SUMMARY: Detailed study of a representative paracolon strain showed that the delayed fermentation of 1% lactose coincided with the development of lactose-fermenting mutants, and that the essential difference between the normal non-lactose-fermenting (lac) organisms and the lactose-fermenting (lac) mutants was one of decreased permeability for lactose. Both types of organism showed the same degree of β-galactosidase activity when grown on lactose-free media; the permeability of the lac organisms was increased by raising the concentration of lactose in the medium. On the assumption that permeability is governed by a specific permease which is genetically distinct from β-galactosidase, a hypothetical scheme based on these two factors together with the presence or absence of hydrogenlyase suggested the existence of six phenotypes among the paracolon bacilli. In an examination of 51 naturally occurring paracolon strains four of these phenotypes were demonstrated. Since the lactose-fermenting paracolon organisms differ from coliform bacilli merely in being less permeable to 1% lactose, whereas the non-lactose-fermenting organisms differ from both the other groups in lacking β-galactosidase activity, it is suggested that the term paracolon should not be applied to this last group but should be restricted to the late-lactose-fermenters. Alternatively, these could be classed within the coliform group.


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