Host Range of Phages

The number of different bacteria that a particular phage can infect defines its host range. Several thousand different phages have been isolated, and the host range of any particular phage is usually limited to a single bacterial species and often to only a few strains of that species. Several factors limit the host range of phage. The two most important are (1) the requirement that the phage must attach to specific receptors on the host cell surface to start infection and (2) the restriction-modification system of the host cell must be overcome. The limited host range means that phages seldom transfer DNA between unrelated bacteria.

13.4 Host Range of Phages 335

Receptors on the Bacterial Surface

Receptor sites vary in chemical structure and location. Receptors are usually on the bacterial cell wall, although a few phages attach to pili and a few others attach to sites on flagella (figure 13.12). Receptor sites can be altered by two distinct mechanisms, thereby creating a resistant cell. First, the receptor sites can be modified by mutation. In any large population of susceptible cells, some will have a modified (mutant) receptor site that confers resistance to any given phage.

Second, as already discussed, some but not all temperate phages that have lysogenized a bacterial cell can alter the cell surface, an example of lysogenic conversion. As a result the original receptor is no longer available. Thus, the prophage protects its host and, in turn, is able to continue replicating inside it.

Restriction-Modification System

Virtually all prokaryotes have two genes located next to each other in the cell that are involved in the restriction-modification system. The restriction gene codes for a restriction enzyme, an endonuclease that recognizes short base sequences in DNA and cleaves the DNA at these sequences. The second gene codes for a modification enzyme, which attaches methyl groups to the bases of the sequence recognized by the restriction enzyme. These methylated bases are not recognized by the restriction enzyme, and so it does not cleave the DNA. This is the mechanism that the cell uses to protect its own DNA from being degraded by the restriction enzyme present in the cell. Its own DNA is methylated and therefore not degraded, but foreign DNA entering the cell will be degraded unless it is methylated first. ■ restriction enzyme, pp. 214,231

The restriction-modification system explains why a recipient bacterium will take up DNA by transformation, conjugation, or transduction only if the donor DNA and recipient cells are of the same species or even subspecies. Recipient cells restrict any DNA if the methylation pattern of the entering donor DNA differs from its own. Foreign DNA from different species, however, does enter recipient cells through the three gene transfer systems. How does this occur in light of the cells having restriction enzymes? How the restriction-modification system functions in keeping most, but not all, donor DNA from being degraded is illustrated by the following story. ■ restriction-modification, p. 214

Phage that infect, multiply, and lyse E. coli strain K-12 can infect the same strain again with a very high efficiency—that is, all of the cells are infected (figure 13.13). These same phage infect the B strain of E. coli with a very low efficiency. If the phage that are released from the few infected B cells are added again to the B strain, however, infection now occurs at a high frequency. These observations can be explained by the presence of different restriction enzymes and different modification enzymes in the E. coli B and K-12 strains. The sequence of bases that is recognized and cleaved by the restriction enzyme of K-12 is different from the sequence of bases cleaved by the restriction

Pilus

Bacteriophage

Pilus

Bacteriophage

100 nm

Tail fiber

100 nm

Tail fiber

Phage Flagella Receptor

Bacteriophage

Flagellum

Bacteriophage

Flagellum

100 nm

Pilus Adsorption

Figure 13.12 Adsorption of Bacteriophages on Various Cell Structures

(a) Bacteriophage on pilus of E. coli. (b) Bacteriophage tail fiber entwined around flagellum. (c) Bacteriophage T4 attached to cell wall of E. coli.

100 nm

Figure 13.12 Adsorption of Bacteriophages on Various Cell Structures

(a) Bacteriophage on pilus of E. coli. (b) Bacteriophage tail fiber entwined around flagellum. (c) Bacteriophage T4 attached to cell wall of E. coli.

100 nm

100 nm

Nester-Anderson-Roberts: I II. The Microbial World I 13. Viruses of Bacteria I I © The McGraw-Hill

Microbiology, A Human Companies, 2003

Perspective, Fourth Edition

Chapter 13 Viruses of Bacteria

E. coliB contains DNA with a different pattern of methylation from E. coli K-12.

E. coliB

Most cells are resistant because the restriction enzyme recognizes DNA as foreign and cleaves it before it can be modified. Occasionally, foreign DNA is methylated, preventing degradation and allowing phage replication.

Infected ^

Infected cell lyses.

Phage grown in E. coli K-12 added to E. coli B and E. coli K-12

E. coliB cells previously resistant to phage grown in strain K-12 now are infected by the phage from lysed E. coli B.

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