234 



lained large colonies on agar with undissolved blood cor- 

 puscles the reason being uncertain (infection with foreign or- 

 ganisms?), but this is the exception. 



In spite of the weak growth on blood agar it has, particu- 

 larly for primary cultivation where growth-promoting organisms 

 exert their favourable influence, proved to be a very useful 

 medium when it is employed with requisite discretion. Duval 

 & Harris (see p. 37), who found Pfeiffer's bacillus practically 

 constantly, used blood agar on which this organism only grew 

 extremely poorly (although the blood agar was „complement- 

 free"). The colonies were often not visible for 48 hours. 



Several authors have used blood agar for the primary cul- 

 ture, and for subcultivation one of the media containing heated 

 blood, mentioned below. Blood agar is just the medium on 

 which the various bacteria generally present in sputum etc. 

 appear most characteristic. It is true this applies especially 

 to Pneumococcal and Streptococcus, while the colonies of 

 Pfeiffer's bacillus are at least just as recognisable on one of 

 the good transparent „haemoglobin" media. On blood agar 

 however a marked increase of growth of Pfeiffer's bacillus 

 will most often be observed around colonies of Staphylococcus, 

 Streptococcus etc. and this picture is extremely characteristic 

 for Pfeiffer's bacillus. 



Fildes, Baker, & Thompson found blood agar almost use- 

 less. Dick & Murray and others found that even after there 

 was good growth on the original plate Pfeiffer's bacillus did not 

 always grow on further cultivation. 



Schottmuller states that blood agar plates must be used in 

 the first 24 hours after they are made, while Levinthal (2) 

 just as decisively maintains that they must be kept at least 24 

 hours before they can be used. It is clear that different factors 

 are operative here. In my experience it does not matter whether 

 the plates are used immediately after preparation or after being 

 kept for some days. If blood agar plates are to be used 

 however, attention should be directed to this point and it 

 should be determined whether the length of time between 

 the preparation of the plates and their inoculation is of im- 

 portance under the given conditions. 



3. The weak growth on media containing whole blood cor- 

 puscles is simply explained by the slight amount of haemoglobin 



