CULTURAL CHARACTERS. GROWTH REQUIREMENTS 



793 



TABLE 51 



Showing Growth of certain Bacterial Species in Peptone Water, with and 

 without x and v factors. 



It may be convenient at this point to indicate the various media that are, at 

 the present time, employed in the study of the hsemophilic bacilli. 



Ordinary blood agar is by no means a satisfactory medium from this point of view ; 

 far better results are obtained with media in which the red cells have been broken up, 

 and their modified contents distributed throughout the medium. The well-known " choco- 

 late " agar, prepared by adding blood to melted agar, raising the mixture to the boiling- 

 point for 3 minutes, and then preparing slopes or plates from the chocolate- coloured mass, 

 is a considerable improvement on the ordinary blood agar plate, but it shares the dis- 

 advantage that the medium is opaque. 



The medium devised by Levinthal (1918) has the great advantage of being colourless 

 and transparent. It is prepared by adding 5 per cent, of defibrinated rabbit or human 

 blood to melted agar in a flask, and raising it to the boiling-point over a flame, with 

 several shakings. The precipitate of coagulated blood and serum protein is allowed to 

 settle, and the clear supernatant fluid is carefully decanted, or may be filtered through 

 sterile glass-wool. The medium may be, for safety, sterilized by a further short heating, 

 but must not be subjected to prolonged sterilization in the steamer. 



Fildes ( 1920) has introduced a peptic digest of blood, which is preserved with chloroform 

 and may be added to broth, or melted agar, as required. This medium, which is trans- 

 parent and has the colour of ordinary broth or agar, gives copious growths of H. influenzce, 

 and inhibits the growth of many other organisms. It is admirably suited for the primary 

 isolation of the influenza bacillus. The abihty to support the optimal growth of ha^mo- 

 phihc baciUi is not a property of blood from all species of animals. For example, on 

 horse blood media, H. pertussis loses its smooth characters more readily than on media 

 made with human or sheep blood (Toomey and Takacs 1938). The influenza bacillus 

 may be even more susceptible to species variations in blood. Thus, Krumwiede and 

 Kuttner (1938) describe thermolabile substances inhibiting the growth of H. infliienizce 

 and H. para-inflnenzce in sheep, goat, bovine and human blood, but not in the blood of 

 the rat, rabbit or guinea-pig. 



For jDrimary culture from such a source as the nasopharynx advantage may be taken 

 of the selective action of peniciUin, which inhibits the growth of Gram -positive cocci and 

 of diphtheroid bacilli but has almost no action on H. influenzce or H. pertussis (see Chajjter 

 77). 



With regard to other conditions of growth, the optimum temperature for 

 H. influenzce is in the neighbourhood of 37° C. The minimal temperature for growth 

 lies between 20° and 25° C. The same conditions hold for other heemophilic bacilli. 

 There is general agreement that H. influenzce grows far better under aerobic than 



