60 BACTERIOLOGY FOR NURSES 



(5) Productive pigment. Potato medium is best for this 



purpose ; if it is not available a little of the growth may 

 be taken from an agar slant with a platinum loop and 

 spread on white paper, when the color will, if present, 

 stand out against the white background. 



(6) Food requirements. Growth on simple media or necessity 



for media containing sugar or body fluids. 



(7) Temperature. Minimum optimum maximum. 



(8) Oxygen requirement. Growth only in the presence of 



free oxygen on the surface. Growth only in the absence 

 of free oxygen at the bottom of stab culture. 



(9) Proteolytic action. Liquefaction of gelatin ; indol forma- 



tion. For the latter test peptone water medium with- 

 out the addition of sugar is employed. The culture is 

 incubated for from four to six days, after which 1 c.c. 

 of a 10 per cent solution of sulphuric acid and 1 c.c. of 

 a 1 in 10,000 solution of sodium sulphite is added. At 

 the point where the acid comes in contact with 

 the medium a pink color appears in the presence of 

 indol. 



(10) Fermentation of sugars. The tests usually employed are 

 for the detection of acid and gas production. The dif- 

 ferent species of bacteria vary greatly in their ability 

 to break down the various sugars; one species may 

 have the power to produce acid from one kind of 

 sugar, acid and gas from another, and yet have abso- 

 lutely no effect upon a third. The medium employed is 

 generally peptone solution to which has been added 

 1 per cent of the sugar chosen and an indicator such 

 as litmus or neutral red. Hiss's serum water is fre- 

 quently used for the detection of acid ; the cleavage of 

 the carbohydrate is indicated not only by a changed 

 color of the indicator but also by the coagulation of 

 the serum. 



The test for acid and gas production may be carried 

 on at the same time in a specially devised fermentation 



