Fig. 31.—Spores in the end of the rod with enlargement of the rod, A, A, A, A.
Fig. 32.—Drumstick spores at the end of the rod.
The fact that the protoplasm is denser and the spore capsule thicker (the percentage of water in each is decidedly less than in the growing cell) gives the spore the property of much greater resistance to all destructive agencies than the active bacterium has. For example, all actively growing cells are destroyed by boiling in a very few minutes, while some spores require several hours’ boiling. The same relation holds with regard to drying, the action of chemicals, light, etc. That the coagulation temperature of a protein varies inversely with the amount of water, it contains, is shown by the following table from Frost and McCampbell, “General Bacteriology”:
| Egg albumin | plus | 50 per cent. water | coagulates at | 56° |
| Egg albumin | plus | 25 per cent. water | coagulates at | 74–80° |
| Egg albumin | plus | 18 per cent. water | coagulates at | 88–90° |
| Egg albumin | plus | 6 per cent. water | coagulates at | 145° |
| Egg albumin | dry | water | coagulates at | 160–170° |
This resistance explains why it happens that food materials boiled and sealed in cans to prevent the entrance of organisms sometimes spoil. The spores have not been killed by the boiling. It explains also in part the persistence of some diseases like anthrax and black leg in pastures for years. From the above description it follows that the spore is to be considered as a condensation of the bacterial protoplasm surrounded by an especially thick cell wall. Its function is the preservation of the organism under adverse conditions. It corresponds most closely to the encystment of certain protozoa—the ameba for example. Possibly the spore represents a very rudimentary beginning of a reproductive function such as is gradually evolved in the higher thread bacteria, the fission yeasts, the yeasts, the molds, etc. Its characteristics are so markedly different, however, that the function of preservation is certainly the main one.
It must not be supposed that spores are formed under adverse conditions only, because bacteria showing vigorous growth frequently form spores rapidly. Special conditions are necessary for their formation just as they are for the growth and other functions of bacteria ([Chapters VI] and [VII]).
CHAPTER III.
CELL FORMS.
Though there is apparently a wide variation in the shapes of different bacterial cells, these may all be reduced to three typical cell forms. These are: first and simplest, the round or spherical, typified by a ball and called the coccus form, or coccus, plural cocci[4] ([Fig. 33]). The coccus may be large, that is, from 1.5µ to 2µ in diameter. The term macrococcus is sometimes applied to these large cocci. If the coccus is less than 1µ in diameter, it is sometimes spoken of as a micrococcus; in fact, this term is very commonly applied to any coccus. When cocci are growing together, many of the cells do not appear as true spheres but are more or less distorted from pressure of their neighbors or from failure to grow to full size after recent division. Most cocci divide into hemispheres and then each half grows to full size. A few cocci elongate before division and then appear oval or elliptical.