TECHNIQUE 



13 



Bacterial cell mcinbrancs arc also stained by dyes of the Sudan, fat soluble 

 group. This probably indicates the presence of a lipoid or lipoprotein com- 

 ponent, which is also bchcvcd to exist in the cell membranes of other organisms, 

 but probably does not mean that the cell membrane should be regarded as 

 predc^minantly lipoid in constitution. 



In addition to the well-known methods of Hiss's and Muir's stains, capsules 

 and slime layers are also stainable by the tannic-acid-violet technique, 

 but these methods give little hint oi the remarkable structure which can be 

 discerned in bacterial capsules by phase-contrast microscopy. 



(Reproduced from the Journiil of General Mi 



Fig. 4 

 CYTOLOGICAL STAINING OF AZOTOBACTER 

 Appearances produced by various staining tecnhiques. 



(1) Nucleated cells stained by lithium carbonate and Giemsa. 



(2) As (1), by magnesium sulphate and thionin. 



(3) As (1), by nitric acid and thionin. 



Comparison of these figures shows positive and negative differential staining of the large 

 perinuclear granules. 



(4) Phase-contrast studies of a, vacuolated and b, nucleated cells. 



(5) Vacuolated cells by nitric acid and thionin. The outlines of the lipid globules alone 

 are stained. 



(6, 7) As (5), stained for lipids by Ziehl-Neelsen and Sudan IV. respectively. In the 

 latter case the lipids have become displaced towards the periphery ; possibly because 

 ♦ of partial solution. 



