CHEESE 627 



lively large quantity. Perhaps even small amounts may affect 

 people with weak digestive organs. The causes of cheese poison- 

 ing are still obscure, and precise observations are desirable. Pos- 

 sibly the presence of bacteria of the Bacillus enteritidis group may 

 be responsible for some cases of cheese poisoning. 



Bacteriologic examination of cheese is accompanied by some 

 difficulties, the most important of which is the proper selection of 

 culture-media. It is obvious that the use of ordinary laboratory 

 media may lead to erroneous results, since some bacteria do not 

 grow on them. Lactobacilli, for example, grow but poorly, some 

 strains not at all, on ordinary media. Eldredge and Rogers, in 

 their studies of Emmenthaler cheese, used a medium prepared in 

 the following manner: 



"(a) Heat skimmed milk to the boiling-point and add -20 c.c. 

 of 10 per cent, lactic acid per liter of milk. 



"(b) Filter, and add to the filtrate: pepton, 1 per cent.; beef 

 extract, 5 per cent.; agar, 1.2 per cent. 



"(c) Heat to the boiling-point until the agar is melted. 



"(d) Correct the reaction to 1.2 per cent., using phenol- 

 phthalein. 



"(e) Cool to 45 C. and add the white of one egg to each liter. 



"(/) Heat to 100 C. for forty-five minutes and filter through 

 cotton. 



"(g) Correct reaction if necessary, tube, and sterilize three suc- 

 cessive days in the Arnold." 



The authors state that cultures of the Bacillus bulgaricus type 

 form good-sized colonies on this medium in four to five days at 

 30 C. 



A suitable medium for growth of bacilli of the Bacillus bulgari- 

 cus type was used by the writer and Hefferan. This is prepared 

 in the following manner: 



Certified milk is skimmed and heated. When near the boil- 

 ing-point a few drops of acetic acid are added to coagulate the 

 casein. The serum is filtered off and enough sodium hydrate 

 solution added to make the reaction 1 per cent, acid to phenol- 

 phthalein. Then 1 per cent, pepton and 2 per cent, dextrose are 

 dissolved. In this solution 1.5 per cent, agar is dissolved by 

 heating, and the medium clarified with the whites of eggs. Then 

 it is tubed and sterilized in the autoclave. At temperatures from 

 37 to 45 C. bacilli of the Bacillus bulgaricus type develop large 

 colonies in about two days. 



For isolation of the majority of bacteria in milk or milk prod- 

 ucts Ayer's casein agar is an excellent medium. The preparation 

 of this medium has been given on page 415. 



Samples of cheese for bacteriologic examination should be 



