130 MODIFICATIONS OF BIOLOGICAL CHARACTERS. 



though carried through successive cultures for a considerable period, 

 did not regain its power to produce the brilliant carmine color which 

 is the most striking character of the species. Katz, in cultivating 

 the phosphorescent bacilli isolated by him from sea water at New 

 South Wales, found that, after being propagated for some time in 

 artificial media, their power to give off a phosphorescent light was 

 diminished or temporarily lost. He also found that two species 

 which when first cultivated did not liquefy gelatin, subsequently, 

 after a year, caused liquefaction of the usual gelatin medium. 



Modification shoivn in Cultures. When bacteria have been 

 subjected to the action of heat or chemical agents, without having 

 their vitality completely destroyed, they often show diminished vigor 

 of growth. Cultures which would ordinarily show an abundant de- 

 velopment within twenty-four hours may not commence to grow for 

 several days. For this reason, in disinfection experiments, it is neces- 

 sary to test the question of destruction of vitality by leaving the cul- 

 tures for a week or -more under favorable conditions as to tempera- 

 ture. In plate cultures or Esmarch roll tubes a few colonies may 

 develop in this tardy way, showing that there was a difference in the 

 vital resisting power of the individual cells, some having survived 

 while the majority were killed. This is well illustrated by Abbott's 

 experiments upon the germicidal action of mercuric chloride as tested 

 upon Staphylococcus pyogenes aureus. Irregularities in the results in 

 experiments in which the conditions were identical having been no- 

 ticed, Abbott inferred that this was due to a difference in the resist- 

 ing power of individual cocci (arthrospores ?). By making cul- 

 tures from colonies which developed from these more resistant cocci, 

 and again exposing the micrococci in these cultures to mercuric chlo- 

 ride in the proportion of 1:1,000 for a longer time and making new 

 cultures from the surviving cocci, and so on, Abbott obtained cultures 

 in which a majority of the cells survived exposure to a solution of the 

 strength mentioned for ten to twenty minutes, whereas in his original 

 culture most of the cocci were killed by this solution in five minutes. 



These changes in vital resisting power enable us to comprehend 

 other modifications which can only be detected by chemical or bio- 

 logical reactions. Thus the reducing power for various substances 

 may be modified by changes in the conditions of environment. And 

 among the pathogenic bacteria changes of a more or less permanent 

 nature may be induced, which are shown by a modified degree of 

 virulence when injected into susceptible animals. 



Attenuation of Virulence may be effected by several methods, 

 all of which depend upon subjecting the cultures to prejudicial in- 

 fluences of one kind or another. 



Pasteur first announced, in 1880, that the microbe of fowl cholera 



