I. Life-Cycle. Colony Formation, Sporing, Papilla Formation.
If any bacterium is sown in the usual way from a fluid suspension on to a plate of suitable culture medium, colonies arise from the multiplication of single bacteria. These colonies grow at their margins ar a rate roughly constant for the species. When the colony has reached a certain size, which is again roughly constant for the species, it ceases to grow or continues growing very slowly. Thus for example colonies of streptococci grow to 1 mm. in diameter, those of coliforms to 3 or 6 mm. The colonies do not continue to grow until they have covered the whole plate. This limitation of growth is not due to the using up of the food supply or to the drying of the medium, since, if a second sowing of the same bacterium is made between the arrested colonies, a new crop of colonies arises.
1 Twort (1907) obtained lactose fermenting variants of B. typhosus after prolonged growth in this sugar. These variations do not however occur frequently and regularly. They are probably examples of mutation from a homozygous strain (Mode A above).
2 The variations mentioned take place in culture where their origin from papillae can be seen. It is likely that exaltation of virulence is an adaptive variation of the same kind occurring in a host.
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