148 DIFCO MANUAL 



and Thompson^ described a heated-blood agar tellurite medium, having the ad- 

 vantage of giving a differentiation of gravis, mitis and intermediate strains. Mor- 

 gan and Marshall^ described a Blood Tellurite Agar which gave a high proportion 

 of positive results, and also permitted differentiation between gravis and mitis 

 types. Wilson^ described a blood agar tellurite arsenate selective medium for C. 

 diphtheriae and used a preserved laked blood. McGuigan and Frobisher^ and 

 Frobisher^ have used a Cystine Tellurite Blood Agar for isolation of C. diph- 

 theriae. Hall^ described a medium prepared from Bacto-Cabbage Infusion Agar, 

 10 per cent defibrinated blood, and potassium tellurite for the isolation and 

 identification of C. diphtheriae. Many other investigators have described similar 

 media using tellurite to inhibit Gram-positive organisms and permit the develop- 

 ment of C. diphtheriae. The advantage of these media lies in the fact that C. 

 diphtheriae , if present in relatively small numbers, would not be overgrown by 

 streptococci, staphylococci or other contaminating forms. 



Bacto-Dextrose Proteose No. 3 Agar, enriched with Bacto-Tellurite Blood 

 Solution, produces a selective medium for C. diphtheriae. Staphylococci and 

 streptococci are generally inhibited, while the diphtheria bacilli develop black 

 or grayish black colonies in 18-24 hours. Occasionally a strain of staphylo- 

 coccus is encountered which grows on this medium, producing black colonies. 

 These, however, may be readily recognized as cocci by microscopic examination. 

 Confirmation of typical colonies is made by microscopic examination of the 

 cells and by testing their fermentation reactions. The morphology of C. diph- 

 theriae grown upon tellurite medium varies somewhat from that obtained upon 

 Loeffler Medium. On the selective medium the organisms are generally club 

 shaped, many are barred and only a few show the bipolar staining characteristic 

 of C. diphtheriae on Loeffler Medium. Confirmation may be made by transfer- 

 ring the black suspected colony to Loeffler Medium on which typical morphology 

 can be demonstrated as soon as there has been sufficient growth. 



Wilson^ used a preserved laked blood in the preparation of his medium. Neil^ 

 likewise used this preserved laked blood very satisfactorily in his Tellurite Choc- 

 olate Agar. Bacto-Tellurite Blood Solution, as discussed on page 277 contains 

 potassium tellurite in blood so that upon the addition of 5 ml. to 100 ml. of sterile 

 Dextrose Proteose No. 3 Agar a selective medium for C. diphtheriae, will be 

 obtained. The use of 4 ml. of Bacto-Tellurite Blood Solution per 100 ml. of agar 

 will give a less selective medium, while 6 ml. will produce a more inhibitive 

 medium. For ordinary purposes 5 ml. of the Tellurite Blood Solution will give 

 satisfactory selectivity without marked inhibition of the diphtheria bacillus. 



Chocolate tellurite media prepared from Bacto-Dextrose Proteose No. 3 Agar 

 and Bacto-Tellurite Blood Solution are recommended for a variety of purposes 

 including the diagnosis of diphtheria, typing of strains and determination of 

 virulence and are especially useful in release of cases and detection of carriers. 

 It is recommended that some non-selective medium, Loeffler's for example, be 

 run in conjunction with the selective tellurite medium for detection of C. diph- 

 theriae in acute cases. 



To rehydrate the medium, suspend 40 grams of Bacto-Dextrose Proteose No. 3 

 Agar in 1000 ml. of cold distilled water and heat to boiling to dissolve the medium 

 completely. Distribute in flasks and sterilize in the autoclave for 15 minutes 

 at 15 pounds pressure (121°C.). Add 5 ml. of Bacto-Tellurite Blood Solution 

 to each 100 ml. of sterile melted agar at 70-80° C, under aseptic conditions. 

 Heat at 75-80° C. until it has the appearance of a "chocolate agar" and then 

 cool to 50° C. Distribute into sterile tubes or plates as desired. For best results 

 inoculate the medium the same day as prepared. 



For a plain Dextrose Agar, use without the addition of Bacto-Tellurite Blood 

 Solution. Final reaction of the medium without enrichment will be pH 7.4. 



