The Growth and Respiration of Bacterial Colonies

Young colonies of two swarming organisms, Bacillus subtilis and Proteus vulgaris, grew about as quickly on solid media as in liquid culture whilst four non-swarming organisms, Bacillus cereus, Enterobacter cloacae, Escherichia coli and Staphylococcus albus, all grew slower on solid than in liquid media. Oxygen uptake by young colonies of B. subtilis, followed manometrically, increased exponentially at about the same rate as unrestricted aerobic growth. All other colonies demonstrated accelerating respiration which was either not strictly exponential or, in the case of S. albus, definitely biphasic, with a fast then a slow exponential rate of increase. Actual and potential respiration was determined for each species by measuring oxygen uptake before and after resuspending the colony in liquid medium. The ratio of actual to potential respiration was largest in the flat, spreading B. subtilis and smallest in the small, hemispherical S. albus. Calculations suggest that oxygen penetrates between 31 and 41 μm into colonies of B. cereus, Ent. cloacae and E. coli and only 9 μm into colonies of S. albus.