THE DIFFERENTIAL STAINING OF BACTERIAL SPECIES 37 



Two colonial characteristics may be noted here. Certain motile bacilli swarm 

 in continuous films of growth on the surface of solid media, a property which is 

 displayed characteristically by members of the Proteus and Clostridium groups 

 (see Chapters 27 and 36). Other motile bacilU, notably among aerobic and anaerobic 

 sporing bacilli, form rounded colonies in which the constituent bacilli are moving 

 in a clockwise or anticlockwise direction, imparting a rotating motion to the whole 

 colony. As a result of the rotation, the colony wanders over the surface of the 

 solid medium, usually leaving a track in the form of a double line of bacilli, appar- 

 ently thrown off from the sides of the advancing colony (see Muto 1904, Roberts 

 1935, Smith and Clark 1938, Euss-Miinzer 1938, Shinn 1938, Turner and Eales 

 1941, Fuller and Norman 1943). 



The character of the colonies produced by a given bacterial species is of great 

 importance in identification, and is, for this purpose, best considered in conjunction 

 with other growth-characters, such as the appearances noted in cultures in liquid 

 media, or in the so-called stab-cultures, in which inoculation is made by thrusting 

 a platinum wire axially into a column of solid medium, such as agar or gelatin. 

 Further description of colony structure is therefore deferred to Chapter 13. 



Staining Reactions. — The staining reactions of bacteria, and particularly their 

 response to certain differential stains, might well be regarded as more in the nature of 

 microchemical tests, than methods of demonstrating structure. It would, there- 

 fore, be more logical to consider staining reactions in connection with the physiology 

 of the cell than with its morphology, but the way in which a given bacterium 

 reacts to special stains constitutes such an important part of the general picture 

 which we form of it, that it is convenient to deal with this matter in the present 

 chapter. 



Reactions to General Bacterial Stains. — Quite apart from the presence of the 

 metachromatic granules, or of other granular material, different bacterial cells 

 may differ in the way in which they take up stains. In some cases the whole 

 cell may be uniformly coloured. In some cylindrical forms the ends of the bacillus 

 may be deeply stained, while the central portion may be almost colourless. This 

 constitutes the so-called polar staining, which is very characteristic of the plague 

 bacillus, and is found in many other bacterial species. Sometimes the cell may 

 stain unevenly throughout its length, so that barred or beaded forms may occur, 

 a feature which is very characteristic of the diphtheria bacillus and many allied 

 organisms, and which occurs to a less marked extent with many acid-fast bacilli. 

 In some cases, in which barred or beaded forms are found, these conditions are 

 associated with the presence of metachromatic granules, but these granules are 

 not the essential cause of such irregularities in staining. It seems clear that, in 

 such forms, there is an unequal distribution throughout the cell of those con- 

 stituents of the cytoplasm which take up the stain employed, and it has frequently 

 been suggested that they are indicative of plasmolytic changes. It seems doubtful 

 whether plasmolysis can be invoked as a general explanation of this phenomenon, 

 though it is true that there is a general tendency for the frequency of granular 

 and barred forms to increase with the age of the culture, and to be notably scanty 

 in very young cultures. They are, however, so plentiful in young and actively 

 growing cultures of certain species that it seems impossible to regard them as 

 resulting entirely from degenerative changes. 



The Differential Staining of Bacterial Species. — In the application of numerous 

 stains, mordants, and differential decolorizing reagents to the study of bacteria, 



