CULTURAL CHARACTERISTICS 



597 



B 





FIG. 284. Different appearances 

 presented by the same streptococcus 

 (A) when grown on agar and (T 



be the specific cause of scarlet fever, is characterized by its tendency to form long 

 chains massed together like staphylococci. 



Many authors regard these differences in form as accidental. Marmorek who, in 

 common with many other observers, holds that all streptococci are identical, has 

 shown that the number, shape and arrangement of 

 the cocci in the chains can be altered at will by 

 modifying the composition of the culture medium 

 (fig. 284). 



Practically all bacteriologists are now agreed 

 that these purely morphological differences are 

 insufficient to justify a differentiation into 

 species. 



Staining reactions. Streptococci stain readily 

 with the ordinary basic aniline dyes and are 

 gram -positive. 



It is to be noted however that there are streptococci 

 which do not stain by Gram's method. Von Lingel- 

 sheim, for instance, described a gram-negative 

 streptococcus, Etienne found a similar strain in a 



, -, T . -, . P \-r*-J wiicjj. givs vr 11 \ju. agai an\i t jj i wucj 



case of sore throat, and other instances of gram- grown in serum-broth. (AfterZenoni.) 

 negative streptococci have been recorded. Lemoine 



isolated a gram-variable streptococcus from a case of erysipelas. [But Andrewes 

 and Horder, and Gordon, found no gram-negative streptococci during their researches 

 provided they used decently vigorous cultures. The reaction to Gram's stain 

 depends partly upon whether Gram's method or the individual observer's modi- 

 fication be employed and partly upon 

 the age of the culture. It is far from 

 uncommon to find dead and dying cocci 

 in a chain gram-negative (Andrewes).] 



2. Cultural characteristics. 



Conditions of growth. Streptococci 

 are facultative aerobic organisms [and 

 grow anaerobically quite as well as 

 aerobically (Andrewes).] Growth 

 takes place between 20 and 46 C., 

 the optimum temperature being 37- 

 38 C. [Andrewes and Horder point 

 out that all pathogenic streptococci 

 grow best at the temperature of the 

 body but while some are capable of 

 vigorous growth at 20 C. others are 

 altogether incapable of growth at that 

 temperature.] 



Streptococci require a neutral or 

 slightly alkaline medium and grow 

 better on media containing serum or 

 blood than on the ordinary laboratory 

 media. 



Characters of growth. Broth. A 

 typical pathogenic streptococcus does 

 not render broth turbid. After incubating for 24 hours at 37 C. the growth 

 takes the form of a light flocculent precipitate adhering to the walls of the 

 tube which, increasing in amount for the next 3 or 4 days, ultimately falls 

 to the bottom forming a somewhat abundant greyish deposit : the reaction 

 of the medium is then distinctly acid (lactic acid). 



FIG. 285. Strepto- 

 coccus pyogenes. Stab 

 culture in gelatin (5 days 

 at 22 C.). 



FIG. 286. Strepto- 

 coccus pyogenes. Surface 

 culture on agar (4 days 

 at 37 C.). 



