ABSTRACT
Large numbers of bacteriophages have been found wherever potential host bacteria are found; e.g., in soils, lakes, hot springs and deep sea vents, or associated with commensal bacteria inhabiting plants and animals; phage ecology is discussed in some depth in Chapter 6. About 10 times as many phages as bacteria have been detected in seawater samples, leading to estimates of about 1032 phages on earth (Bergh et al., 1989; Wommack and Colwell, 2000). Phages may be incredibly varied in their properties; e.g., their hosts, genetic content, regulatory mechanisms, physiological effects, and most of the genes in the larger phages correspond to nothing yet described in the currently available databases. There is wide interest in how viruses arose; i.e., whether they are representatives of early pre-cellular forms of life or are sophisticated forms of selfish genes derived from modern genomes, how they acquired their special properties and genes, and how they relate to each other and to cellular genomes. The evidence so far indicates that the tailed phages, at least, are of very ancient origin. As discussed below, some phageencoded enzymes like T4 thymidylate synthase appear to have diverged from the precursor of their bacterial and eukaryotic relatives before those two diverged from each other. So far, we have only begun to examine an infinitesimal sample of the phages infecting common, culturable bacteria. We can only look with fascination at the variety of tailed phages in aqueous and soil habitats, where most of the bacteria cannot yet be cultured to use as hosts for isolating phages. The various groups of tailless phages have been less widely studied; they seem to be less numerous, but they also are harder to distinguish from nonviral elements and algal viruses in the environment.
