells/mm. on GSA. Colony diameter increased linearly at 0·0585 mm./h. on TSA and 0·0594 mm./h. on GSA, but the modes of growth were very different. Colonies on TSA increased in both diameter and height while those on GSA increased mainly in diameter, so colony volumes on the two agars differed. Colony diameters of and eight Gram-negative rods also increased linearly with time. We propose that a linear increase of colony diameter with time represents the steady state growth of bacterial colonies.


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