406 INFLUENZA. 



in the sputum are very minute rods not exceeding 1.5 //, 

 in length and .3 //, in thickness. They are straight, with 

 rounded ends, and sometimes stain more deeply at the ex- 

 tremities (Fig. 98). The bacilli occur singly or form clumps 

 by their aggregation, but do not grow into chains. They 

 show no capsule. They take up the basic aniline stains 

 somewhat feebly, and are best stained by a weak solu- 

 tion of carbol-fuchsin applied for 5 to 10 minutes. They 

 lose the -stain in Gram's method. They are non-motile, 

 and do not form spores. 



Cultivation. The best medium for the growth of the 

 influenza bacillus is blood-agar (see page 47), which was 

 introduced by Pfeiffer. He obtained growths of the 

 bacilli on agar which had been smeared with influenza 

 sputum, but he failed to get any sub-cultures on the agar 

 media or on serum. The growth in the first cultures he 

 considered to be probably due to the presence of certain 

 organic substances in the sputum, and accordingly he tried 

 the expedient of smearing the agar with drops of blood 

 before making the inoculations. The blood of animals is 

 suitable, as well as human blood. In this way he com- 

 pletely succeeded in attaining his object. The colonies of 

 the influenza bacilli on blood agar appear within twenty- 

 four hours, in the form of minute circular dots almost 

 completely transparent. When numerous, the colonies are 

 scarcely visible to the naked eye, but when sparsely 

 arranged they may reach the size of a small pin's head. This 

 size is generally reached on the second day. The bacilli 

 die out somewhat quickly in cultures, and in order to keep 

 them alive sub-cultures should be made every four to five 

 days. By this method the cultures may be maintained for 

 an indefinite period. They also grow well on agar smeared 

 with a solution of haemoglobin ; growth on the ordinary 

 agar media is slight and somewhat uncertain. A very 

 small amount of growth takes place in bouillon, but it is 

 more marked when a little fresh blood is added. The 

 growth forms a thin whitish deposit at the bottom of the 

 flask. The limits of growth are from 25 to 42 C., the 



