BACTERIOLOGICAL STUDY OF EETAIL ICE CEEAM. 15 



COLON BACILLI IN ICE CREAM. 



Since the presence of colon bacilli has been understood in water 

 analysis to indicate fecal contamination, many investigators and 

 boards of health apply the same tests to milk, and naturally then to 

 ice cream with the same idea. 



In water analysis lactose-bile fermentation tubes are used for the 

 examination for colon bacilli. By using different dilutions the min- 

 imum number oi gas-forming bacteria in a given amount of water 

 may be determined. This preliminary test has to be followed by 

 confirmatory tests in which cultures are isolated and their character- 

 istics studied in order to prove the presence of colon bacilli. In our 

 work we have used this method to some extent, but have endeavored 

 to prepare a synthetic medium which would restrict the growth of 

 the majority of bacteria found in ice cream and at the same time 

 would allow colon bacilli to develop and produce characteristic 

 reactions. During the experiments 53 different combinations were 

 used, as shown in Table 10. 



Asparagin was used as a source of nitrogen in almost all media. 

 Throughout our work we used medium No. 1, which was made as 

 follows: Agar, 1.5 per cent; asparagin, 0.3 per cent; sodium dibasic 

 phosphate, 0.1 per cent; lactose, 1 per cent; and 2 per cent of a satu- 

 rated neutral solution of litmus. This medium proved the most 

 satisfactory of all, and when tested with 10 different strains of Bacillus 

 coli it was found that they all grew well. Medium No. 13, recom- 

 mended by Dolt, did not prove satisfactory, as several of the strains 

 of coli did not develop well on this medium. Medium No. 53, com- 

 posed of agar 1.5 per cent, lactose 1 per cent, ammonium malate 0.2 

 per cent, sodium acid phosphate 0.02 per cent, and 2 per cent satu- 

 rated neutral solution of litmus, proved very satisfactory in a few ex- 

 aminations, but as this medium was developed late in the work it has 

 not been thoroughly tested. The value of the other media will not 

 be discussed here but will be reserved for a later continued study Of 

 the subject. 



When ice cream was plated on litmus-lactose-asparagin agar, 

 colon bacilli proved to be about the only organisms which grew' and 

 formed acid. The colonies were quite distinctive in their appearance 

 and with little practice could readily be counted. Occasionally there 

 developed on the plate a few very small acid colonies which were not 

 gas formers, and sometimes a few nonacid-forming colonies were 

 observed. The majority of bacteria in ice cream did not grow on 

 this medium, ;ii least not in is hours at 37° C. (97.5° F.), and it was 

 possible in plate as l<>w as one-tenth of ;i cubic centimeter when 

 looho wo dilution had to be used with infusion agar. 



