778 



METHODS OF INVESTIGATING BACTERIA. 



Mode of 

 specimens. 



staining of 



bacteria. 



Cover glass 

 preparations. 



If we are investigating pathogenic organisms we must 

 remember that numerous fungi are constantly growing 

 on the surface of healthy living animals, on the skin, in 

 the cavity of the mouth, &c. After death these organ- 

 i sm s rapidly spread, in the first place, into all the 

 superficial parts of the body; specimens taken with the 

 view of investigating the presence of pathogenic organ- 

 isms must therefore never be taken from the impure 

 surface, even in the case of living animals ; and after 

 death we must always try, if possible, to obtain the 

 specimens from the interior of the organs by making a 

 series of fresh cuts with a heated knife. 



The direct microscopical observation (sometimes after 

 the addition of a half per cent, soda solution, a mixture 

 of glycerine and water, or a solution of acetate of potash 

 1 to 10) without further aids only gives results at all 

 satisfactory in the case of the larger fungi, or of cultiva- 

 tions of bacteria ; if other objects, such as cells, nuclei, 

 or fragments of nuclei, crystals, and amorphous in- 

 organic materials, are present in the preparation we 

 may not be able to recognise the bacteria, even under 

 very high powers of the microscope. Hence in almost 

 all cases where it is important to examine the preparation 

 accurately it becomes necessary to stain the micro- 

 organisms. The bacteria take up certain colouring 



. , . , * - 



materials with extraordinary energy, and we can usually 

 so manage the staining that the micro-organisms alone- 

 are coloured, or at least these alone are markedly 

 stained, while all the other objects in the preparation 

 are but faintly, or not at all, tinged. And where we 

 wish to demonstrate the absence of micro-organisms 

 from some material it is only possible to do so with any 

 degree of certainty by the aid of methods of staining. 

 The treatment of the preparations with the view of 

 staining them differs according as we have to do with 

 fluids or with animal organs. 



Fluids are in the first place dried on a cover glass in 

 a yer ^ ^^ i ayerj a sma n drop being placed on the 

 glass by means of a recently heated platinum wire, and 

 spread out over it by a few circular movements, or still 



