144 BACTERIA IN RELATION TO DISEASE 



according to the metatrophic or paratrophic habit of the germ. Strictly- 

 parasitic forms, such as the tubercle bacillus, diphtheria bacillus, and gono- 

 coccus, require the most nourishing media (see Chaps. VI and XVI). Long 

 continued growth in artificial cultures weakens almost all pathogenic bac- 

 teria ; decrease of virulence and morphological changes (involution) showing 

 that nutrient media outside the body can never entirely replace the living 

 tissues. The attenuation can be brought about and increased by many 

 means (see p. 28) and, within certain limits, can be graduated with great 

 precision. It is this possibility of varying the virulence of pathogenic 

 bacteria that underlies a good deal of modern sero- therapeutics and pro- 

 tective inoculation. 



The fact that the spores and dried vegetable cells of pathogenic bac- 

 teria are in many cases possessed of high powers of resistance renders it 

 possible that the dried secretions and excretions of patients may become 

 a source of infection for the healthy. The tubercle bacillus, for instance, 

 is possessed of its full virulence even after lying for two, or even three, 

 months dried up in dust. As far as the strictly parasitic forms are con- 

 cerned the dry resting-stage is the only form in which they are met with 

 outside the animal body. They have never been found growing and 

 multiplying, for disease products are the only vehicle by means of which 

 they are spread abroad and, once outside the body, even in situations that 

 afford abundant food, they invariably succumb to the rapidly-growing 

 metatrophic species. 



But of the many bacteria that are capable of causing pathological 

 changes in the living tissues a large number are of metatrophic habit and 

 able to thrive outside the body. Some grow quickly, others slowly, some 

 have very humble requirements as regards food-stuffs, others are more 

 pretentious, so that the ability to live saprophytically is very different in 

 different cases. All of them, however, represent a much greater danger 

 for mankind than do the obligatory parasites. Not only the pathological 

 products, such as sputum or pus, are possible sources of infection, but 

 all substances, such as milk or impure water, that offer a nutrient sub- 

 stratum and permit these metatrophic forms to multiply, become carriers 

 of disease. 



The detection and isolation of pathogenic germs from among a mixture 

 of species, such as occur in contaminated drinking-water, is often enough 

 a task of the greatest difficulty, and can be carried out successfully only by 

 those who have had long familiarity in the technical details involved. In 

 many cases experiments on animals are necessary to decide whether a given 

 form is pathogenic or not. 



Infection of the uninjured body by bacteria most frequently takes 

 place in those organs which communicate with the exterior, particularly 

 in those that, like the respiratory and alimentary tracts, are the passages 



