138 



The blood agar is often dark-coloured or blackish in an 

 area stretching some millimetres beyond the edge of the cul- 

 ture, and is very ill-defined. When a number of different 

 strains are inoculated on the same plate, the variable intensity 

 of the dark colouration developed often produces a rather 

 variegated picture. The great majority of the typically-growing 

 strains can produce a dark colouration but many of the atypical 

 do not possess this power. In some cases however. I have 

 seen atypical cultures give rise to a very dark colouration. 



All transitions from deep colouration to none at all can 

 be seen. The colour depends largely on the intensity of the 

 growth, so that the best growing strains on a blood agar plate 

 are most likely to blacken the medium. If the preparation 

 of the blood agar is such that a relatively rich growth occurs, 

 the colouration is also deeper. It may be seen around a part 

 of the culture where there is increased growth from symbiosis 

 with a foreign colony of bacteria, while elsewhere there is 

 no change in the colour of the medium'. Two strains which 

 grow equally well however, may behave rather differently as 

 regards dark colouration. 



The blackening of blood agar is therefore not an essential 

 character of Pfeiffer's bacillus. Its dependence upon small 

 changes in the constitution of the blood agar and the gradual 

 transitions have prevented me from' utilising it in the descrip- 

 tion and classification of the strains. 



Another phenomenon which may also be of rather variable 

 development is the differing tendency of the various strains 

 to spread on the surface of blood agar. It is not superfluous 

 to review the great differences which may be observed in 

 this respect, for it is usually proclaimed as characteristic of 

 Pfeiffer's bacillus (as a distinction from Bordet's bacillus), 

 that when inoculated in narrow streaks it spreads a bit on 

 both sides. This is commonly the case, but it is practically 

 confined to the „typical" strains and then to a variable ex- 

 tent. Further the constitution of the medium has some in- 

 fluence on the spreading of the growth. On fresh blood agar 

 which has been prepared without undue heating and which on 

 account of the intact condition of the red blood corpuscles 

 only gives slight growth, the strains will not be able to spread 

 much along the surface either. On slightly heated blood agar 



