24 THE MORPHOLOGY OF BACTERIA 



and growth of the organism or the fitness of the culture medium. This 

 phenomenon is rarely or never met with among the pathogenic bacteria. 



Branching. Among the individual organisms comprising a culture 

 in artificial media of tubercle, diphtheria or glanders bacilli, and to a 

 lesser extent of other bacilli, a certain number appear as definitely 

 branched rods: the typical organism in each instance does not exhibit 

 branching. Branching has also been demonstrated in the spirilla. 1 

 Bacillus bifidus appears habitually as a rod-shaped organism with 

 bifurcated ends in artificial media, although it is an unbranched 

 bacillus in its normal habitat, the intestinal tract of nurslings. Occa- 

 sionally, bacteria, as the tubercle bacillus, may exhibit branching in 

 the animal body as well as in cultures, although less commonly. 



The cause of this branching is unknown, and at least two theories 

 have been advanced in explanation of it: each theory has a certain 

 amount of evidence in its favor. One theory assumes that branching 

 is the result of unfavorable environmental conditions, and it has been 

 shown that old broth cultures of diphtheria bacilli contain branched 

 organisms; young cultures contain few or no branched forms. The 

 assumption is that old cultures contain harmful products of metab- 

 olism which cause the diphtheria bacillus to assume branched forms. 

 The second theory asserts that the appearance of branched forms 

 among bacteria demonstrates a relationship between them and higher 

 organisms, which are habitually branched. Bacteria, according to 

 this theory, exhibit branching as a part of their normal development. 



Branching does not necessarily take place under conditions which 

 would appear to be unfavorable or partially inimical to their growth, 

 and, on the other hand, it may be observed occasionally when environ- 

 mental conditions should be favorable for development. It appears 

 to be reasonable to assume that branching may be a normal develop- 

 mental process in the life history of the organism, although the phylo- 

 genetic significance of branching is as yet undetermined. 



C. SIZE AND WEIGHT OF BACTERIA. 



Size. The unit of measurement for microscopic objects is the 

 micron (//), which is 0.001 of a millimeter, or approximately 25500 of 

 an inch, in length. Bacteria are the smallest known living organisms 

 which have been seen with the microscope. Measured with this unit, 

 they exhibit considerable differences in size. The average sized pus- 



1 Reichenbach, Centralbl. f. Bakteriol., 1901, xxix ,553. 



