DEFINITIONS AND LIMITS S5 
their presence is usually to be explained on the ground that they are 
secondary invaders. 
A smaller group of bacteria are parasitic, that is, they exist upon 
the bodies of living plants, animals or men. Many of them are rarely 
met with in Nature far removed from their respective hosts. Their 
activities are not usually in opposition to those of their host and their 
presence is therefore unnoticed. They may become invasive, how- 
ever, whenever the natural barriers, which ordinarily suffice to keep 
them out, are impaired. 
From the parasitic bacteria there has been gradually evolved a 
small but formidable group of organisms, the pathogenic bacteria, 
whose activities are in partial opposition to those of their host. The 
pathogenic bacteria, like the parasitic bacteria, require a living host, 
but they differ from the parasitic forms in that they actually invade 
the tissues of their hosts and induce progressive (contagious) disease 
from host to host. 
There are no sharply definable limits between these three groups of 
bacteria, the saprophytic, parasitic and pathogenic; the latter appear 
to have arisen from the former by a process of e\'olution. Certain 
general modifications in the general types of chemical activity mani- 
fested by these groups are discernible, however, which are partly the 
result and partly the cause of their change in environment as they 
have passed from a saprophytic to a parasitic existence. Prominent 
among these modifications and activities is a gradual decrease in the 
intensity with which the parasitic and pathogenic bacteria act upon 
their environment. 
The essential function of the saprophytic bacteria in Nature is to 
effect a rapid, deep-seated degradation of organic matter to simple 
compounds; these organisms decompose a relatively large amount 
of substance in a relatively short time. They are active chemically 
and many of them form highly resistant spores which enable them 
to survive prolonged periods of environmental vicissitude. The 
habitually parasitic bacteria, on the other hand, which exist upon the 
bodies of living animals, and the progressively pathogenic bacteria 
which develop within the tissues of animals are not subjected to 
extremes of temperature and food supply; they rarely or never form 
spores. The chemical activity of these organisms is usually much 
less pronounced than that of the saprophytic bacteria.^ Indeed, 
intense chemical activity would be incompatible with their continued 
parasitic existence, for the damage to their host would be insupport- 
able. The parasitic and pathogenic bacteria do not, for example, 
produce widespread liquefaction of the tissues, even when large 
numbers of them are actually growing in the body of the host. The 
growth of invasive organisms in the animal body is characterized 
rather by subtle changes in the composition of the tissues of the 
1 Theobald Smith: Am. Med., 1904, 8, 711. Kendall: Boston Med. and Surg. 
Jour., 1913, 169, 749; Jour. Infec. Dis., 1923, 32, 341. 
