332 BIOLOGY OF PNEUMOCOCCUS 



ble of calling forth a specific immune response, other attempts 

 were made to rob the cell of the power of invasion and infection 

 without impairing its antigenic qualities. For a long time for- 

 maldehyde in the form of formalin has been employed for the pur- 

 pose of killing and preserving pathogenic organisms, but it is not 

 clear when it was first used to prepare pneumococci for the pur- 

 pose of immunization. Tao (1932) 13so reported that the antigenic 

 potency of formalinized cultures was greater than that of heat- 

 killed cultures in causing the development of agglutinins and pro- 

 tective antibodies in rabbits and mice. Formalin in a concentration 

 of 0.3 per cent was added to the cultures and the treatment of the 

 animals was limited to one subcutaneous and two or three intra- 

 venous injections. 



Pico and Negrete 1089 prepared immunizing antigens for horses 

 with 0.05 per cent formalin. The authors attributed the keeping 

 qualities of the vaccine to the action of formaldehyde in rendering 

 the soluble specific substance insoluble in water, but it has been the 

 experience of the writers of the present volume that concentrations 

 of formalin as high as 0.3 per cent fail to prevent the gradual lysis 

 of pneumococci suspended in salt solution. In Ferguson's 436 experi- 

 ence in actively immunizing white mice and rabbits with both heat- 

 killed and formalinized vaccines, it appeared that the latter were 

 only slightly superior to heat-killed cultures. Formalin was used in 

 a strength of 0.2 per cent, and suspensions were made in salt solu- 

 tion containing 0.5 per cent phenol. There was a tendency of killed 

 pneumococci to autoWze with a loss of antigenic power accom- 

 panying the disintegration of the cell. For this reason, as well as 

 because of their better immunizing effect, Barnes and White gave 

 their choice to heat-killed vaccines, since these vaccines undergo 

 less deterioration than those prepared with formalin. 



According to Dubos,* formalin added in a concentration of 0.3 

 per cent to suspensions of living pneumococci prevents multiplica- 

 tion but fails to inhibit the activity of intracellular enzymes. The 



* Personal communication, 1936. 



