4 INFECTION AND RESISTANCE 



practically synonymous respectively with "parasitic" and "sapro- 

 phytic." But, as we shall see, although as a rule a micro-organism 

 must be parasitic to possess pathogenic powers, some of the true 

 saprophytes or so-called half-saprophytes may be pathogenic under 

 certain conditions, and the terms do not cover each other absolutely. 

 It is reasonable to suppose that all micro-organisms were origi- 

 nally in the condition which we designate by the term "saprophytic." 

 By this term we imply that these germs maintain themselves only 

 upon dead organic matter and do not thrive in or upon the living 

 animal tissues. The class of saprophytes is widely distributed and 

 constitutes, of course, the most important group of bacteria in na- 

 ture, since upon the activities of these germs depends the unlocking 

 of nitrogen and carbon from the organic complexes in the dead bod- 

 ies and waste products of animals and plants. Such bacteria if 

 strictly saprophytic, that is, entirely unable to maintain themselves 

 upon living tissues, have little importance as producers of disease, 

 or, expressed in technical terms, have little "pathogenicity." Nev- 

 ertheless, there are cases in which strict saprophytes may cause dis- 

 ease by lodging upon and growing in animal tissues which have 

 been killed by other causes, so-called necrotic areas ; and these, still 

 being in relation with the body as a whole through the blood and 

 lymph channels, furnish an area of saprophytic growth from 

 which products of putrefaction or even bacterial poisons may be 

 absorbed. While, as a rule, the disease following the invasion 

 of necrotic tissue such as gangrenous amputation stumps, old 

 unhealed sinuses, diabetically gangrenous areas, etc., may be caused 

 by a large variety of saprophytic bacteria, there are a few very 

 important and specifically pathogenic bacteria which are, strictly 

 speaking, saprophytes. Thus the form of meat poisoning caused 

 by the Bacillus botulinus is due entirely to the poison formed by 

 this bacillus outside of the body within the substance of the dead 

 foodstuff, and disease ensues as the result of subsequent ingestion of 

 this poison with the food. In the same way the tetanus bacillus and, 

 less strictly speaking, the diphtheria bacillus, at least in its ordinary 

 mode of attack, are rather closer to the class of saprophytes than to 

 that of the parasites, since neither of these bacteria, under usual 

 -circumstances, invades the substance of the tissues beyond the point 

 of initial lodgment, causing disease only by the production of 

 ispecific poisons, a condition known as "toxemia" or intoxication. 

 The tetanus bacillus, moreover, is not usually capable of maintain- 

 ing itself and multiplying even at the point of initial lodgment 

 unless the tissues have been injured by trauma or irritated by the 

 presence of foreign bodies. Bacteria of such characteristics, there- 

 fore, though pathogenic that is, incitant of disease remain never- 

 theless essentially saprophytes living upon the dead animal tissues, 

 not invading the living cells or body fluids. It is true that invest!- 



