188 BIOLOGY OF PNEUMOCOCCUS 



are ordinarily chosen for the isolation of pneumococci, the pre- 

 liminary testing of antigenic substances, and for the determination 

 of type-specificity and potency of diagnostic and curative immune 

 serums. Albino strains are preferable to strains of pigmented or 

 wild varieties because of their lower resistance to infection. Pure- 

 line races, with their inherited uniformity of susceptibility, would 

 be ideal types for routine investigative purposes were it not for 

 their present scarcity and prohibitive cost. That susceptibility or 

 resistance to infection are transmissible characters and that the 

 breeding of races with either high or low degrees of susceptibility 

 can be accomplished by proper selection has been demonstrated by 

 Irwin and Hughes 670 in the case of the rat for bacteria of the en- 

 teric group. 



The extreme susceptibility of the mouse to subcutaneous, intra- 

 peritoneal, or intravenous injection of Pneumococcus is shown by 

 the rapid invasion of the blood stream and the death of the ani- 

 mals without localization of the infection. Introduced by any of 

 these routes Pneumococcus may be highly infective for mice. When 

 the virulence of a strain of Pneumococcus has been exalted by suc- 

 cessive mouse passage, the cultures used for intraperitoneal injec- 

 tion into a mouse may be so diluted that, although the amount used 

 for inoculation may yield only one or even no colonies, infection 

 will frequently follow. In fact, Wamoscher 1478 has demonstrated by 

 micromechanical isolation of single cells that one pneumococcus 

 may suffice to infect a mouse. It is for this reason, therefore, that 

 the mouse is so admirably adapted for the detection of pneumo- 

 cocci in infective material, for determining virulence, for testing 

 the immunizing action of antigens, and for measuring the protec- 

 tive power of specific immune serum. 



An observation concerning the dominance of one type of Pneu- 

 mococcus in causing general infection in the mouse following the 

 injection of an inoculum containing pneumococci of several types 

 was described by Etinger-Tulczynska. 368 When mixtures of equal 

 parts of cocci of different types were administered either by the 



