MODES OF BACTERIAL ACTION 25 



instruments; (d) Koch's steam steriliser, by means of which a 

 crate is slung in a metal cylinder, at the bottom of which water is 

 boiled; (e) the autoclave, which is the most rapid and effective 

 of all the methods. This is in reality a Koch steriliser, but with 

 apparatus for obtaining high pressure. The last two (d, e} are used 

 for sterilising the nutrient media upon which bacteria are culti- 

 vated outside the body. Blood serum would, however, coagulate 

 at a temperature over 60 C. (124 F.), and hence a special steriliser 

 has been designed to carry out fractional sterilisation daily for a 

 week at about 55 C.-58 C. 



Modes of Bacterial Action 



In considering the specific action of micro-organisms, it is desir- 

 able, in the first place, to remember the two great functional divisions 

 of saprophyte and parasite. A saprophyte is an organism that 

 obtains its nutrition from dead organic matter. Its services, of 

 whatever nature, lie outside the tissues of living animals. Its life 

 is spent apart from a " host." A parasite, on the other hand, lives 

 always at the expense of some other organism which is its host, in 

 which it lives or upon which it lives. There is a third or inter- 

 mediate group, known as " facultative," owing to their ability to act 

 as parasites or saprophytes, as the exigencies of their life may 

 demand. 



The saprophytic organisms are, generally speaking, those which 

 contribute most to the benefit of man, and the parasitic the reverse, 

 though this statement is only approximately true. In their relation 

 to the processes of fermentation, decomposition, nitrification, etc., we 

 shall see how great and invaluable is the work which saprophytic 

 microbes perform. Their result depends, in nearly all cases, upon the 

 organic chemical constitution of the substances upon which they are 

 exerting their action, as well as upon the varieties of bacteria them- 

 selves. Nor must it be understood that the action of saprophytes is 

 wholly that of breaking down and decomposition. As a matter of 

 fact, some of their work is, as we shall see, of a constructive nature ; 

 but, of whichever kind it is, the result depends upon the organism and 

 its environment. This, too, may be said of the pathogenic species, 

 all of which are in a greater or less degree parasitic. It is well 

 known how various are the constitutions of man, how the bodies of 

 some persons are more resistant than those of others, and how the 

 invading microbe will meet with a different reception according to the 

 constitution and idiosyncrasy of the body which it attacks. Indeed, 

 even after invasion the infectivity of the special disease, whatever it 

 happens to be, will be materially modified by the tissues. When we 

 come to turn to the micro-organisms which are pathogenic parasites 



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