CHAPTER II 



THE NATURE AND IDENTIFICATION OF BACTERIA 



Bacteria 



Bacteria are simple unicellular organisms which, multiply, 

 often very rapidly, by binary fission. The majority possess 

 no chlorophyll, though bacterial chlorophyll does occur in 

 the photosynthetic organisms (see p. 86). No nucleus is 

 visible in the bacterial cell if this is examined by the usual 

 methods, although the application of paiticular staining 

 techniques will reveal the presence of what are usually called 

 " nuclear structures " in the cytoplasm. 



Size 



Large bacteria such as Clostridium welchii may have a 

 length as great as S-bfi (1/x = 0-001 mm.) and, at the other 

 end of the scale, we have various Micrococci with a diameter of 

 300-500 m/x (Im/x = 0-001 /z). The larger organisms thus 

 approximate in length to half the diameter of tJie red blood- 

 corpuscle or of the yeast-cell. At the other end of the scale 

 it is difficult to draw a line between the smaller bacteria and 

 the larger viruses, especially if we take into account the 

 Rickettsia which, in size and properties, fall between the true 

 bacteria, which can exist outside the cells of a host, and the 

 viruses which can multiply only within the cells of a host. 

 The frontispiece shows the gradation in size from CI. welchii, 

 streptococci, rickettsia, and viruses large and small, to protein 

 molecules. Most bacteria can be cultivated in laboratory 

 media of varying complexity, but in general it is true to say 

 that the smaller the organism, the poorer its synthetic powers 

 and, consequently, the more parasitic it becomes, until in the 

 ultimate stages shown by the viruses a sufficiently nutrient 

 medium is supplied only in the interior of the living cells of 

 a host. A particle having the size of the Foot-and-Mouth- 

 disease virus is sufficiently large to accommodate about 50-100 

 protein molecules only. 



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