CHAPTER II. 



GENERAL CHARACTERISTICS OF BACTERIA RELATION TO OTHER 

 MICRO-ORGANISMSMORPHOLOGY AND STRUCTURE. 



BACTERIA comprise the most important of the groups of micro- 

 organisms which have in common the ability to invade the living tissues 

 of animals and plants, and so become involved in the production of 

 disease. The micro-organisms of some of the groups are undoubtedly 

 animal structures, while others are clearly minute plants. Bacteria 

 are such primitive forms that their differentiation is not marked, being 

 related to both plants and animals, but their resemblance to plants 

 seems to be so much closer that they are assigned to the vegetable 

 kingdom. The bacteria are able to obtain their nourishment from 

 much simpler chemical substances than the animal cells, yet they 

 cannot use some of the substances which are assimilable by the green 

 plants. Structurally and morphologically they are apparently extremely 

 simple, although biologically they are very variable. Some bacteria 

 are endowed with motility, others lack it. The majority are reproduced 

 by transverse division, and in some respects they resemble the fungi; 

 hence called by Xaegeli " fission fungi, or schizomycetes." They are 

 also somewhat allied to the lower alga?, especially in their ability to 

 use simpler inorganic substances to build up higher compounds, but 

 differ from them in not having chlorophyll. A few varieties of unicel- 

 lular organisms resemble bacteria in all their known characteristics, 

 except that they possess chlorophyll or substances similar to it. Others, 

 still, which have no chlorophyll, are able, in the absence of light, to 

 build up organic substances synthetically. The motile bacteria are 

 closely related to the protozoa, some of which also invade animal 

 tissues. The latter belong mostly to the animal kingdom and have 

 a very wide distribution. 



The bacteria are, therefore, a great class of micro-organisms which 

 have relation on one or more sides to other classes. There are wonderful 

 differences in the conditions of life and nutrition, which suit the different 

 varieties. We meet with bacterial life between and 75 C. Some 

 live only in the tissues of men, others in animals, and by far the greater 

 number in dead organic matter. For some free oxygen is necessary to 

 life, for others it is a poison. 



Bacteria may be defined as extremely minute, unicellular vegetable 

 micro-organisms, which reproduce themselves with exceeding rapidity, 

 usually by transverse division, and nourish themselves without the aid 

 of chlorophyll. They have great powers of adapting themselves to 



