When a pure bacterial culture is attacked by a bacterial virus, the culture will clear after a few hours due to destruction of the sensitive ceIls
by the virus. However, after further incubation for a few hours, or sometimes
days, the culture will often become turbid again, due to the growth of a bac-
terial variant which is resistant to the action of the virus. This variant can be
isolated and freed from the virus and will in many cases retain its resistance
to the action of the virus even if subcultured through many generations in the
absence of the virus. While the sensitive strain adsorbed the virus readily, the
resistant variant will generally not show any affinity to it.
The resistant bacterial variants appear readily in cultures grown from a
single cell. They were, therefore, certainly not present when the culture was
started. Their resistance is generally rather specific. It does not extend to
viruses that are found to differ by other criteria from the strain in whose pres-
ence the resistant culture developed. The variant may differ from the original
strain in morphological or metabolic characteristics, or in serological type or in
colony type. Most often, however, no such correlated changes are apparent,
and the variant may be distinguished from the original strain only by its re-
sistance to the inciting strain of virus.
The nature of these variants and the manner in which they originate have
been discussed by many authors, and numerous attempts have been made to
correlate the phenomenon with other instances of bacterial variation.