The origin of some of the streptococci found in milk / by L.A. Rogers and Arnold O. Dahlberg.

  • Lore Alford Rogers
Date:
1914
Catalogue details

Licence: Public Domain Mark

Credit: The origin of some of the streptococci found in milk / by L.A. Rogers and Arnold O. Dahlberg. Source: Wellcome Collection.

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    The cultures were incubated for seven days at 30° and titrated cold against twentieth-normal sodium hydrate with phenolphthalein as an indi- cator. From the results so obtained is subtracted the titration of a blank, and the result is expressed as the percentage of normal acid. Some objec- tion may be raised against the use of 30° C. as an incubation temperature rather than the more common one of 37°. The lower temperature was adopted because practically all streptococci will grow at this temperature, while a few grow at 37° slowly or not at all. The fermentation produced by the streptococci is in almost all cases so marked that there is very rarely any question about the presence or 7 a A. MEAN L6, FO, AND FC ON DEXTROSE. | B. EAN LE ANDO EY ON CLYCERIN. / Q 400 O. MEAN ET ANDO HW OV GLYCERIN. / AN) 0. (7EAN ER, £0, Pl, EY, OG ANDO NR / Un OW GLYCERIN. &50 N 300 & S < 250 S Sie! N N 4.50 - eee ae ae 7 #2 g F 5s Co vA oe 9 TO 17 2 3 O44 Yrs. Fic. 3.—Curve showing the typical rate of fermentation of dextrose and glycerin. absence of the fermentation. Of all the substances we have used gly- cerin forms an exception to this rule. The fermentation proceeds slowly and in seven days may be slightly above or slightly below 1 per cent normal acid, the point selected as marking the line between fer- mentation and no fermentation. This is illustrated by Table I, which shows the progressive rate of fermentation by typical cultures. Three cultures fermenting dextrose are included to show the usual course of the fermentation in the more easily fermented sugars. Each titration was made from a separate tube. A study of this table shows that the 12 cultures may be divided into three quite distinct types on the basis of the rate of fermentation of glycerin. ‘This is shown more clearly in figure 3, in which the average titrations for each of the three types are plotted. Two of these cultures fermented the glycerin with comparative
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