NOTE ON THE STAINING OF FLAGELLA 
By ALFRED MacCONKEY 
It is a generally accepted idea that staining the flagella of bacteria is difficult 
to accomplish, and that a ' mordant ' is necessary to fix the dye in the substance of 
the flagellum. Is this idea correct ? A ' mordant ' is defined as ' a substance used 
to fix colours in stuffs.' Is this what takes place in the process of staining flagella ? 
At first sight this would appear to be the case, for we find that flagella will not 
usually stain unless a ' mordant ' be used. A little consideration, however, raises 
doubts as to the correctness of this interpretation of the process. If the ' mordant ' 
acts merely as a fixing agent it should not leave much effect upon the size of the 
cilia, which should, within limits, appear always of the same thickness ; and the 
depth of colour should depend upon the degree of concentration of the ' mordant ' 
and stain, upon their inter-reaction, and upon the length ot time the film was 
subjected to their action. But, in practice, we find that this is not quite the case. 
The thickness of the flagella depends upon the strength of the ' mordant ' and upon 
the length of time it is allowed to act ; and the depth of colour depends upon the 
thickness of the substance to be stained. If the flagella are very fine, the strength 
of the staining fluid and the duration of its application make very little difference, 
for the flagella will always appear faintly stained ; whereas, if the flagella be thick, a 
comparatively brief application of the stain will dye them a deep colour. 
These facts suggest two explanations of the action of ' mordants.' In the first 
place, the action may be due to the deposition of a precipitate upon the outside of the 
flagellum, this precipitate being dyed in the subsequent staining process. 
In the second place, the ' mordant ' may cause the flagellum to swell and 
become thicker. Thus it would produce a greater depth and breadth of substance 
for the stain to act upon. 
We know that, to be visible, a line must have a certain breadth, and it is 
quite conceivable, considering the size of bacteria, that their flagella may be too fine 
to be seen even when stained. Of these two explanations, I am inclined to favour 
the second for the following reasons : — 
I In all the usual methods, the flagella when stained appear to be thicker 
than we suppose them to be in reality, and the organisms themselves are larger than 
when stained in the ordinary way. 
