PATHOGENICITY FOR EXPERIMENTAL ANIMALS 199 



entities as lobar pneumonia and bronchopneumonia, and those of 

 low virulence were associated with minor illnesses or with disease 

 occurring in persons already debilitated. Whittle denied the exist- 

 ence of any relation between serological type and pathogenicity. 



Gundel and Wasu (1931) 578 reported that the virulence of a 

 given type of Pneumococcus reached a maximum at the height of 

 the disease process and that in the complications of lobar pneu- 

 monia in man, such as meningitis and otitis media, only strains of 

 the highest virulence were found, the organisms usually belonging 

 to the fixed serological types. Further discussion of the infectious- 

 ness for man of pneumococci of the various serological types will 

 be found in the following chapter. 



NUMBERS OF COCCI REQUIRED TO INFECT 



Among reports on the quantitative determination of the infec- 

 tive ability of pneumococcal strains there may again be mentioned 

 the observations of Wamoscher (1926). 1478 L T sing a strain of Type 

 III Pneumococcus, counting the number of cocci in the inoculum 

 by means of the Peterfi micromanipulator, and injecting the or- 

 ganisms subcutaneously into white mice, the author found that ap- 

 proximately one-quarter of the number of mice so treated suc- 

 cumbed to pneumococcal infection in two to four days after re- 

 ceiving an inoculation of one pneumococcus. The percentage of 

 fatal infections rose with an increase in the number of organisms 

 injected. Using only freshly isolated strains of pneumococci grown 

 in broth from the culture obtained from a single mouse passage 

 after isolation from a human source, and injecting the culture in- 

 traperitoneally into white mice, Gundel and Wasu noted marked 

 differences in the virulence of strains within a type, although, as a 

 rule, representatives of Types III, II, I, and IV possessed degrees 

 of virulence in the order named, the average minimal lethal dilution 

 for all Type III strains being 1 to 100,000,000, for Type II cul- 

 tures 1 to 10,000,000, for Type I cultures 1 to 1,000,000, while 



