BACTERIOLOGICAL DIAGNOSIS 297 



The modern methods of research are so elaborate 

 and so complex that no single observer can hope 

 to obtain an intimate working acquaintance with 

 more than a small section of a special subject, 

 though he should possess sufficient acquaintance 

 with the literature of those subjects which border 

 on his own to criticize usefully the work of others 

 and to broaden his own horizon. 



The bacteriologist, who investigates those bac- 

 teria which produce disease in man, devotes 

 years to the accurate study of a certain group or 

 groups of such organisms. A group of bacteria 

 comprises many species, superficially resembling 

 each other very closely, but differing in their actions. 

 Some produce disease, others are harmless parasites, 

 and others again may have no connection with 

 living bodies. The typhoid-colon group, for ex- 

 ample, includes hundreds of species, indistinguish- 

 able from each other under the microscope, but one 

 species produces typhoid or enteric fever in man, 

 another paratyphoid fever, another dysentery, 

 another food-poisoning; and others are responsible 

 for diseases of various kinds in animals, while 

 others are constant, and perhaps necessary, in- 

 habitants of the intestines -of men and animals. 

 Though these species are microscopically indis- 

 tinguishable, they can be divided into subgroups 

 by their methods of growth under artificial con- 

 ditions. Thus subgroups containing typhoid bacilli 

 and species culturally resembling them, dysentery 



