When a histochemical stain for β-galactosidase activity is applied to growth of Gram-negative bacteria on agar medium, the pigmentation is non-uniform and capable of revealing internal colony organization into different cell types. Use of an strain with a thermo-sensitive repressor indicates that colonies expand by addition of new cells at the periphery and that older cells which have synthesized β-galactosidase early in development remain in the centre. Mixed inocula of different strains show clonal exclusiveness as they proliferate outwards. transposons can create genetic fusions that place β-galactosidase expression under a variety of regulatory systems. Stained surface cultures of and strains carrying insertions in plasmids reveal a variety of flower-like staining patterns. These patterns display both clonal (i.e. sectorial) and non-clonal (circular and radial) features which are heritable within a given strain. The non-clonal aspects of the patterns reflect phenotypic differentiation without genetic change. These observations indicate that bacterial growth on agar surfaces is a highly regulated process similar, in many respects, to the development of specific multicellular tissues and organisms.


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