The morphology and physiology of bacteriophages as revealed by the electron microscope

Abstract

SYNOPSISThe electron microscope can reveal details of both the infective process and the morphology of bacteriophages. Various standard specimen‐preparation methods are available for these studies: negative staining is the most valuable method for morphological aspects and the sectioning of bacteria for studying intracellular multiplication. Phages fall into six distinct morphological groups. Detailed examination of these reveals a variety of forms showing variations within each group. All phages have one thing in common, the possession of symmetry; the heads have recognisable geometrical shapes and the tails, when present, possess morphological subunits usually clearly arranged in a regular fashion.Phage morphology is important not only as a fundamental characteristic, but also as a major criterion for taxonomic purposes. However, some phages, which are morphologically similar, can infect bacterial hosts in different genera or families so that additional criteria such as host range have to be used in considering a possible system for the classification of bacteriophages.The electron microscope is particularly suitable for providing information about the adsorption and intracellular multiplication stages of the infective process. Shadowing and negative staining show that tailed phages adsorb tail first, and that tail‐less ribonucleic acid phages attach to bacterial fimbriae in large numbers. Sections of infected bacteria reveal cytological changes within the cell during intracellular multiplication.In addition, the electron microscope can be used to investigate the natural habitats of phages. In one of these a particle with an unusual “life cycle” was found associated with the genus Bacillus. It normally exists in the prophage state in a lysogenic host. It is active on indicator strains, but only kills them and cannot multiply within them.These various aspects are reviewed in detail with particular reference to information provided by the electron microscope.