262 MEDICAL BACTERIOLOGY 



the same species, and show lessened virulence after periods of vegetative exist- 

 ence, particularly so if some of the conditions of environment have been detri- 

 mental to their development. Thus streptococci are observed which when 

 isolated from a lesion of man are very virulent for rabbits and perhaps also viru- 

 lent for guinea-pigs but after several weeks or months of cultivation on artificial 

 media become avirulent for guinea-pigs and only slightly virulent for rabbits. 

 If a large dose of such attenuated streptococci is injected into a rabbit, disease 

 develops and organisms from the afflicted rabbit directly transferred to a second 

 rabbit show increased virulence, and by several passages from rabbit to rabbit 

 the original high degree of virulence may be restored. 



Exactly what change occurs in bacteria that results in alteration of virulence 

 is not clear, but some of the obvious changes manifested by some bacteria that 

 have acquired immunity are suggestive. 



ACQUIRED IMMUNITY OF BACTERIA 



Normally rat blood is capable of destroying large numbers of anthrax bacilli. 

 If a minute quantity of rat serum is added to a broth culture of anthrax bacilil 

 and gradually increasing amounts are added to subcultures, eventually subcul- 

 tures will grow abundantly in pure rat serum. This immunity is inherited by 

 subcultures grown in plain broth and it is specific. 



Danysz demonstrated that anthrax bacilli immune to rat serum have 

 developed a mucous sheath or capsule. This mucous capsule unites with the 

 germicidal substance in rat serum and makes it innocuous for anthrax bacilli. 



Metchnikoff, Trommsdorf and others have shown that under analogous 

 conditions streptococci, tubercle bacilli, the typhoid bacillus and cholera vibrio 

 develop a mucous capsule which immunizes them against substances normally 

 germicidal, including arsenic and bichloride of mercury; that many of these 

 acquired immunities become permanent and are transmitted to future genera- 

 tions, that some, at least, of the bacteria retain all the faculties and virulence 

 after acquiring immunity that they possessed before. Furthermore, the 

 acquired immunity of bacteria is specific, i.e., a bacterium that has acquired 

 immunity to the serum of one species of animals is just as susceptible to the sera 

 of other species of animals as it was before; anthrax bacilli that have acquired 

 immunity to formaldehyde are just as susceptible to mercury and phenol as 

 anthrax bacilli in general. 



In acquiring active, specific immunity, some microorganisms, by the same 

 process, acquire other new faculties. Effront has shown that when brewers' 

 yeast is accustomed to living in media containing quantities of hydrofluoric 

 acid germicidal to other yeasts, its power to produce alcohol is augmented and 

 its susceptibility to chemical germicides other than hydrofluoric acid increased. 



Although there is not sufficient data upon which to base broad generaliza- 

 tions, collected observations indicate that as a rule vegetative existence lessens 

 and parasitic existence increases the virulence of bacteria; that when changed 

 from a favorable to an unfavorable environment bacteria either lose some of 

 their faculties or acquire new; virulence being exalted in some instances and 



