Bacteriophages

phages, bacterial viruses that cause the disintegration (lysis) of bacteria and other microorganisms. Bacteriophages reproduce in cells, lyse them, and enter other—generally young—growing cells. Transmissible lysis of bacteria (anthrax bacilli) was first observed in 1898 by the Russian microbiologist N. F. Gamaleia. In 1915 the English scientist F. Twort described the same phenomenon in suppurative staphylococci. In 1917 the French scientist F. d’Hérelle called the lytic agent passing through bacterial filters a “bacteriophage.”

Structure and chemical composition. The particles of many bacteriophages consist of a round, hexagonal, or rod-shaped head 45–140 nanometers (nm) in diameter and a process 100–200 nm thick (see Figure 1). Other bacteriophages do not have a process; some are round, others threadlike, 8 x 800 nm in size. The head contains mostly deoxyribonucleic acid (DNA; the length of a strand greatly exceeds that of the head and attains 60–70 microns, and the strand is tightly coiled within the head) or ribonucleic acid (RNA) and a small amount (about 3 percent) of protein and some other substances. The process has the shape of a hollow tube surrounded by a sheath containing contractile proteins similar to muscular proteins. The sheath in some bacteriophages can contract, revealing part of the shaft. At the end of the process in many bacteriophages there is a basal plate with several spinelike or other projections. Long delicate strands proceed from the plate to help attach the phage to the bacteria (see Figure 1). The membranes of the head and process consist of proteins. The total amount of proteins is 50–60 percent of a phage particle; the nucleic acids account for 40–50 percent. Every bacteriophage has specific antigenic properties differing from the antigens of the host bacterium and other phages. There are antigens common to several phages (especially those containing RNA).

Figure 1. Diagram of the structure of a particle of the T2 bacteriophage of Bacillus coli

Occurrence. Bacteriophages exist for most bacteria, including pathogenic and saprophytic, and also for actinomycetes (actinophages) and blue-green algae. They are found in the intestines of man and animals, in plants, soil, bodies of water, sewage, manure, and so on. Bacteriophages of soil microorganisms influence the course of the microbiological processes at work in the soil (denitrification, ammonification, nitrogen fixation).

Reproduction. The bacteriophage attaches itself to a bacterial cell by its process and dissolves the cell wall by releasing an enzyme. The contents of the head then pass through the canaliculus of the process into the cell, where the phage’s nucleic acid halts the synthesis of bacterial proteins, DNA, and RNA; then the synthesis of phage nucleic acid and then of phage proteins begins. Some of these proteins are enzymes; others form the membrane of the mature phage particle. Smaller, spherical phages enter bacteria without the participation of the process. If a bacterial cell is infected simultaneously with phage particles differing from one another in a number of properties, then among the offspring—in addition to particles similar to the parents—there will be particles in which these properties occur in a new combination. This is due to the fact that phage reproduction is characterized by recombination—the exchange of fragments of strains of nucleic acid, which is the carrier of hereditary information. Particles of large phages destroy the bacteria in leaving them, whereas the particles of small and threadlike phages leave living bacteria. Some bacteriophages are highly specific and capable of lysing the cells of only one particular species of microorganism (monophages), and others can lyse the cells of various species (polyphages).

Bacteriophages are of two types: virulent bacteriophages cause the lysis of cells and formation of new particles; mild (symbiotic) bacteriophages are adsorbed by the cell and penetrate it but do not cause lysis—they remain in the cells in a latent, noninfectious form (prophage). Cultures containing latent phage are called lysogenic. Lysogenesis is transmitted to the offspring of bacteria. A lysogenic culture may contain two, three, or more phages; it is generally resistant to the phages present. (Only a few cells are lysed and liberate mature phages.) By exposing a lysogenic culture to ultraviolet or X rays, hydrogen peroxide, and some other substances, it is possible to increase substantially the number of cells liberating phages (so-called bacteriophage induction). Lysogenesis is widespread among all species of bacteria and actinomycetes. In a number of cases many properties of a lysogenic culture (toxicity, bacterial mobility, and so on) depend on the presence therein of certain prophages. Many mutations of bacteriophages have been described. These are associated with change in their lytic activity, structure of the particles and colonies, resistance to unfavorable factors, and other properties. Bacteriophages play a major role in microbial variability and evolution. There are various mechanisms by which they act on the cells. They can markedly alter the nitrogen-fixing ability of Azotobacter, the toxicity and antigenic properties of pathogenic bacteria, and so on.

Practical significance. Some phages (alone or combined with antibiotics) have been used to prevent (phagopro-phylaxis) and treat (phagotherapy) a number of bacterial infections of man (dysentery, typhoid, cholera, plague, staphylococcal and anaerobic infections, and others) and animals. However, antibiotics and other chemotherapeutic agents proved to be more efficacious than phages, and their use for therapeutic purposes has narrowed. Bacteriophages are successfully used to determine the species of bacteria and actinomycetes. They may harm the production of antibiotics, amino acids, milk products, bacterial fertilizers, and other aspects of microbiological synthesis. They are important in theoretical studies on genetics and molecular biology.