Characterization of a nocardiophage for Nocardia restrictus.

Abstract

Summary A nocardiophage, called Ri, was isolated from soil samples. It lysed Nocardia restrictus and a few other species of Nocardia, but none of the eight streptomycetes tested. On N. restrictus phage Ri produced plaques very irregular in shape, size and turbidity; replacement of NaCl by Ca(NO3)2 in the medium increased the uniformity of plaque morphology and gave higher counts. Electron microscope examination of the phage particles revealed a hexagonal head, 75 nm. in diameter, and a long, flexible, non-contractile tail, 330 × 10 nm., bearing a round terminal plate; a collar was sometimes visible in particles with empty heads. The phage nucleic acid is most probably double-stranded DNA. Maximum viability of phage was at pH 7; the survival was less than 0·001 % for an exposure of 30 min. at pH 5; about 20% survived at pH 6, and 10% at pH 8. When exposed to 55° or to ultraviolet irradiation, phage Ri showed similar survival curves, and was very sensitive to both treatments. Adsorption of phage to its host was rapid and efficient (80% in 5 min.); the latent period was 28 min., the rise period occurred between 28 and 37 min., and the burst size was 61. Mutation of N. restrictus to resistance to phage Ri was very rare; the apparent mutation rate was 3·4 × 10−11, which can be partly attributed to the lower growth rate of the resistant mutants. In chromium-shadowed preparations, N. restrictus appeared as short rods, 1·3 × 0·5 μ., dividing by transverse fission, showing no lateral branching characteristic of the genus Nocardia, and very few chains (never longer than 2 or 3 cells); thin sections of infected cells showed a few intracellular phage particles.