GROWTH CHARACTERS 



563 



in identification, particularly in recently isolated strains. Many strains of pneumo- 

 cocci, for instance, give characteristic " draughtsman " colonies, with an entire, 

 sharply raised edge and a central depressed area. A Type III pneumococcus 

 frequently gives a characteristic watery, or mucoid, colony. One type of hsemolytic 

 streptococcus gives easily recognizable minute, clear colonies, and so on. 



Many species grow poorly in gelatin ; and in a gelatin stab such growth as occurs 

 is mainly confined to the track of the needle. The only streptococci, apart from 

 some of the anaerobic species, that are known to liquefy gelatin are certain strains 

 of enterococci. When liquefaction occurs it is usually infundibuliform in type. 



In broth or other liquid media many species of streptococci give a granular growth, 

 the medium remaining clear and the granular masses collecting as a powdery deposit, 

 or adhering to the sides of the tube. Although this type of growth shows a char- 

 acteristically high frequency among certain species of streptococci, especially when 

 first isolated, it is by no means constant ; and the degree of granularity may vary 

 over a wide range. In some cases a granular deposit may be associated with a 



Fig. 121. — Str. pyogenes. 



Surface colony on blood agar plate, showing 

 zone of haemolysis round colony { X 8). 



Fig. 122. — Str. jtneumonice. 



Surface colonies on blood agar plate, showing 

 zones of discoloration and partial hi^emo- 

 lysis round colonies ( X 8). 



varying degree of turbidity of the medium ; in others the growth, as a whole, may 

 be distinctly though finely granular, the granules remaining dispersed throughout 

 the medium. Any one strain may undergo marked changes in this respect on 

 subculture ; if a strain which, on first isolation, gives a markedly granular growth, 

 is subjected to repeated subculturing at short intervals, it is often possible to 

 produce a diffuse growth within a limited number of generations. Granularity 

 is usually more evident in cultures grown at 37° C. than at 22° C. In all cases the 

 type of growth in a fluid medium is closely associated with the character of chain 

 formation. A strain which is forming long chains will give a typically granular 

 growth ; if the growth becomes diffuse it will be found that the average chain 

 length has diminished. Those species in which the diplococcal form predominates, 

 as for instance Str. pneumonice or enterococci, usually give diffuse, non-granular 

 growths in fluid media. A description of the variations in morphological appear- 

 ance, colonial form, and type of growth in broth met with among the streptococci 

 is given on page 594. 



