THE IDENTIFICATIONS OF MICRO-ORGANISMS 359 



differences in the chemical constitution of different organisms, or of different parts 

 of the same organism. For studying the morphology, it is advisable to use a weak 

 stain ; otherwise so much dye may be absorbed as to alter the appearance of the 

 organism. Gram's stain is of great value, in that it serves to divide all bacteria 

 into one or other of two classes — the Gram-positive and the Gram-negative. The 

 Ziehl-Neelsen method of staining is likewise of value, since it serves to distinguish 

 the acid-fast from the non-acid-fast bacilli. Numerous other stains are used for 

 special purposes, such as the demonstration of flagella, capsules, spores, and meta- 

 chromatic granules. 



By a study of the morphology and the staining reactions, it is generally possible 

 to identify the group to which a given organism belongs. In certain instances, 

 when the origin of the organism is known, it is possible to make a presumptive 

 diagnosis of its actual identity ; though it should be clearly understood that, in 

 medical bacteriology, a provisional identification of this kind is valid only if in 

 consonance with a clinical diagnosis established on other grounds. Thus acid-fast 

 bacilli in the cerebro-spinal fluid of a patient with clinical symptoms of meningitis 

 may provisionally be identified as tubercle bacilli ; Gram-negative diplococci in 

 the pus of an infant with ophthalmia neonatorum are probably gonococci ; and' 

 Gram-negative bipolar-staining ovoid bacilli in the gland-juice from a patient with 

 an inguinal bubo, in an area where plague is prevalent, may provisionally be regarded 

 as plague bacilli. As a rule, however, it is impossible to identify an organism by 

 morphology and staining alone. 



C. Cultural Reactions. — Under this heading must be included a study of the sur- 

 face, and often of the deep, colonies formed on solid media, and of the type of growth 

 in fluid media. Nutrient agar is the usual medium on which colony formation 

 is studied, but if the organism fails to grow on agar, then some other medium must 

 be chosen. The colonies are best examined after 24 hours' incubation at 37° C., 

 and again at intervals for a week. In describing them, particular attention should 

 be paid to their shape, size, elevation, structure, colour, transparency, surface, 

 edge, consistency, and emulsifiability ; differentiation into central and peripheral 

 areas should also be noted. The type of growth following a streak inoculation on 

 an agar slope should be studied, attention being paid particularly to the profuseness 

 of growth, to the elevation, colour, surface, and edge, and to any change in the 

 medium itself. The type of growth in a gelatin stab culture should also be studied, 

 and notes made of the degree and extent of the growth, the presence of a surface 

 growth, the presence or absence of liquefaction, and if liquefaction occurs, of the 

 particular type which it assumes (see Chapter 13). In any systematic examina- 

 tion, the growth should be studied on certain special media such as Loeffler's 

 serum, glycerine potato, and coagulated egg. 



The cultural reactions of the different groups of bacteria are fairly distinctive, 

 and even within a given group there may be differences between the members. 

 Some organisms moreover have a characteristic form of growth, which enables 

 them to be distinguished from morphologically similar organisms. As a rule, 

 however, a study of the cultural reactions merely indicates the group to which a 

 given organism belongs ; it does not distinguish between the different members. 

 It serves to confirm the conclusions reached from the examination of the morphology 

 and staining reactions. 



D. Resistance. — Organisms vary considerably in their resistance to inimical 

 agencies. Roughly speaking, three classes may be distinguished : 



