Bradley (1967), in a classic review paper, summarised the principles of phage morphology and outlined six basic morphological types (fig. 1). The tailed phages, Bradley's groups A-C account for some 96% of all phages isolated to date and as discussed below belong to the order Caudovirales. Only phages in Group A have contractile tails. All tailed bacteriophages have a nucleic acid core surrounded by a protein coat. 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.
Phages attacking lactic acid bacteria belong to Groups A, B and C and contain double stranded DNA. Phages in Groups D and F contain single stranded DNA, however, Group E phages contain single-stranded RNA.
The majority of lactococcal phages can be classified within Group B. A small number, see Lembke et al.,1980 and 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.
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.
Phages e.g. KSY1 (courtesy of Maija-Lisa Saxelin), 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.
The C-type phage, KSY1 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.
Dr. 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, Dr. Aprea has subjected this phage to detailed genetic and morphological examinations.
Phages are currently classified on the basis of their nucleic acid type, single or double stranded DNA or RNA and on morphology. Tailed phages belong to the order Caudovirales. This order has three families, the Mycoviridae with contractle tails (Bradley's Group A), the Siphoviridae with long non-contractile tails (Bradley's Group B) and the Podoviridae with non-contractile short tails (Bradley's Group C). These phages contain linear, double stranded DNA.
Further information on phage classification is summarised in table 1.
The B6, filamentous phage for Propionibacterium freudenreichii, discussed previously, belongs to the Inoviridae family.
Ackermann, H.-W. (2007) 5500 Phages examined in the electron microscope. Arch. Virol. 152, 277–243.
Bradley, D.E. 1967. Ultrastructure of bacteriophages and bacteriocins. J. Bacteriol. 31, 230-314.
Chopin, M.C., Rouault A., Ehrlich S. D., Gautier M. (2002). Filamentous phage active on the gram-positive bacterium Propionibacterium freudenreichii. J. Bacteriol. 7, 2030-3.
How to cite this article
Mullan, W.M.A. (2002). [On-line]. Available from: https://www.dairyscience.info/index.php/site-links/56-writing-research-reports-egpapers-theses.html . Accessed: 18 August, 2022. Revised December 2012.