304 MICROBIOLOGY 



broth composed of ordinary meat infusion or meat-extract 

 broth with addition of 0.5 per cent glacial acetic acid and two 

 per cent glucose. A transfer to litmus milk was made after 24 

 hours. The large amount of acid seems to restrain other bac- 

 teria from multiplying and interfering with the growth of 

 Bact. bulgaricus. The litmus milk tubes were then used for 

 plating in mjlk-serum agar, to which we also added 0.5 per 

 cent glacial acetic acid. The plates were incubated at 42 C, 

 Microscopic colonies are usually visible in 24 hours, and are 

 discernible to the naked eye after 48 hours. They are small 

 and resemble colonies of streptococci, but may be distinguished 

 from these under a hand lens by their slight irregularity of 

 contour. Under the No. 3 objective they have the character- 

 istic wooly appearance of Bact. bulgaricus. If present in the 

 original material, they occur on such a plate in nearly pure 

 culture, other forms having been suppressed by the high tem- 

 perature and by the acidity of the agar. The litmus milk 

 tubes themselves are, after 48 hours, an indication of the 

 presence of Bact. bulgaricus in the material added to them. 

 The color of the litmus caused by Strept. lacticus is a light 

 pink, but that caused by Bact. bulgaricus soon becomes a deep 

 vivid pink, which is quite distinctive. We were soon able to 

 tell at a glance from a series of milk tubes which were likely 

 to prove positive for this organism. On whey-agar (one per 

 cent acid to phenolphthalein) plates at 37 C. to 42 C. the 

 24-hour colonies are minute and almost indistinguishable to 

 the eye from streptococcus colonies. If numerous they show 

 on the plate like fine grains of sand. Under the microscope the 

 edges of even the smallest colonies are seen to be broken or 

 fringed and the 48-hour colonies are characteristically densely 

 matted, wooly at the center, and branching at the edges. The 

 colonies remain small and are mostly under the surface. On 

 acetic-acid agar they appear readily and numerously, but re- 

 main minute and are smaller than streptococcus colonies. On 

 plates from the gastric juice of a case of carcinoma two some- 

 what different types could be distinguished, one being the 

 same as usual, the other more dense. 



"On whey-agar slants the cultures show a good develop- 



