TEXT-BOOK OF BACTERIOLOGY. 123 



there the necessary material is wanting; in one case pathogenic 

 effects are possible, in another they do not occur. The albuminous 

 substances seem to be necessary for the formation of most of the 

 toxines, and therefore there will be only a chance of success in 

 studying these peculiar products if we breed our bacteria on media 

 rich in albumin. 



Another circumstance deserves mention here. Under some cir- 

 cumstances, the excretions of different micro-organisms, which sep- 

 arately are harmless, may unite and develop poisonous qualities. 

 Thus, for instance, we know from the researches of Roger that so 

 harmless a bacterium as Micrococcus prodigiosus, which alone is 

 almost entirely innocuous, can, in conjunction with another bac- 

 terium, which is also non-pathogenic for the animal in question, be- 

 come a cause of disease. A number of other observations all point 

 the same way, although in most of them other factors also oper- 

 ate. All the facts adduced thus far and the reasons for variations 

 in the agency of the bacteria apply, in general, only to those kinds 

 which excrete their poisonous products outside the body exclusively, 

 which are unable to grow and thrive in the body, and which, there- 

 fore, must bring with them the poisonous products required to pro- 

 duce disease, without reckoning on a further increase of the same 

 within the body. These bacteria need not be themselves trans- 

 mitted : it suffices to employ the substances which they have formed, 

 and we may deprive such a culture of its living inmates without 

 diminishing its pathogenic strength. 



It is true a certain precaution is necessary. If the germs be 

 destroyed by submitting the food medium to heat and thorough^ 

 sterilizing it, the very delicate products of excretion are generally 

 also destroyed; they separate into their component parts and 

 lose their peculiar properties. It is therefore better, in all cases in 

 which it is desired to exclude the bacteria and study the soluble 

 substances alone, to separate the two by way of filtration through 

 clay cells. This is a process first employed by Pasteur. 



The latter, in conjunction with Chamberland, has constructed 

 tube-shaped filters of burnt china-clay, or kaolin, which is sure to 

 retain all micro-organisms. At first, as Sirotinin has shown, part 

 of the dissolved substances are retained by the walls of the filter. 

 But after a short time this ceases, and for our purposes the isola- 

 tion of the products of excretion these filters are extremely useful. 

 With the last-mentioned kind of bacteria it can be proved that it 

 makes no difference whether we employ cultures with or without 

 germs, and this gives us a most reliable insight into the nature of 

 the operations of these so-called toxic micro-organisms. 



