58 PHYSIOLOGY OF NUTRITION 



If it is desirable to exclude all organic compounds, colloid silica may be 

 used as a matrix instead of gelatine. Great latitude is permissible in the 

 composition of the peptone-sugar-meat solutions that are used. Almost 

 every laboratory has its own special recipes, dictated either by whim or by 

 the results of long experience. In many cases the media are unnecessarily 

 overloaded with nutriment. Besides meat, hay, straw, plums, potatoes and 

 many other substances may be used to make nutrient infusions. The wort 

 of beer is often used, and, as an opaque solid culture-medium, potatoes are 

 often employed. For practical details the technical handbooks must be 

 consulted. 



Apart from characters such as pigmentation or the production of gas, 

 the modes of growth of different bacteria on the same substratum are 

 sufficiently varied to be of some use in the differentiation of species. Too 

 much importance, however, must not be attached to these features. In 

 fluid media, two chief types of growth are seen. In the one the liquid 

 remains clear, in the other it becomes turbid. When the fluid remains 

 clear we know that we have to do with non-motile bacteria with a tendency 

 to filamentous growth. They collect on the walls and bottom of the tube 

 in flocculent masses, which may be dislodged by shaking (Anthrax, Strepto- 

 coccus). When such non-motile forms need much oxygen (B. tuberculosis 

 for instance), they form a thick membrane, sometimes smooth, sometimes 

 wrinkled, over the surface of the clear liquid. All isolated, non-filamentous 

 forms, particularly the motile species (Cholera, Typhoid), cause the medium 

 to become uniformly turbid. The turbidity varies from a dense milky 

 opacity to a faint opalescence only visible on shaking. Strongly aerobic 

 species form a pellicle on the surface of the fluid ( V. cliolerae, B. subtilis). 



By culture upon gelatine we are able to divide the bacteria into two 

 groups, those which, by means of a peptonising enzyme, liquefy the gelatine 

 and those which do not. The great majority of known forms belong to the 

 former class ; B. typhi, B. coli, Streptococcus ; Lactic bacilli, &c., are examples 

 of the latter. Further differences of growth on gelatine are brought out by 

 the methods of ' plate ' and ' stab ' culture. In plate culture the melted 

 gelatine is inoculated with a minute quantity of bacteria, or of substances 

 containing bacteria, and then poured out in a thin layer upon glass. There 

 it coagulates, and the germs it contains multiply. The germs being fixed 

 produce around themselves sharply-defined colonies, each colony having 

 arisen from a single germ or from a few stuck together. The young; 

 colonies in particular show many characteristic features ; colour, form, and 

 contour, consistence and lustre, and the structure as seen under a low- 

 power lens are all useful points. A detailed account will be found in 

 Lehmann and Neumann's atlas (30), and is needless here. The following 

 description of the colonies of two well-known bacteria will show the nature 

 of the differences involved better than an enumeration of all the known 



