358 METHODS OF OBTAINING PURE CULTURES 



on the most important media ; these will be found of great value for future com- 

 parison. For studying the properties of an organism the following scheme is 

 suggested. 



A. Morphology. — Under this heading we include the shape and size of the 

 organism, its arrangement, motility, the number and distribution of flagella, the 

 shape and situation of spores, and capsule formation. It is impossible to study 

 all these properties on a single medium ; motility for example should be looked for 

 in a young rapidly growing broth culture, preferably not more than 6 to 8 hours 

 old ; flagella are sought for on a young agar culture ; spores in a culture that has 

 been growing for some days ; capsules in a pathological exudate, and so on. The 

 shape and size of the organisms are subject to considerable variation, and it is 

 important to gain some idea of the extent of this variation. With a few exceptions, 

 such as the corynebacteria, most organisms are larger in a young than in an old 

 culture (Henrici 1926). Measurements, for example, of Saltn. typhi-murmm in a 4- 

 hours' culture on agar showed that the average size was 2-35 /u X 0-79 jli ; in the 

 same culture after 26 hours the size was only 1*13 ju X 0*49 ju. In volume the 

 organisms from the young culture were over five times that from the old. On 

 further incubation the average size decreased still more (Wilson 1926). When 

 taking measurements of a given strain it is therefore important to record the age of 

 the culture from which they were taken. Even in one and the same preparation 

 the individual organisms may vary considerably in size and shape ; this may be so 

 marked as to justify the term " pleomorphic." Thus coccoid, bacillary, and 

 filamentous forms may all be present together ; or besides the usual rods there 

 may be club forms, navicular forms, granular forms, large bloated forms, shadow 

 forms, and so on. Moreover, the appearance of the organisms is often considerably 

 influenced by the type of medium on which they are grown. Chain formation, for 

 example, is more evident in liquid media than on solid. The typical morphological 

 appearance of the diphtheria bacillus is seen best on Loeffler's serum ; on agar the 

 organisms tend to be more solid and less granular. The nature of the medium 

 often influences the production of spores and of capsules. Some organisms, such 

 as B. anthracis, form spores readily in artificial culture, but never do so in the 

 animal body. On the other hand, capsules are quite frequently found in the body, 

 but less often in artificial culture. The arrangement of the organisms should be 

 carefully studied ; if they are cocci, they may be arranged singly, in pairs, tetrads, 

 packets, clusters, or chains ; if bacilli, they may be arranged singly, in pairs end- 

 to-end, in bundles, chains, clusters, or in Chinese-letter forms in which the in- 

 dividual bacilli lie more or less at right angles to each other : if vibrios, they may 

 be arranged singly, in S-forms, semicircles, in wavy chains composed of S-forms 

 strung end-to-end, or they may present the fish-in-stream appearance. Though 

 most organisms show two or three types of arrangement, it is usual for one of these 

 to be predominant ; this comes to be regarded as the typical arrangement. It 

 cannot be emphasized too strongly that the morphology of bacteria is subject to 

 variation, depending on the age of the culture, the nature of the medium, the 

 particular strain used, the temperature of incubation, and a number of other 

 factors ; the extent of this variation can be learnt only by experience. 



B. Staining Reactions. — The morphology of bacteria may be studied in a 

 hanging-drop preparation, by dark-ground illumination, or in stained films. By 

 each of these methods different information may be gained. Staining methods, 

 in addition to revealing the morphology of the organism, may render evident 



