Introduction to Bacteriophages
Viruses that infect bacteria are called bacteriophages or simply phages. The plural form "phages" is used when referring to different species (e.g., lambda and T4 are both phages). When referring to one or more virions of the same species, the word phage is used; thus, a bacterial cell may be infected by one or more lambda phage. The most commonly studied phages have a roughly spherical icosahedral capsid to which a tail is attached. The tail may be long or short, contractile or noncontractile. Other kinds of phage have tailless heads or filamentous structures. The genetic material of most phages is double-stranded DNA (dsDNA), although some single-stranded DNA (ssDNA), single-stranded RNA (ssRNA), and double-stranded RNA (dsRNA) phages are known. Enveloped forms are rare. Other characteristics that are useful for classification include molecular weight, genomic base composition (G þ C content), antigenic specificities of the capsid, and species or strain of susceptible host cells (host range). Host restriction is the ability of a bacteriophage to replicate in only certain strains of bacteria.
EXAMPLE 11.1 Several bacterial species synthesize a site-specific endonuclease enzyme that can digest any foreign DNA containing the specific nucleotide sequence that constitutes the recognition site of the enzyme. According to the restriction and modification model proposed by W. Arber, such a bacterium would also contain a methylase enzyme to modify (by methylation) these same sequences in its own DNA, and thus protect it from digestion by endogenous endonuclease. Foreign DNA from a different source, such as an infecting phage particle, would not have these recognition sites methylated and hence would be destroyed (and thus restricted from surviving in that strain) by the host's endonuclease.
The nucleic acid from a single phage particle typically infects a bacterial cell, replicates itself many times, produces viral proteins to make numerous viruses, and ruptures (lyses) the cell to release several hundred progeny phage. Repetitions of this reproductive process can cause a turbid bacterial culture to rapidly become clear owing to lysis of the host cells. If a dilute solution of phage is plated on a confluent growth of bacterial cells ("lawn") on nutrient agar in a Petri dish, a cleared area, or "hole," will develop around each position where a phage particle was deposited. These holes, called plaques, contain millions of progeny phage that have been released from lysed cells. By counting the number of plaques on a plate, and knowing the amount and dilution of the phage suspension added to the plate, one can estimate the total number of phage particles or the phage titer in the original phage solution.
Bacteriophage Life Cycles
Most phages (such as phage T4 that infects E. coli) have only a lytic cycle in which they kill the host cell in the production of progeny phage. Such phages are said to be virulent. A few phages (such as phage lambda that also infects E. coli) have a lysogenic cycle in which they may either act as a temperate phage (nonvirulent) or enter a lytic cycle.