426 THE DIPHTHERIA BACILLUS GROUP 
a well-marked turbidity. The diphtheria bacillus grows well in milk, 
producing an initial acid reaction during the first two or three days 
of incubation, followed by the gradual development of an alkaline 
reaction.^ No gross changes, however, are produced in the milk, even 
with prolonged cultivation. The growth on potato is very slight 
provided the reaction of the potato is alkaline; no growth at all takes 
place on acid potato. 
The diphtheria bacillus is an aerobic, facultatively anaerobic organ- 
ism. Its limits of growth are 17° C. as a minimum, 43° C. as a maxi- 
mum, with the optimum at 37° C. Ten minutes exposure to 60° C, 
five minutes at 70° C, or one minute at 100° C. readily kills diph- 
theria bacilli. The organisms are occasionally transmissible through 
milk, and in this connection it should be remembered that the ordinary 
method of heating milk in an open vessel will not certainly kill diph- 
theria bacilli, for, as Theobald Smith^ pointed out many years ago 
Fig. 61. — Bacillus diphtheii;r liraiifhing. X 800. 
a scum forms on the free surface of the milk, consisting of casein and 
lime salts, which is a non-conductor of heat. Within this membrane 
the diphtheria bacilli may resist a long period of heating. Diphtheria 
bacilli exposed to heat enclosed in a false membrane, as for example, 
those taken from the throat, may resist an exposure of 95° to 100° C. 
even for an hour. Organisms dried in this membrane may remain 
viable at low temperatures, protected from sunlight, from three to 
five months. Naked germs are readily killed by antiseptics in the 
ordinary concentrations, but those exposed to the action of antiseptics 
protected in membranes may resist for some time. Hydrogen peroxide 
is said to be particularly germicidal for the diphtheria bacillus. 
Products of Growth. — r7?6'?«?crt/.—B. diphtheria^ produces acids, chiefly 
lactic, together with smaller amounts of acetic and formic acid from the 
fermentation of glucose and maltose. Lactose, saccharose and man- 
1 Kendall. Day and Walker: Jour. Am. Chem. Assn., 1914, 36, 1950. 
2 Jour. Exp. Med., 1899, 4, 217. 
