SIZE AND WEIGHT OF BACTERIA 25 
Theobald Smiths has described a pleomorphic bacillus associated in 
pure culture with extensive lobar pneumonia in calves. In the exu- 
date it is a minute bacillus. In culture it appears in three forms: a 
bacillus, a coccus-like body, and a conglomerate actinomyces-like 
colony with peripheral clubs. Lignieres and Spitz^ have described a 
somewhat similar organism. 
Branching.— Among the individual organisms comprising a culture 
in artificial media of tubercle, diphtheria or glanders bacilli, and to a 
Cesser 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.^ 
Bacillus bifidus grows 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 exliibit 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 theofy assumes that branching 
is the result of unfavorable environmental conditions, and it has 
been showm that old broth cultures of diphtheria bacilli contain 
branched organisms. This is said to be associated with an increase in 
the acidity of the medium; young cultures contain few or no branched 
forms. The assumption is that old cultures contain harmful products 
of metabolism 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, accord- 
ing to this theory, exhibit branching as a part of their normal de^'elop- 
ment."* 
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 (^i), which is 0.001 of a millimeter, or approximately 2^5 ^oo of 
1 Jour. Exp. Med., 1918, 28, .33.3. 
2 Actinobacillose, Rev. Soc. med. arg., 1902, 10, 105. 
•^ Reichenbach: Centralbl. f. Bakteriol., 1901, 29, 553. 
* See Dunbar: Zur Frage der Stellung der Bakterien, Hefen und Schimmelpilze im 
System, Munich, 1907. 
6 See Bujwid: Centralbl. f. Bakteriol., 1889, 6, 630. Wilson: Jour. Path, and 
Bacteriol., 1906, 11, 394. 
