On conditions necessary to obtain a clean milk supply and on methods of testing cows' milk in relation to standards of cleanness : report to the Sub-committee on clean milk / by S. Delépine, June 16th, 1918.

Auguste Sheridan Delépine

Date:: 1918

Credit: On conditions necessary to obtain a clean milk supply and on methods of testing cows' milk in relation to standards of cleanness : report to the Sub-committee on clean milk / by S. Delépine, June 16th, 1918. Source: Wellcome Collection.

16/116 page 14

$When the milk is added to the jelly the t\^'o should be tlioroughly mixed before being poured on to the plate, and the temperature of the melted jelly should not be above blood heat. In some American laboratories the measured quantity of milk is poured direct into the dish in which the gelatine plate is made. I dou])t whether uniformly satisfactorj^ results can be obtained by such a hasty procedure. It is true that minute accuracy is not necessary to obtain results capable of general application in administrative work. On the other hand, any slackness in bacteriological work is liable to degenerate into carelessness, the results of which may lead to gross injustices and other unpleasant consequences The results obtained are greatly influenced by the culture media used, the temperature at which they are incubated, and the duration of the incubation. The methods adopted in various countries and laboratories differ materially, so that the results obtained in various places are not absolutely comparable. This must always be kept in mind in connection with the fixing of standards. The culture media used in my laboratory are (a) peptone-bouillon- gelatine (nutrient gelatine) and (b) peptone-bouillon-litmus-lacto,se-agar (litmus lactose agar). The first of these media has been used in most bacteriological laboratories, including my own, for more than thirty years, and when due attention is paid to its reaction and the qualitj^ of the peptone and of the gelatine it may be looked upon as one of the most valuable standard media, owing to the greah number of observations for w^hicli it has been used by leading bacteriologists all through the wnrld. Nutrient gelatine is used to cultivahe bacteria at a temperature of 20° C., wTich is that of a warm room. Many of the air, water, and soil bacteria grow wnll at that temperature, and are capable of producing in tw^o days colonies large enough to be visible to the naked eye ; in the case of a fairly large number of bacteria three days are necessary to obtain this result, and there are some slow -growing bacteria which become visible only at the end of several more days. Most of the bacteria capable of growing on nutrient gelatine at 20° produce colonies clearly recognisable in three days. It has, therefore, been customary to enumerate the colonies at the end of 72 hours, the number recognisable at the end of 48 hours does not bear a constant relation to the number of those visible at the end of 72 hours, but some observers are satisfied with the results of the shorter incubation; their figures are very much low'er than those of other bacteriologists, and do not represent accurately the bacterial contents of milk. In some laboratories only the bacteria visible to the naked eye are counted; in others, my owm included, a microscope magnify¬ ing 10 to 15 times is used. There are many common and pathogenic bacteria wdiich do not grow on gelatine under the conditions described above: several thousand tubercle bacilli might be present in a sample of milk without any sign of then presence being found by the ordinary plate method.$