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.óBacteria 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 Zeitschriftfur Hygiene, xxii., Heft 2, 1896, p. I.