Summary: The existence of the plasmid incompatibility group D was reaffirmed as a result of compatibility experiments done on plasmids R687, R711b, R778b and R840 which were previously tentatively accepted as constituting the group. The group was further delineated by the isolation of a phage, phage D, which adsorbed specifically to IncD plasmid- encoded pili produced by K12 strains and strains of and harbouring one of these plasmids. Plaque formation, like that of phage pilHα, was temperature sensitive in that plaques formed at 26 °C but not at 37 °C. Plaques were fairly clear, regular in outline and varied from pinpoint to about 1.5 mm in diameter on hosts where plaques were detected, but on the other hosts the plaques were more turbid and often irregular in outline. The phage did not plate (or propagate) on IncD plasmid-carrying strains of or The phage had an isometric hexagonal outline with a diameter of about 27 nm. It contained RNA and resembled two other RNA-containing phages, M and pilHα, by being sensitive to chloroform. It adsorbed to the sides of the very distal ends of the shafts of IncD plasmid-coded pili.


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