FORM AND STRUCTURE OF MICROBES 55 



Bacteria — The term "microbes" in current language 

 most frequently signifies bacteria. The bacteria belong to the 

 lower plants, are unicellular, contain no chlorophyll, and are 

 almost all incapable of taking up carbon from the carbonic 

 acid of the air. They are incapable of life except in the 

 presence of ready made organic matter, and are in consequence 

 confined either to a saprophytic existence (moulds on a fruit) 

 or to a parasitic (as in the case of the typhoid bacillus in the 

 intestine). Scattered as they are throughout nature the 

 bacteria are the chief agents in the decompositions and re- 

 combinations of organic and living matter. 



The usual classification of bacteria depends on their external 

 form and is merely provisional. The round bacteria are called 

 micrococci or cocci. The 

 streptococci are, as Pas- ^ 

 teur described them, like • , 

 rosaries or necklaces ; the 

 staphylococci are like 

 bunches of grapes. Ac- 

 cording as the micrococci 



in their multiplication 

 J. .J . ^ ., Fig. 24. — Principal bacterial forms. I. 



divide in two or three ^ Cocci.-2. Diplococci.-3. Strepto- 

 planes in space, they ap- cocci. — 4. Staphylococci. — 5. Sar- 



„„„. r.__„„^«j ;« tV-Q .^,,1 cinK. — 6., Bacteria. — 7. Bacilli. — 



pear arranged m the mul- g vibrios.-g. Spirilla.-io. Spiro- 



berry form or in the form chaetes.--ii. Clostridium. 



of a sarcina or woolpack. 



The bacilli are long bacteria, the ends of which are sharply 

 cut ; the bacterium., properly speaking, has the ends rounded 

 like a spindle or shuttle. The curved bacteria are known as 

 vibrios, spirilla, and spirochsetes. 



The bacterial cell is clothed by a membrane and is not 

 naked like an amoeba. It is this sheath, resistant and elastic 

 up to a certain point, which permits a corkscrew spirillum to 

 maintain its form in spite of its movements, and the motile 

 and flexible bacteria to recover their shape after distortion. 

 The membrane takes on colours different from the protoplasm. 

 It is not of the same composition in all the bacteria. Some- 



