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.