572 BIOLOGY OF PNEUMOCOCCUS 



suitable mediums for growth and by selecting appropriate doses of 

 culture. In considering the variables in the mouse protection test, 

 Felton 405 placed the greatest emphasis on the culture employed and 

 cited the killing power and antibody-neutralizing ability of the or- 

 ganisms as factors of first magnitude. There can be no doubt that 

 variations in cultures contribute to lack of uniformity in the re- 

 sults of tests. Although many laboratories employ the so-called 

 original Neufeld Type I strain, conditions under which cultures 

 are maintained are certain to induce changes of greater or lesser 

 extent in the organisms. In order to minimize variations in cultures 

 of the various types used it would be well to have an international 

 central supply from which representative strains could be issued 

 and to establish standard conditions under which cultures should 

 be maintained in different laboratories. A similar suggestion was 

 made by Felton and is worthy of general consideration. 



Variability in the outcome of potency tests on antipneumococcic 

 serum is introduced by the mice employed. Uniformity in suscepti- 

 bility, the numbers used for each determination, the age, weight, 

 and sex of the animals are all factors to be considered. Petrie and 

 Morgan found that the sex of the animals appears to have no sig- 

 nificant influence on the results of titrations. Furthermore, the 

 weight of mice had no effect on the mortality-rate, but heavier ani- 

 mals tended to show a longer survival time. Although little work 

 relating to the effect of age of mice has been done, it is preferable 

 to use young stock. Of greater importance is the degree of uni- 

 formity in susceptibility of the mice to pneumococcal infection. It 

 has been customary in titrating serum to employ, when possible, 

 white mice from homogeneous breeding stock. Even when this is 

 done, some mice, or groups of mice, may exhibit unequal suscepti- 

 bility to the infecting doses of culture. Petrie and Morgan found 

 that from 5 to 10 per cent of the animals used possessed an innate 

 resistance to small doses of pneumococci. 



In attempts to decrease the discrepancies introduced by the test 

 animal, it has become customary to increase the number of mice 

 used per dose of serum. In the older methods where only two or 



