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Some of the rod-shaped varieties are from 1/12,000 to 1/8,000 of an inch in length, and average about 1/50,000 of an inch in diameter. It has been calculated that a space of one cubic millimeter would contain 250,000,000 of these minute organisms, and that they would not weigh more than a milligram. Bacteria are propagated in a very simple manner.
But how to distinguish them has been a puzzle. Microscopists have come to look upon the differences in shape, multiplication, and formation of spores as furnishing data sufficient to enable them to divide the bacteria into genera. The genus Bacillus, for instance, is the name given to all rod-shaped bacteria which form endogenous spores, etc.
All bacteria producing Streptococcus chains form a single genus Streptococcus, and all which divide in three division planes form another genus, Sarcina, etc. The rod-shaped bacteria also differ somewhat, but to a less extent. They almost always divide in a plane at right angles to their longest dimension. Occasionally a rod species may divide lengthwise, but this is rare.
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