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The Type A Phages of Salmonella typhimurium: the Significance of Mixed Clones Arising from Singly-Infected Bacteria
Abstract
Summary: Salmonella typhimurium Q1, when grown in standard medium at a temperature of 37°, is a uninuclear organism in which nuclear division precedes cell division, so that some 25 to 30% of the bacteria show various stages of nuclear division. When incubated at a lower temperature (e.g. 25°) nuclear division outruns cell division, and binucleate and quadrinucleate forms appear. In experiments at 37°, infection of Q1 or superinfection of Q1 (A1 a) with temperate phage A1 b temporarily arrests bacterial division, producing a lag period of 30–45 min. Decision to lyse is taken from 1 to 9 min. after exposure to infection, and the ensuing productive development leading to the release of a brood of temperate phage particles is an immediate sequel of this decision. Decision to lysogenize, as evidenced by immunity to a heavy challenge by homologous virulent phage, is taken within one minute of infection. 60 to 70% of bacteria infected or super-infected with a single phage particle produce pure clones of lysogenized, prophage-substituted, or doubly lysogenized bacteria. It is suggested that the mixed clones of uninfected and converted bacteria derived from the remaining 30 to 40% may result from the lysogenization of one-half of a nucleus already embarked on the process of division; evidence favouring this theory is discussed. It is concluded that, contrary to current belief, and under the conditions of our experiments, the infective material of a phage particle which has penetrated a sensitive bacterium proceeds with little delay either to become integrated with the chromosome as prophage or to undergo productive development and produce lysis, and does not remain for one or more bacterial generations an unattached cytoplasmic inclusion.
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© Society for General Microbiology 1964