HEMOLYSIN PRODUCTION 796 



are slightly opaque, and viewed by obliquely transmitted light they are iridescent. 

 This iridescence is, perhaps, their most characteristic property. The surface is 

 smooth and slightly mucoid in appearance. The edge is entire. 



These " smooth " strains are described by Pittman as readily giving rise to 

 " rough " variants producing colonies of varying granularity and differentiation, 

 the organisms composing them being in the form either of short bacilli, or of 

 longer rods or threads. The rough variants were never capsulated. 



The relation of Pittman's " smooth " strains to the strains isolated by other 

 workers from infective conditions, or from the normal respiratory tract, raises a 

 problem in terminology which is at the moment difficult to solve. Her observations 

 have been confirmed, in whole or in part, by several subsequent workers, and there 

 can be little doubt that the strains she has differentiated correspond to a form of 

 H. influenzcE that is frequently associated with acute infections in man. If, however, 

 we accept Pittman's colonial differentiation as a criterion separating " smooth " 

 from " rough " strains, we must include among our " roughs " many, probably the 

 majority, of those strains that give, on Fildes' or Levinthal's agar, colonies of the 

 type that earlier observers had generally regarded as " smooth," and that consist of 

 morphologically typical cocco-bacillary organisms. These difficulties may be 

 resolved if we accept three forms of H. influenzae, the mucoid capsulated M form 

 with a characteristic specific soluble substance, an S form without a capsule, and 

 R forms (Chandler, Fothergill and Dingle 1937, 1939) (see p. 799). The balance 

 of evidence is, we think, in favour of accepting Pittman's nomenclature, but this 

 question will be discussed more fully in relation to antigenic structure. 



H. canis forms colonies that are at first indistinguishable from those of H. 

 influenzcB, but as they grow older they become larger and distinctly more opaque. 



H. pertussis, when grown on Bordet-Gengou medium, also forms colonies that, 

 during the early stages of growth, may resemble those of H. influenzcB. But when 

 incubation is prolonged beyond 24 hours the colonies become larger, more opaque, 

 and greyish in colour, a form that is never assumed by H. influenzcB. They are also 

 smoother, more shining and more distinctly dome-shaped. The combination of 

 slight opacity, greyness of hue, and shining surface, gives them an appearance that 

 has not inaptly been compared to that of a bisected pearl. They have also been 

 compared to drops of mercury ; but this overstates their metallic appearance. 

 A confluent row of colonies has been compared to an " aluminium streak," and this 

 simile again is not inapt. As already noted, H. bronchisepticus grows much more 

 readily than other members of the group, forming quite well developed colonies 

 on plain nutrient agar within 24 hours. 



In liquid media, such as Fildes' broth, the majority of strains of H. influenzcB 

 give rise to a uniform turbidity, with or without a slight powdery deposit. Some, 

 on the other hand, give a flocculent deposit, with a varying degree of turbidity of 

 the supernatant fluid. There is, as would be expected, a correlation between 

 morphology and type of growth in a fluid medium. Cocco-bacillary strains give a 

 uniform turbidity. Many of those showing long bacilli, or twisted and convoluted 

 threads, give flocculent growths. 



H. canis gives a diffuse growth with a slight deposit. So does H. pertussis. 



Resistance. — H. influenzcB is killed by exposure to a temperature of 50-55° C. 

 for 30 minutes. H. canis and H. pertussis behave similarly. 



Hsemolysin Production. — Before considering the fermentation of carbohydrates, 

 or other substrates, it will be convenient to discuss the hgemolytic activity of the 



