DEHYDRATED CULTURE MEDIA 155 



dilutions of the specimen are prepared and 0.01 ml. amounts spread by a glass 

 spreader, over the surface of the Mitis Salivarius Agar containing tellurite. Plates 

 are incubated for exactly 24 hours at 37 °C. S, mitis produces small or minute 

 blue colonies. Some S. mitis colonies may be more easily distinguished with a 

 longer incubation. S. salivarius produces blue, smooth or rough "gum drop" colo- 

 nies 1-5 mm. in diameter, depending on the number of colonies on the plate. 

 Enterococci form colonies dark blue or black in color, shiny, slightly raised, 1-2 

 mm. in diameter. These organisms, few of which are pathogenic, may be readily 

 differentiated from S. mitis and S. salivarius, particularly when viewed by re- 

 flected light. Beta hemolytic streptococci resemble S. mitis. Other types of 

 streptococci have not been studied on this medium. Chapman^ reported that 

 Erysipelothrix rhusiopathiae produce colorless circular convex colonies. He also 

 reported that when coliform organisms do grow they produce brown colonies; 

 however, generally they are not only inhibited but are actually killed. Spreaders 

 are rarely observed. Molds grow after 2 days incubation. Using this medium 

 Chapman was able to demonstrate pathogenic streptococci in about 95 per cent 

 of fecal specimens from chronic invalids. Pathogenicity of these streptococci was 

 determined culturally according to the method described by Chapman^ -^ using 

 hexylresorcinol. 



Bacto-Mitis Salivarius Agar, to which Bacto-Chapman Tellurite Solution has 

 been added, duplicates the medium as described by Chapman.^-^ Comparative 

 tests have shown this medium to be satisfactory for the isolation of streptococci 

 and enterococci from grossly contaminated specimens such as body secretions and 

 excretions. 



To rehydrate the medium, suspend 90 grams of Bacto-Mitis Salivarius Agar 

 in 1000 ml. cold distilled water and heat to boiling to dissolve the medium com- 

 pletely. Sterilize in the autoclave for 15 minutes at 15 pounds pressure (121°G.). 

 Cool to 50-55 °C. and just prior to pouring the plates add exactly 1.0 ml. of 

 Bacto-Chapman Tellurite Solution (discussed in detail on page 277). Prepare 

 plates with 25 ml. medium per 95 mm. diameter plate. Do not heat the medium 

 after the addition of the Tellurite Solution. Final reaction of the medium will 

 be pH 7.0. 



One pound of Bacto-Mitis Salivarius Agar will make 5 liters of medium. 



1 Am. J. Digestive Diseases, 13:105:1946. * J. Bact., 42:653:1941. 



2 Trans. N. Y. Acad. Sciences, 10:45:1947. ^ J. Bact., 48:113:1944. 

 'J. Infectious Diseases, 67:113:1940. * Personal Communicatioa. 



BACTO 



AZIDE BLOOD AGAR BASE (B409) 



DEHYDRATED 



Bacto-Tryptose 10 g. 



Bacto-Beef Extract 3 g. 



Sodium Chloride 5 g. 



Sodium Azide 0.2 g. 



Bacto-Agar 15 g. 



Bacto-Azide Blood Agar Base is a selective medium for the isolation of strepto- 

 cocci from stools, sewage and other specimens. It is also suggested as a selective 

 medium for the isolation of staphylococci. The medium may be employed with 

 the addition of blood permitting the production of typical hemolytic reactions by 

 the streptococci, or may be used without added blood. 



The addition of sodium azide to culture media as a selective agent has been 

 suggested by a number of investigators. Edwards^ in 1933 used a liquid medium 



