CHAPTER II 
GENERAL INFORMATION ABOUT BACTERIA 
Bacteria belong to the vegetable kingdom, being minute 
unicellular organisms. 
I. Bacterial Forms 
Normally bacteria are of three different kinds so far 
as their form is concerned: 
1. Cocci (plural from Greek coccos, meaning kernel), 
which are round or oval; these usually occur either in 
clusters or pairs, being called diplococci (meaning double 
cocci) in the latter case. 
2. Bacilli (plural from Latin bacillus, meaning “a little 
rod”), which are straight, rod-shaped organisms, whose 
ends may be square or convex more frequently) or con¬ 
cave (less frequently). 
3. Spirilla (plural from Latin spirillum, meaning a 
coil) which are curved or comma-shaped organisms (see 
Fig. 1). 
Of these three varieties the bacilli are by far the most 
frequent. 
The Size of Bacteria.—The unit of bacterial measure¬ 
ment is a micron (a Greek word meaning small) ; it is a 
thousandth part of a millimeter or 1/25000 of an inch, 
and is usually represented by the Greek letter /z. Cocci 
usually vary from .1 /x to 2 fx in diameter, the average 
size being about .1 /z in diameter. Bacilli vary from 1 
/z to 3 or 4 fx, some being very large, as, e. g., the an- 
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