BACTERIOLOGICAL STUDY OF EETAIL ICE CREAM. 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 of 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, at least not in 48 hours at 37° C. (97.5° F.), and it was 
possible to plate as low as one-tenth of a cubic centimeter when 
2 dilution had to be used with infusion agar. 
