794 HEMOPHILUS 



under anaerobic conditions. Statements with regard to its ability to develop 

 under strictly anaerobic conditions are somewhat contradictory (see Kristensen 

 1922). Fildes (1921) states that H. influenzce gives good initial anaerobic growth 

 on a suitable medium, but quickly dies out. 



Cultural Reactions. Type of Growth. — The type of colony given by H. influenzcB 

 on solid media varies widely with the kind of medium employed. On blood agar it 

 forms tiny transparent, pin-point colonies, sometimes flat, and tending to become 

 confluent, sometimes more convex, and with less tendency to confluence. On a 

 more favourable medium, such as Levinthal's agar, and especially Fildes' agar, 

 the colonies are far larger. After 24 hours' incubation they attain a diameter of 

 0-5-0-8 mm. They are circular in outline, raised and dome-shaped, with a slightly 

 splayed-out, entire edge. The surface is usually smooth ; the colony is trans- 

 lucent ; there is little differentiation ; and the growth emulsifies easily (Fig. 168). 



On further incubation, and in many cases during the first 24 hours of growth, 

 the colony becomes differentiated into a central portion with a granular or con- 

 toured surface, an intermediate flattened portion, and a sharply bevelled periphery 



f3\ fmS 



\ 



r^f^ 



Fig. 168. — H. influenzae. Fig. 169. — H. influenzce. 



Colonies on Fildes' agar after 24 hours ( X 8). Colonies on Fildes' agar after 48 hours ( X 8). 



with a narrow splayed-out edge. Between the 24th and 48th hours there is usually 

 a considerable enlargement of the colonies, which may attain a diameter of 1-1-5 mm. 

 This increase in size results in the formation of a flatter colony, retaining a raised 

 central boss, sometimes smooth, sometimes granular or contoured (Fig. 169). Some 

 colonies may be, from the start, flatter and more granular ; others may remain 

 raised, conical and smooth, with little central differentiation. Kristensen (1922). 

 lays considerable stress on such colonial differences, especially those which develop 

 on blood agar, and apparently regards them as more important than cellular differ- 

 ences in morphology, in distinguishing " typical " from " atypical " strains. Smith 

 (1931), in the study referred to above, noted a definite but not absolute correlation 

 between morphology and colony form. The morjihologically typical strains tended 

 to give, on Fildes' agar, smooth colonies with little differentiation. The morpho- 

 logically atypical strains, or variants, tended to produce a more granular colony, 

 with earlier and more considerable differentiation. 



The " smooth " strains described by Pittman (1931) have distinctive colonial 

 characters. Levinthal's agar gives rather better differentiation than Fildes'. Smooth 

 strains give relatively large colonies, sometimes attaining 3 mm. in diameter. They 



