432 INFLUENZA. 



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

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

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

 tremities (Fig. 112). 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 solution 

 (i : 10) 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 49), 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 57^-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 the lower 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, incubated at 37 C, ap- 

 pear 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 



