Fine structure and radiation resistance in Acinetobacter: a comparison of a range of strains.

Nine strains of the Gram-negative bacterium, Acinetobacter, showed a wide variation in resistance to ionizing radiation; all gave sigmoid survival curves, with D10 values for the exponential portion ranging from 70 to 460 J kg"" (7-46 krd). The fine structure of these strains was studied by electron microscopy. Results for a resistant strain were described earlier and the present paper gives comparative results for the other 8 strains. The mode of division varied, 5 strains dividing predominantly by constriction of all the layers of the cell wall, while the other 4strains showed ingrowth of thick septa. These 4 included the 3 most resistant strains and 1 strain of intermediate resistance. The arrangement of surface layers was the same as that usually found in Gram-negative bacteria. In 1 strain an extra layer was visible outside the outer membrane; this layer does not appear to influence radiation resistance since it is lacking in another strain of similar resistance. The layer of wrinkled material, previously observed in the resistant strain between the outer membrane and the intermediate dense layer of the cell wall, in negatively stained preparations of isolated cell walls, was seen in 5 other strains of intermediate and high resistance, while in 3 sensitive strains finely granular material appeared to occupy a corresponding position in the cell wall. These observations suggest that morphological features, such as the wrinkled layer of the cell wall, and possibly the mode of cell division, may influence the radiation resistance of Acinetobacter strains, but their function is not yet known.