38 GENERAL PHYSIOLOGY OF BACTERIA 
probable that the reaction of the medium plays a not unimportant part 
in determining the conditions for sporulation. Spores are rarely, if 
ever, produced within the intact tissues of the animal body: if the 
tissues are exposed to the air, however, particularly postmortem, spore 
formation may take place. No bacteria progressively pathogenic for 
man are known to form spores. 
The unusual resistance of mature spores to desiccation, to exposure 
to dry and moist heat, and to disinfectants may be due either to their 
low content of water, for spores contain less than one-half the water 
contained in the normal vegetative cell, to the relatively thick refractile 
spore membrane, or to unusual concentration of fatty and lipoidal 
substances. Experiments by Lewith^ would suggest that the relative 
desiccation of the contents of spores as compared with the moisture 
content of the vegetative organism would be the most plausible expla- 
nation of their resistance to heat without apparent injury. He found 
that egg albumen (dried) suspended in 5 per cent of water coagulated 
at 145° C; suspended in 18 per cent of water, coagulation took place 
at 90° C; with 25 per cent of water, at 80° C; and in a considerable 
volume of water (amount not stated) coagulation occurred when the 
temperature reached 56° C. 
^W °o^.^ 
.« 
Fig. 4. — Germination of bacterial spores. 1, by absorption of spore membrane; 
2, equatorial germination; 3, polar germination. 
The resistance of spores to physical conditions varies somewhat 
according to the organism in which they are formed. Generally 
speaking, however, several minutes' exposure to the temperature of 
boiling water (100° C.) may fail to kill them. Dry heat is less effec- 
tive than moist heat, for an exposure of 160° C. for one and a half 
hours is required to certainly sterilize glassware containing spores. 
Ten to 15 pounds live steam pressure for fifteen minutes is required 
to effect sterilization of liquids and organic matter in general. Direct 
sunlight will kill spores after days of exposure. 
Germination of bacterial spores takes place when they are placed 
in a suitable nutritive environment in which the temperature, moisture 
and oxygen relations are favorable. The vegetative cell breaks 
through the spore membrane, apparently after the latter has lost its 
refractility, and reproduction by fission proceeds anew, and persists 
until environmental conditions again lead to sporulation. 
1 Arch. f. exp. Path. u. Pharmakol., 1S90, 26, 341. 
