118 OUTLINES OF BACTERIOLOGY 



curved. When stained with methylene-blue the bacilli assume a deep 

 blue colour, and inside them granules of a still deeper colour are also 

 visible. The latter give the rods a characteristic beaded appearance. 

 Occasionally, also, the ends of the rods are swollen after staining, 

 sometimes so much so that a club-shaped form is assumed. 



When a cut is made in the skin by an object which is contaminated 

 with earth or dung, a germ called Bacillus tetani sometimes gains 

 entrance into the body and gives rise to the disease called Lock-jaw. 

 The bacteria are localised in the neighbourhood of the wound, but the 

 poisons secreted by them infect the whole body. The rods are usually 

 4-5 /x long and rather narrow, being only about *4 ^ in breadth (Fig. 71) 



FIG. 71. Bac. tetani. Showing cilia. FIG. 72. Bac. tetani. Showing spores. 



They form spores readily, which are usually situated at one end of 

 the rod, giving the latter a very characteristic drum-stick appearance 

 (Fig. 72). The toxin of this bacillus is extremely powerful, as even 

 0*0005 milligramme of an impure preparation has been found sufficient 

 to kill a mouse. The wound from the contaminated object may be 

 very small, in fact, may consist of a mere abrasion of the skin, but if 

 these bacteria once get settled on that point it is not long before the 

 whole body is poisoned. This disease is most prevalent among people 

 who work in byres, in farmyards and on the soil, where the bacillus is 

 most plentiful. Fortunately, though abundant, the bacillus only rarely 

 gets a hold, because before full effect can be produced there must 

 be either symbiosis with another organism, such as Staphylococcus 

 pyogenes aureus, or else some stimulating cause in the shape of a 

 mechanical irritant. The irritants in this case are the presence of 

 lactic acid, or the presence of soil or small splinters of wood in the 

 wound. 



A much dreaded parasite is the " comma bacillus " or the "cholera 

 spirillum." The individuals of this germ are not rod- but comma- 

 shaped (Fig. 73). Here, again, the individuals are found only in one 

 part of the body, whilst the poison secreted by them permeates the 

 whole system. They are normally motile, and measure each about 



