16 THE BIOLOGY OF BACTERIA 



temperature and moisture. Outside the human body it has been the 

 endeavour of bacteriologists to provide media as similar to the above 

 as possible, and containing many of the same elements of food, in 

 order that the life-history may be carried on outside the body and 

 under observation. By means of cover-glass preparations for the 

 microscope we are able to study the form, size, motility, flagella, 

 spore formation, and peculiarities of staining, all of which characters 

 aid us in determining to what species the organism under examination 

 belongs. By means of artificial nutrient media we may further 

 learn the characters of the organism in " pure culture," * its favour- 

 able temperature, its power or otherwise of liquefaction, of curdling 

 of milk, or of gas or acid production ; its behaviour towards oxygen ; 

 its power of producing indol, pigment, and other bodies ; as well as 

 its thermal death-point and resistance to light and disinfectants. It 

 is well known that under artificial cultivation an organism may be 

 greatly modified in its morphology and physiology, and yet its 

 conformity to type remains much more marked than any divergence 

 which may occur. 



Nutritive Medium.f The basis of many of these artificial media is broth. 

 This is made from good lean beef, free from fat and gristle, which is finely minced 

 up and extracted in sterilised water (one pound of lean beef to every 1000 c.c. of 

 water). It is then filtered and sterilised. To provide peptone beef-broth, ten 

 grammes of peptone and five grammes of common salt are added to every litre of 

 acid beef-broth. It is rendered slightly alkaline by the addition of sodium car- 

 bonate or sodium hydrate, and is filtered and sterilised. In glycerine-broth 6 to 8 

 per cent, of glycerine has been added after filtration, in glucose-broth 1 or 2 per cent, 

 of grape-sugar. This latter is used for anaerobic organisms. The use of broth as 

 a culture medium is of great value. It is undoubtedly the best fluid medium, and 

 in it may not only be kept pure cultures of bacteria which it is desired to retain for 

 a length of time, but in it also emulsions and mixtures may be placed preparatory to 

 further examination. Gelatine consists of broth solidified by the addition of 100 

 grams of best French gelatine to the litre. Its advantage is twofold : it is trans- 

 parent, and it allows manifestation of the power of liquefaction. When we speak 

 of a liquefying organism we mean a germ having the power of producing a pepton- 

 ising ferment which can at the temperature of the room break down solid gelatine 

 into a liquid. Grape-sugar gelatine is made like grape-sugar broth. Agar was 

 introduced as a medium which would not like gelatine melt at 25 C. , but remain 

 solid at blood-heat (37'5 C. ; 98 '5 F.). It is a seaweed generally obtained in dried 

 strips from the Japanese market. Ten to fifteen grammes are added to every litre 

 of peptone-broth. Glycerine and grape-sugar may be added as elsewhere. Blood 

 agar is ordinary agar with fresh sterile blood smeared over its surface. Blood serum 

 is drawn from a jar of coagulated horse-blood, in which the serum has risen to the 

 top. This is collected in sterilised tubes and coagulated in a special apparatus (the 

 serum inspissator). Potato is prepared by scraping ordinary potatoes, washing in 

 corrosive sublimate, and sterilising. It may then be cut into various shapes con- 

 venient for cultivation. Upon any of these forms of solid media the characteristic 



* A "pure culture" is a growth, in an artificial medium outside the body, of one 

 species of micro-organism only. 



f The facts here given are obviously only general indications. The accurate 

 preparation of medium is of vital importance in Bacteriology, and for its accomplish- 

 ment text-books should be consulted (Eyre's Bacteriological Technique, 1 25-1 74). 



