BIOLOGY OF BACTERIA. 57 



Milk usually contains bacteria, entering it from the 

 dust of the dairy, possessing this power. In the process 

 of peptonization the milk may become bitter, but need 

 not change its original reaction. As the peptonization 

 progresses the milk very often becomes poisonous, espe- 

 cially to individuals under two years of age, and may 

 bring about a fatal enterocolitis or "summer complaint." 

 The disease does not only occur in consequence of toxic 

 substances formed from the split-up albumins, or from 

 the presence of metabolic products of the bacteria, but, 

 as Liibbert has shown, 1 from the presence of the bacteria 

 themselves. One reason that the enterocolitis caused in 

 this way comes on in summer is that it is only in un- 

 usually warm weather that these bacteria are able to 

 grow luxuriantly. 



Sometimes the properties of coagulation and digestion 

 of milk are valuable aids in the separation of different 

 species of bacteria. 



12. Production of Disease. Bacteria which produce 

 diseases are known as pathogenic; those which do not, 

 as non-pathogenic. Between the two groups there is no 

 sharp line of separation, for true pathogens may be culti- 

 vated under such adverse conditions that their virulence 

 will be entirely lost, while at times bacteria ordinarily 

 harmless may be made toxic by certain manipulations or 

 by introducing them into animals in certain combina- 

 tions. The diseases produced are the result of the sum 

 of numerous activities exhibited by the bacteria. For 

 example, it may be that a microbe, having effected its 

 entrance into an animal, grows with great rapidity, 

 completely blocking up the blood- and lymph-channels, 

 so that the proper circulation of these fluids is stopped 

 and disease and death must result. Perhaps more com- 

 mon than this is a local establishment of the organisms, 

 with a resulting inflammation, due partly to the presence 

 of the foreign organisms, and partly to their toxic me- 

 tabolic products. More often, however, the pathogenic 



1 Zeitschrift fur Hygiene, xxii., Heft 2, 1896, p. I. 



