154 PATHOGENIC ORGANISMS 



129. In the microscopical examination, the streptococci having 

 the typical form of Str. lacticus (elongated cocci, usually in 

 pairs) should be ignored and a search made for the picket fence 

 variety only. These, on staining with methylene blue, usually 

 appear in chains with solidly stained portions at right angles 

 to the longitudinal axis; capsules are usual but are not invari- 

 ably foimd. Some observers attach more significance to the 

 long-chain types, but in view of the numerous cases in which the 

 short-chain types have been associated with pathological con- 

 ditions, it would appear to be good policy to attach equal 

 significance to both varieties. The property of chain forma- 

 tion is undoubtedly a variable one and is profoundly modified 

 by the composition of the medium and general environment. 



In the indirect method, the sample is diluted as in the exam- 

 ination for fffical streptococci and the various dilutions seeded 

 into dextrose broth. After incubation for forty-eight hours 

 at 37° C, the cultures are examined for chain formation by 

 making a smear or a hanging drop preparation; from the 

 smallest quantity containing typical chains the approximate 

 number of streptococci can be calculated. If desired, the broth 

 cultures can be plated out on nutrient agar or gelatine, and the 

 organisms isolated in pure culture. The quickest and most 

 satisfactory method of examination for pathogenic streptococci 

 is by plating on blood agar. Ruediger*^ as early as 1912 

 suggested the differentiation of Str. pyogenes from Str. lacticus 

 by the hsemolytic properties of the former and since that date 

 several workers have demonstrated that haemolysis is a usual 

 property of the pathogenic streptococci. All hsemolytic strains, 

 however, are not pathogenic. 



The best technique is to add various dilutions of the sample 

 to 10 c.cms. of meat infusion agar containing 1 c.cm. of horse 

 blood and then pour into Petri plates. These are incubated 

 at 37° C. and examined after twenty-four and forty-eight hours 

 for hsemolysis. Those colonies showing a clear, transparent, 

 colourless zone are transferred to broth and finally inoculated in 

 the usual Gordon test media, viz., dextrose, saccharose, raf- 



