MACROFORMATIONS 1 37 



E: THE MEDUSA-HEAD COLONY 



(1.2,3,6) 



In most bacteria the colony, however complex its structure, is an accidental 

 growth, each cell of which is equivalent to all the others. The colony docs 

 not alternate with the swarm, but is itself the swarm, or vegetative mass. 

 Where conditions are such that motile bacteria may exercise their motility, 

 no colony is formed at any stage ot culture. Colonies are not confined to 

 conditions of artificial culture, although there they appear in their most 

 perfect form. 



The type of bacterial colony whose structure was first recognised, although 

 not understood, was the so-called " medusa-head " colony of the anthrax 

 bacillus. This form of colony, which consists of long, coiled, bacillary threads, 

 is common to all bacteria of what we have termed rough morphology. In 

 the case of the anthrax bacillus, and similar, large, spore-bearing bacilli, it is 

 easily seen with a hand lens, whereas the much smaller size of, tor instance, 

 lactobacilh, renders it less obvious, so that its presence, except in these large 

 genera, was long unsuspected. 



Although it has long been known that the virulent anthrax bacillus possesses 

 this type of colony, whereas the smooth colonies of the avirulent anthrax 

 vaccine were composed of individual bacilh, the obvious corollary that the 

 respectively virulent and avirulent smooth and rough colonies ot BcKk'nacca' 

 might possess the same type of structure in each case, escaped attention until 

 much later. This was partly due to the studies which several different workers 

 made upon the mode of cell division in smooth and rough strains ot bacteria 

 (Chapter V), which, by crediting rough bacteria with a " snapping " mode 

 of division, erroneously claimed that the rough colony was composed of 

 zig-zag chains of bacilli. This error, which has persisted tor a quarter of a 

 century, and is included in nearly all text-books of bacteriology, would 

 never have arisen if these observations had been checked by examination of a 

 rough colony, in situ, for it can readily be seen that the structure of a rough 

 colony of a typhoid bacterium, and the " medusa-head " colony ot the anthrax 

 'bacillus are identical. 



