C

Short non-contractile

tails

PO34, KSY1

From Brusow, 2001;Desiere et al., 2002;McGrath et al., 2004;and Stanley et al., 2003

From Brusow, 2001;Desiere et al., 2002;McGrath et al., 2004;and Stanley et al., 2003

Based on morphological distinctions, there are three main types of bacteriophages: morphotypes A, B, and C, with the B or Siphoviridae group being the most common type that infect lactic acid bacteria.The group B phages have long tails with heads that have either a small isometric (B1 sub-group), prolate (B2), or elongated (B3) shape (Figure 1). The lactococcal phages generally belong to the B1 or B2 group, whereas the phages that infect Streptococcus ther-mophilus are all in the B1 group. Furthermore, most of the lactococcal phages that have been characterized share structural similarity to three specific lactococcal phages: c2 (a B2 morphotype), 936, and P335 (morphotype B1).Therefore, it is common to refer to other phages as being represented by one of these three types. For example, phages of the P335 type are considered to be the most common type infecting industrial fermentations that rely on lactococci (Durmaz and Klaenhammer, 2000).

Bacteriophage genomics has now become one of the most powerful means for characterizing and classifying lactococcal and related phages. Sequence analyses of bacteriophage

72 Microbiology and Technology of Fermented Foods Box 3-1. Bacteriophages of Lactic Acid Bacteria (Continued)

72 Microbiology and Technology of Fermented Foods Box 3-1. Bacteriophages of Lactic Acid Bacteria (Continued)

Figure 1. Electron photomicrographs of representative lactococcal bacteriophages. All were all isolated from cheese factories. Photos provided courtesy of Sylvain Moineau.

genomes (more than thirty have been sequenced) have revealed that gene clusters are arranged in modular fashion (Figure 2).That is, the genome consists of modules that code for particular functions, such as replication, structure, or assembly. In addition, these modules are transcribed either during the early or late stages of the phage replication cycle. In some phages, a middle gene cluster also exists. Phage classification, then, can be based on similarities in the genetic organization of different phages. Moreover, it is now recognized that the ability of lytic bacteriophages to undergo frequent recombination is likely due to the modular nature of the phage genome. Importantly, recombination by phage may result in the acquisition of new genetic information, including genes that enable the phage to counter host defense systems (Rakonjac et al., 2005).

Early ori Late r-0

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