266 INTRACELLULAR LUMINESCENCE 



various degrees of brightness as compared to the parent strain under 

 similar conditions, have long been known (Beijerinck, 1912, 1915) 

 and have been studied further in recent years (Doudorolf, 1942; 

 Giese, 1943; McElroy and Farghaly, 1948; Miller, Farghaly, and McEl- 

 roy, 1949; McElroy and Friedman, 1951). 



With the genetic potentiality for luminescence, the intrinsic bright- 

 ness, i.e., the average intensity of light emitted per cell, may vary 

 enormously at different stages of the growth cycle, or with environ- 



— 50 



1 1 1 1 1 \ T 



2 4 6 8 10 12 14 



TIME IN HOURS 



LUM. INT. • • 

 CELL COUNT »o 



28* C 



— t 



Fig. 1. Rate of increase in cell numbers and in luminescence intensity during 

 aerobic growth of a species of luminous bacteria in 3% NaCl-nutrient broth 

 at 28° C. (After Baylor, 1949.) 



mental factors such as temperature of incubation and composition of 

 the medium. Beijerinck (1915) noted that, at relatively high tem- 

 peratures, growth of Pliotobacteriiim splendidum will take place, 

 accompanied by very little luminescence. When dark cultures, grovm 

 at above optimal temperatures, are placed at a lower temperature 

 favorable for luminescence, light does not immediately appear, al- 

 though subcultures grow and luminesce at the lower temperature. 

 Along with these studies Beijerinck made the interesting observation 



