Conditions for the Existence of Bacteriophage

Bacteriophages (phages) are viruses whose hosts are bacteria. The ecology of bacteria and their phages has been studied less thoroughly than that of many higher organisms. The experimental data on their behavior in the laboratory are as extensive as for almost any other living things. Experimentation has been oriented primarily at answering basic biological questions, with phage serving as a convenient tool for technical reasons. Several facts have emerged from it, however, which are important to an understanding of the position of phages in the biological world and their' evolution and survival in nature. We shall first describe some general properties of phages, and then list some specific facts which are now sufficiently well established to serve as a basis for critical discussion. We shall then attempt an analysis and evaluation of the possible factors which may be responsible for the perpetuation of phage. It is convenient to start by classifying phages into two types: Virulent phages can reproduce only by destroying their host. Temperate phages are in addition capable of multiplying indefinitely as intracellular parasites within the same cell lineage. The distinction can be made only with respect to a given host species; a phage may be temperate on one host and virulent on another (Bertani, 1958). Also, virulent phages can arise as mutants of temperate phages. When a virulent phage infects a bacterium susceptible to it, the cell dies, produces more phage, and lyses. When a temperate phage infects a sensitive cell, the result is sometimes the same as with a virulent phage. However, in a fraction