PART 111. 



NON-PATHOGENIC BACTERIA. 



No very sharp line can be drawn between pathogenic bacteria 

 on the one hand, and non-pathogenic bacteria on the other; for 

 many kinds of bacteria which occur as saprophytes under 

 ordinary circumstances may multiply in the body and cause 

 injury under certain conditions. Bacillus prodigiosus, an or- 

 ganism which is given to students in the beginning of their 

 studies in bacteriology, on account of its harmlessness, pro- 

 duces lesions in experiment animals when injected into the 

 peritoneal cavity. But those organisms may be fairly classed 

 as non-pathogenic which are not found to cause spontaneous 

 disease, and which do not cause disease in animals when in 

 troduced in small amounts. 



The number of species of non-pathogenic bacteria is very 

 large. Eisenberg* describes 376 species of bacteria, mostly 

 non-pathogenic. Sternbergf enumerates 489 species, includ- 

 ing the pathogenic varieties; but the majority, of course, are 

 nonpathogenic. FliiggeJ considers about 500 species of bac- 

 teria. Migula§ recognizes nearly 1300, and Chester || about 

 800 species. Probably some of the bacteria which have been 

 described as distinct species are in reality not different; but, on 

 the other hand, it is also probable that a still larger number of 

 spices have not been described at all — ^how many, it is impos- 



*Bakteriologische Diagnoslik. 1891. 



tManual of Bacteriology. 1893. 



XDie Mikroorganismen. 1896. 



ifSystem der Bakterien. 1900. 



||Manual of Determinative Bacteriology. 1901. 



267 



