TEXT-BOOK OF BACTERIOLOGY. 127 



bacillus when bred continually and uninterruptedly on our arti- 

 ficial foods lose the capacity of effecting the changes from which 

 they take their names, and Hueppe even speaks of this as a " loss 

 of virulence " in these micro-organisms. 



It lies in the nature of things that this diminution of virulence 

 takes place gradually, proceeding step by step, and not with one 

 great leap. Therefore we are often able to interrupt the process 

 at a certain stage, or even to retrograde and undo the work already 

 done. The best, and indeed the only, means to this end is to give 

 back to the partially-weakened cells their natural conditions, and 

 endeavor to reaccustom them once more to their former way 

 of life. 



In the case of pathogenic species, we first attempt to inoculate 

 them into the animals most susceptible to them. If this fails, we 

 can have recourse to the already-mentioned way of increasing the 

 natural susceptibility of an animal artificially. Should this prove 

 successful and the micro-organism once more establish itself on its 

 natural soil, one may reckon with some degree of certainty on its 

 recovering its former powers; but it is evident that this is not an 

 increase of the virulence originally given to it by nature. 



In direct contrast to the phenomena hitherto discussed is a 

 second mode of diminishing the virulence of micro-organisms which 

 leads to the same final results. That which brings about the 

 diminution is, here, not the long-continued infiuence of altered (but 

 not necessarily worse) conditions of life; it is the action, for a short 

 time, of influence directly prejudicial to the bacterial protoplasm. 

 In fact, all the means employed to produce an artificial diminution 

 of virulence in pathogenic bacteria are such as, if applied in a 

 slightly stronger form, would cause the destruction of the cells and 

 would kill their contents. 



Thus we breed bacteria on media to which a certain quantity 

 of antiseptic or disinfecting substance has been added, but which 

 just allows the microbes to exist and grow upon them. Such, for 

 instance, is the process of Eoux and Chamberland for diminishing 

 the virulence of the anthrax bacillus, by cultivating it in a bouillon 

 with the addition of bichromate of potassium (1 : 5,000 to 1 : 2,000), 

 and that of Toussaint by mixing about 1^ of carbolic acid with 

 blood containing anthrax bacilli. 



In a similar manner — i.e., as a disadvantageous form of nutri- 

 tion — the organism of certain animals is found to act; namely, 

 those animals which are insusceptible, or but little susceptible, to 

 the particular kind of bacteria which we desire to weaken. Thus 

 the bacilli of swine erysipelas lose their virulence to a certain ex- 



