440 BACTERIOPHAGES 



when logarithms of survivors are plotted against time of exposure to, or 

 dosage of, an inactivating agent. The slope of this line is the (fractional) 

 rate of inactivation and measures sensitivity of the phage or bacterium in 

 terms relative to dosage. Accumulated dosage is often expressed in 

 multiples of the average lethal dose or hits, since e~" survival signifies an 

 average of n hits per phage or bacterium. 



Ghost: A phage particle that appears empty in the electron microscope. In 

 several instances ghosts are known to lack nucleic acid. 



Host-induced modification: A variant property of phage particles strictly deter- 

 mined by the kind of bacterium in which they last grew. Such modifica- 

 tions are considered nonheritable since they disappear when the phage is 

 transferred to a different host. They form one class of phenotypic varia- 

 tions. 



Host range: The range of action of a phage measured in terms of the varieties 

 of bacteria in which it can grow. Host range is often, but not always, 

 determined by success or failure of adsorption. 



Immunity: Resistance to infection that does not result from failure of the phage 

 to adsorb and inject, characteristically associated with the lysogenic state. 



Indicator bacteria: Bacteria sensitive to a specified kind of phage, usually also 

 resistant to other specified kinds. 



Induction: The transition from the prophage to the vegetative state in lysogenic 

 bacteria, occurring spontaneously or after experimental application of 

 inducing agents. 



Infection: Used loosely to indicate attachment of phage to bacterium with at 

 least some effects on the cell. One speaks, perhaps improperly, of infec- 

 tion by ghosts of T2 because the cell may be killed, though injection and 

 phage growth are impossible. 



Infective center: A plaque-forming particle that may be either a productively 

 infected bacterium or a free phage particle. When one or the other is 

 meant, the term infective center should not be used. 



Injection: Entrance of phage DNA into bacterium. 



Latent period: Elapsed time between infection or induction and lysis of cells 

 in one-step growth. Latent periods of individual cells vary over a range 

 called the rise period. When reported without qualification, latent 

 period means minimum latent period. 



Lysis: Dissolution of an infected bacterium, recognized by release of phage 

 particles, by loss of turbidity of a bacterial suspension, or by microscopic 

 changes of diverse kinds. 



Lysis from without: Abortive infection accompanied by prompjt lysis of a cell, 

 typically following excessive multiple infection. 



Lysogenic bacterium: A bacterium capable of multiplying indefinitely in tlie 

 infected condition. In lysogenic cultures phage is produced only by ex- 

 ceptional cells that lyse. 



