VINEGAR-MAKING 235 
ordinary bacilli. At high temperatures they grow out in long 
slender threads, sometimes very long, without any traces of 
divisions. These threads may subsequently break up into short 
elements. At low temperatures they have a peculiar habit of 
forming long threads with rounded swollen centers. When these 
threads break up into short forms only the thick, swollen centers 
remain undivided. This character is such a peculiar one that 
it places these acetic bacteria in a class by themselves. These 
Fic. 47.—Acetic acid bacteria, showing long rods and rounded swollen centers. 
different varieties are distinguished not only by slight differences 
in structure, but also by some important differences in relation 
to condition, and in their power of forming acetic acid. They 
vary in the amount of acetic acid they will produce under similar 
conditions. For example, B. acett produces 1.27 per cent. of 
acid at 59°F., while under the same conditions B. pasteurianum 
will produce 0.08 per cent. But, more important still, is the fact 
that the temperatures which favor the different varieties are 
not the same. JB. aceti, for example, produces a good fermenta- 
tion at a temperature as high as 42°F., whereas B. pasteurtanum 
at the same temperature will hardly multiply, and produces 
no fermentation. Some of the species produce the maximum 
effect more quickly than others, and some may begin to destroy 
