CYTOLOGICAL ANALYSIS OF ULTRAVIOLET-IRRADIATED ESCHERICHIA COLI III

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Abstract
Cytological changes following the action of UV irradiation, of furacin and of hypertonic NaCl-agar are described for E. coli strain B and its resistant mutants B/r and B/F. UV irradiation is followed by aggregation of nuclear chromatin in the centers and along the axes of the cells and growth with cell division in all 3 strains. In strain B the sequence goes on to long filaments, skeins and radially enlarged cells which only return to normal morphology 6 or more hours after exposure. In strains B/r and B/F, shorter filaments are formed and the culture returns sooner to normal morphology. The survivors of cultures plated on critical concentrations of hypertonic NaCl-agar show equivalent aberrations, as do strains B and B/r on exposure to sublethal concentrations of furacin. In strain B/F, brought to high resistance to furacin by prolonged growth in the drug, minimal changes follow exposure to any tolerated dose of furacin. The initial aggregation of nuclear chromatin is compatible with a mechanism described by others for micro-organisms subjected to a variety of injurious agents. This alteration in configuration of chromatin, inhibition of cell division, appears to be a secondary consequence of primary injury. Differences between strain B and its mutant resistant strains (except resistance of B/F to furacin) seem therefore to be greater reparative forces in the resistant strains rather than invulnerability to the primary injury. Available evidence suggests that initial UV injury is sustained by nuclear DNA and perhaps other sites, and that the cytological changes are secondary consequences. Strain B/F may have mutated to true invulnerability to this drug.