164 ◾ Fundamental Food Microbiology
isometric heads, for example, TP901-1). Genome sequence analysis indicates that the species in
the same family as well as those in different families vary considerably. In contrast, the genome
analysis of phages of Streptococcus thermophilus (e.g., sfi 19, sfi 11) indicates a fairly good homology. Phages of several species of Lactobacillus have been identified, and genome sequences of some
species and strains are now available, namely for Lactobacillus delbrueckii (LL-H), Lab. plantarum
(phi-gle), Lactobacillus johnsonii (Lj 965), Lab. gasseri (adh), and Lab. casei (a2). Very few phages
of Leuconostoc and Oenococcus have been isolated, and none have been isolated from Pediococcus. 9
Life Cycle
A phage cannot multiply by itself in food. Instead, it attaches (needs Ca 2+ for adsorption) with its
tail on the surface of a bacterial cell (specific host) and injects its DNA inside the cell cytoplasm.
If it is a lytic phage, the bacterial cell produces a large number of copies of the phage DNA and
phage proteins. Following assembly of the proteins and phage DNA to produce mature phages,
the bacterial cell lyses, releasing the phages (as many as 200) into the environment (Figure 14.3).
They, in turn, attack other bacterial cells. These are lytic phages. Their growth cycles (lytic cycle)
take approximately 20–30 minutes. There are other phages (called temperate phages) that are nonlytic, and their DNA, following injection into the cytoplasm, is integrated with bacterial DNA.
The phage DNA (called prophage) is carried by a bacterial cell DNA (lysogeny state), and as a
bacterial cell multiplies, the prophage also multiplies without showing the presence of the phage
in the bacterial strain (lysogenic strain). As long as the host cell carries the prophage, it is immune
to attack by the same phage. Recent genome studies have shown that the DNA of a host strain can
have five or more types of prophages. 9 However, a prophage can be induced by a physical (such as
UV) or a chemical (such as mitomycin C) agent, causing the phage DNA to separate out of bacterial DNA and resume the lytic cycle (Figure 14.3).
Host Specificity
Bacteriophages against many species of Lactococcus, Streptococcus, Leuconostoc, Lactobacillus, and
Oenococcus oenos that are currently used in food fermentation have been discovered. The phages
are host-specific, and there can be one specific host (strain) for a specific phage to several related
strains for a phage. A bacterial strain can also be the host of many different types of phages. A
Head
DNA
Head-tail connection
Tail sheath
Base plate
Tail fiber
(a)
(b)
(c)
0.2 µm
0.2 µm
Figure 14.2 Picture of bacteriophages (a) schematics of t4 phage, (b) transmission electron
microscopic (teM) picture of siphovirus (thin tail), and (c) myovirus (thick tail). (teM photographs were provided by Dr. Paul ebner and Yanying Pan, Purdue University.)
isometric heads, for example, TP901-1). Genome sequence analysis indicates that the species in
the same family as well as those in different families vary considerably. In contrast, the genome
analysis of phages of Streptococcus thermophilus (e.g., sfi 19, sfi 11) indicates a fairly good homology. Phages of several species of Lactobacillus have been identified, and genome sequences of some
species and strains are now available, namely for Lactobacillus delbrueckii (LL-H), Lab. plantarum
(phi-gle), Lactobacillus johnsonii (Lj 965), Lab. gasseri (adh), and Lab. casei (a2). Very few phages
of Leuconostoc and Oenococcus have been isolated, and none have been isolated from Pediococcus. 9
Life Cycle
A phage cannot multiply by itself in food. Instead, it attaches (needs Ca 2+ for adsorption) with its
tail on the surface of a bacterial cell (specific host) and injects its DNA inside the cell cytoplasm.
If it is a lytic phage, the bacterial cell produces a large number of copies of the phage DNA and
phage proteins. Following assembly of the proteins and phage DNA to produce mature phages,
the bacterial cell lyses, releasing the phages (as many as 200) into the environment (Figure 14.3).
They, in turn, attack other bacterial cells. These are lytic phages. Their growth cycles (lytic cycle)
take approximately 20–30 minutes. There are other phages (called temperate phages) that are nonlytic, and their DNA, following injection into the cytoplasm, is integrated with bacterial DNA.
The phage DNA (called prophage) is carried by a bacterial cell DNA (lysogeny state), and as a
bacterial cell multiplies, the prophage also multiplies without showing the presence of the phage
in the bacterial strain (lysogenic strain). As long as the host cell carries the prophage, it is immune
to attack by the same phage. Recent genome studies have shown that the DNA of a host strain can
have five or more types of prophages. 9 However, a prophage can be induced by a physical (such as
UV) or a chemical (such as mitomycin C) agent, causing the phage DNA to separate out of bacterial DNA and resume the lytic cycle (Figure 14.3).
Host Specificity
Bacteriophages against many species of Lactococcus, Streptococcus, Leuconostoc, Lactobacillus, and
Oenococcus oenos that are currently used in food fermentation have been discovered. The phages
are host-specific, and there can be one specific host (strain) for a specific phage to several related
strains for a phage. A bacterial strain can also be the host of many different types of phages. A
Head
DNA
Head-tail connection
Tail sheath
Base plate
Tail fiber
(a)
(b)
(c)
0.2 µm
0.2 µm
Figure 14.2 Picture of bacteriophages (a) schematics of t4 phage, (b) transmission electron
microscopic (teM) picture of siphovirus (thin tail), and (c) myovirus (thick tail). (teM photographs were provided by Dr. Paul ebner and Yanying Pan, Purdue University.)