GRAM-STAINING ORGANISMS 121 



In order to ascertain whether an organism is or is not stained 

 by Gram's method, it is sometimes useful to mix with it in making 

 the preparation some undoubted Gram -staining organism e.g. 

 if a bacillus, the Micrococcus pyogenes ; if a coccus, B. anthracis 

 or B. subtilis. The admixed organism then serves as an index. 



The following organisms are Gram-positive : B. anthracis. 

 B. diphtherice, B. tetani, B. botulinus, B. tuberculosis, B. smegmatis, 

 B. leprce, B. murisepticus, Actinomyces, B. subtilis, B. mesen- 

 tericus, B. megaterium, B. mycoides, the pyogenic cocci, the strep- 

 tococci, including the pneumococcus, most cocci, yeasts, moulds 

 and streptothrices. B. perfringens, B. cedematis maligni, and B. 

 Chauvcei are usually Gram-positive in the tissues, but tend to 

 become Gram-negative under cultivation. 



The following organisms are Gram-negative : B. typhosus, B. 

 paratyphosus, B. enteritidis, B. dysenterice, B. coli, B. pestis, B. 

 influenzce, B. mallei, B. pseudo -tuberculosis, B. pyocyaneus, 

 B. prodigiosus, the septicsemic bacilli, such as chicken cholera, 

 the spirilla and vibrios, spirochaetes and protozoa, M. gonor- 

 rho3ce, M. meningitidis, M. (B.) melitensis, and M. catarrhalis. 



The Staining of Film Specimens 



To stain films, smear, and impression preparations, 

 the film is flooded after fixing with a drop or two of the 

 solution, or the preparation, if a cover-glass, may be 

 floated, film side down, on the solution contained in a 

 watch-glass ; if it should sink it makes little difference. 

 Various baths or pots can be obtained for staining slides. 

 Warming intensifies the staining properties of all staining 

 solutions, and may be necessary if deep staining is required 

 or if the temperature of the laboratory be low (see also 

 p. 126). When sufficiently stained, the preparation is 

 rinsed in a beaker or tumbler of water, or in a fine stream 

 of water, preferably distilled, to remove the superfluous 

 colour, after which it is dried and may be mounted in a 

 drop of solution of Canada balsam in xylol. The prepara- 

 tion may be dried either by gentle warming over the 

 Bunsen flame after the film has been blotted with filter 



