26 APPLIED BACTERIOLOGY FOR NURSES 



ing their color even after prolonged contact. These 

 are spoken of as ''acid-fast" bacteria. This test is ex- 

 tensively used in identifying tubercle bacilU, for these 

 belong to the acid-fast group. (See Plate 1.) 



Another staining method largely used in identifying 

 bacteria is one devised by Gram. This is carried out 

 as follows: The bacteria, spread and fixed on the slide 

 in the usual way, are stained with a solution of gentian- 

 violet with the aid of a little heat. At the end of 

 several minutes the stain is poured off, and, without 

 washing, the slide treated with a solution of iodin. 

 Following this, the preparation is washed with absolute 

 alcohol and then wdth water. AVhen treated in this 

 way certain bacteria are found to retain the original 

 violet stain, while others lose it during the alcohol treat- 

 ment. This is a very valuable reaction, and is exten- 

 sively used in identifying the germs of meningitis, 

 gonorrhea, etc. Bacteria which retain the violet stain 

 when treated according~to Gram's method are called 

 ''Gram-positive," w^hile those which lose their stain are 

 called "Gram-negative." 



In passing we may say that, in addition to these 

 ordinary staining methods, special procedures have been 

 devised for the demonstration of flagella in motile bac- 

 teria and for staining spores, capsules, etc. 



The teacher is to demonstrate simple staining, explaining the 

 various steps, tubercle staining (Ziehl's method), and Gram's stain. 



For this exercise let the students examine as many stained 

 specimens as possible, to fix in their minds the appearance and 

 relative size of bacteria. 



