LESSON VI. 



STUDY OF GROSS APPEARANCES OF BACTERIAL 



CULTURES. 



Upon development of bacteria inoculated upon a nutrient medium there appear 

 as a result of multiplication of the individual organisms more or less isolated colonies. 

 A colony is a focus of growth, or group of bacteria, recognizable by the eye, unaided, 

 or aided by low powers of the microscope. Should such a colony, as is usually the 

 case, be composed of bacteria of the same type, arising from the multiplication of a 

 single original germ, it may be spoken of as a pure colony; while if there be confused 

 in the group several types, resulting from the merging together of minute foci of de- 

 velopment of closely situated but different germs, it may be termed an impure or 

 mixed colony. In liquid media the chances for diffusion of the organisms by agitation, 

 extension of growth, or motility of the germs are so marked that isolation of the colonies 

 and preservation of their separate characters are exceptional; while upon solid media 

 each separate focus of growth is apt to maintain its isolation and peculiarities, whence 

 important data for the identification of the type may be obtained. The study of 

 isolated colonies is naturally most readily pursued in diffusion cultures on solid media f 

 either in the films of plates, dishes, or rolled tubes, or throughout the mass of the 

 medium, or in dilute smear cultures where the original germs have been well scattered 

 over the surface. The massive growths of stroke and stab inoculations on solids and 

 the unrestrained diffuse growths in liquid media present less characteristic appear- 

 ances, but may in individual cases contribute important information for the same end. 



An organism does not preserve an identity of appearance of its colonies and massive 

 growths upon the various media, or when influenced by modifications of its other 

 life conditions; and the variations produced by development upon different media 

 and in different atmospheres and at different temperatures are likewise of importance 

 in identification of similar but unlike types from each other. With a view of estab- 

 lishing these points of peculiarity it is customary, therefore, to study the appearances 

 of growth of a given bacterium upon various media (potato, gelatine, agar, serum, 

 bouillon, and milk), in each instance noting the characteristics shown by the different 

 forms of inoculation, as well as the influences exerted by variations of temperature 

 and of atmosphere upon the development in each. For complete study of all its 

 peculiarities the organism should have been grown in agar by smear, stroke, stab, 

 and diffusion (plate and in the mass in a tube) ; similarly in gelatine ; by smear and 

 stroke inoculation on potato and blood-serum ; and by diffusion in bouillon and milk. 

 Preparations of each of these should be submitted to at least two temperatures 

 that of the room and that of the body; and, similarly, examples of each should be 

 placed in anaerobic conditions as well as in the ordinary air. 



In every investigation the rate of growth (time of appearance of visible growth), 

 optimum temperature, preference of nutrient medium and of atmosphere should be first 

 noticed and recorded. Thereafter the appearances of the cultures in each medium are 



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