Bacteriophage morphology

All bacteriophages have a nucleic acid core surrounded by a protein coat. The nucleic acid may be composed of DNA or RNA and can be single or double stranded. Phages active against lactic acid bacteria are approximately tadpole or sperm shaped and have a distinct head terminating in a tail with a hollow core.

Bradley(1967), in a classic review paper, summarised the principles of phage morphology and outlined the basic morphological types which are still valid (fig. 1). Note phages with a contractile tail are found only in Group A. Phages attacking lactic acid bacteria belong to Groups A, B and C. Phages in Groups A, B and C contain double stranded DNA. Phages in Groups D and F contain single stranded DNA where Group E phages contain single-stranded RNA.

The majority of lactococcal phages can be classified within Group B. A small number, I am aware of two (Lembke et al.,1980; Saxelin et al.,1979) have been classified within Group C. While one report of a phage for Lc. lactis C10 with a contractile tail has been reported (Group A) this has not been confirmed. Both prolate polyhedral and isometric head shapes have been found.

Examples of phages with prolate and isometric morphology are shown in figs 2 and 3 respectively, namely ØC2(W) and Ø712. Isometric phages for lactococci have been reported to have heads ranging from 45-65 nm and tails ranging from 100-250 nm. Prolate phages are generally smaller with head sizes ranging from 55-65 x 40-48 nm and tails ranging from 80-110 nm. Prolate phages generally produce high levels of lysin and have a broader host range than isometric phages for lactococci.

Phages e.g. KSY1, fig 5, may also possess complex tail appendages, collars and other structural components. These can be difficult to visualise and require specialist staining techniques. While these complex structures can be helpful in differentiating similar phages for a particular strain they have limited value in phage classification.

Professor Teuber's group at Kiel in Germany have extensively studied the morphology of lactococcal phages and fig. 4 ( Lembe et al., 1980) provides examples of typical morphotypes. Note the phage in fig 4.A belongs to Bradley's Group C, the others belong to Group B. The phages in fig 4.B and 4.C have prolate heads whereas the others, with the exception of 4.A, have isometric head morphology.

Another example of a type C phage, KSY1, courtesy of Maija-Lisa Saxelin, is shown in fig 5. This unusual lactococcal phage has been shown to cause defects in a Finnish cultured milk product called 'Viili'. The KSY1 phage appears to code for an enzyme that hydrolyses the capsule or slime layer of lactococci involved in Viili production.

Phages for Str. thermophilus, lactobacilli and leuconstocs have also been isolated and studied, however, these are not as well documented as lactococcal phages. Interestingly both lactobacilli and leuconstocs are hosts to both Group A and B phages. Phages for Str. thermophilus have been reported to have isometric head morphology and to belong to Bradley's Group B.

Giuseppe Aprea working with the author on artisanal buffalo milk starters, isolated the first phage reported for Lact. brevis. Following work with Professor Gerald Fitzgerald at UCC in Ireland, Giuseppe Aprea has subjected this phage to detailed genetic and morphological examinations. The latter have revealed the first C type phage for Lact. brevis.

Discovery | Phages for lactic acid bacteria | Bacteriophage lysins |Enumeration | Isolation and purification | Storage lactococcal lysates | Industrial significance | Control

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Mullan, W. M. A. (2002).   [On-line] UK: Available:  Accessed:

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