Infection of Competent Mycobacterium smegmatis with Deoxyribonucleic Acid Extracted from Bacteriophage B1

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A relatively competent state of Mycobacterium smegmatis for infection with deoxyribonucleic acid (DNA) extracted from phage B1 was found in the late log phase of bacterial growth. This state of the culture was used in quantitative studies on the infectivity of the DNA. The buoyant density of B1 DNA was 1.728 g/cc in CsCl, and 1 μg of the DNA produced 84 infective centers, the phage equivalent of which was 1.5 × 10−8. The infectivity was destroyed by catalytic amounts of deoxyribonuclease but not by specific B1 antiserum. Tween 80, which prevents phage adsorption, did not prevent DNA infection. The response of plaque-forming ability to DNA concentration suggested that two or more molecules are required to initiate an infective center. The low efficiency of DNA infection in mycobacteria was considered to be caused by a limiting population of competent cells in the culture employed; in this experiment less than 10−5 of the cells were infected with DNA. A typical cycle of infection was observed, although the latent period was prolonged and the burst size reduced after DNA infection. The transition of B1 DNA infection to deoxyribonuclease insensitivity had a lag period of about 10 min, and increased linearly with a velocity of about 0.24 infective centers per min per μg of DNA. Half of the infective titer was inactivated by heating at 92 C for 15 min. The melting temperature was about 96 C. Species barriers were not crossed by B1 DNA; however, the DNA was infectious for a B1-resistant mutant of the host.

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