TEXT-BOOK OF BACTERIOLOGY. 17 



grow only in the living- bodies of the higher organisms, at whose 

 expense and generally to whose detriment they live as genuine 

 parasites, and without which they cannot exist at all. 



These are called the strictly parasitical bacteria, and the first 

 mentioned, which do not thrive in living organisms, are called the 

 strictly saprophytic bacteria. The boundary is, indeed, not im- 

 passable. There exists a whole class of bacteria which can find the 

 eonditions for life as well outside other living beings can live sapro- 

 phytically as they can by penetrating into foreign organisms and 

 there live as parasites; these are the semi-parasitical or semi- 

 saprophytlc bacteria. 



Besides the conditions already mentioned, there are some other 

 factors which are of importance in the life of the bacteria. 



A certain degree of warmth is quite necessary for their plentiful 

 development. Warmth is a mighty spring- in the clock-work of all 

 organic life, and its influence on the bacteria is unmistakable. 



The temperature required by micro-organisms for their pros- 

 perity is, it is true, very different for the different species. In g-en- 

 eral, however, we may say that the limits between which the bac- 

 teria can exist are from 40 C. down to about 10 C., yet but few 

 of them can exist throughout this entire range. By far the greater 

 number are restricted to a much narrower compass, within which, 

 again, there is another still narrower range, an optimism of tem- 

 perature, at which they thrive most quickly and luxuriantly. 



There is a particularly noticeable difference here between the 

 strictly saprophytic and the strictly parasitical bacteria. The 

 former find the best conditions for their growth in the average 

 temperature of our summer months or the medium temperature 

 of rooms i.e., about 24 C. This limit some of them can hardly 

 pass ; that is, they die in a higher temperature, and are therefore 

 incapable of living parasitically in the warm-blooded creatures and 

 there causing- disease. 



The parasitic species, on the other hand, develop most favorably 

 and quickly at the temperature of incubation i.e., at about 35 to 

 40 C. Some micro-organisms which cling 1 very tenaciously to their 

 parasitic habits, which only with difficulty and by artificial means 

 can be induced to grow on our culture media outside of living bodies, 

 obstinately refuse to bate anj^thing of their due temperature. The 

 tubercle bacillus, for instance, only thrives at a constant tempera- 

 ture of 37 C., and a slight departure therefrom produces defective 

 development. 



If the temperature sinks below or rises above the proper degree 

 the bacteria fall into a state of insensibility and inactivity, from 



