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These phages are mainly used for the purposes of classification and identification.
Two kinds of phage effects are observed when phage suspensions are spotted on agar plates seeded with susceptible strains:

i) True lysis: appearing as clear zones in the bacterial lawn of test strains. Phage propagation is verified by lysis after the transfer of material from lytic zones to agar plates seeded with the respective strains or by the demonstration of single plaques after dilution of high-titre phage suspensions.

ii) Clearing effects (Ref. 3504): appearing as turbid zones in the bacterial lawn of the test strains. Phage propagation cannot be demonstrated using the procedures described above.

These effects occur only if high-titre phage suspensions (ca. 109 - 1011 pfu/ml) are used [pfu = plaque forming units].

Both kinds of effects have been proven to be taxon specific (Ref. 3504, 3505). Sets of phages are needed for classification and for identification by phage typing because a single phage, as a rule, does not affect all strains of a taxon under study. Such sets have been selected from the phages available, with the aim that any one strain of a genus or species under investigation is affected by at least one of the phages of the appropriate set.

The data presented here and the conclusions drawn depend on the numbers and kinds of phages available, the numbers and kinds of strains and taxa tested, and on the present state of bacterial taxonomy.