24 PRINCIPLES OF BACTERIOLOGY 



varied conditions. They have no nucleus, strictly speaking, but con- 

 tain a substance which resembles nuclear material. 



Those bacteria which depend entirely upon a living host for their 

 existence are known as strict parasites; those which can lead a sapro- 

 phytic existence, but which usually thrive only within the body of a 

 living animal, are called facultative parasites. The strict saprophytes, 

 which represent the large majority of all bacteria, while they destroy 

 refuse are not only harmless to living organisms, but perform many 

 exceedingly important functions in nature, such as the destruction of 

 dead organic matter and its preparation for plant food through decom- 

 position, putrefaction, and fermentation. The parasites, on the contrary, 

 are harmful invaders of the body tissues, exciting by their growth and 

 products many forms of inflammation and disease. 



Relationship of Bacteria to Other Micro-organisms. Bacteria are 

 allied to moulds on the one hand, and to yeasts on the other. They 

 have no differentiation into root, stem, and leaf. They resemble fungi, 

 in that no sexual reproduction occurs, and that their mode of multipli- 

 cation is usually by simple division. From such facts as these we may 

 show their relations and build up a classification as follows : 



Thallophyta (lower plants with no distinction between root and stem). 



I 



Forms with chlorophyll Forms without chlorophyll, 



(algae, etc.). 



Multicellular ; spores in differentiated Unicellular ; spores frequently absent, 



spore-bearing organs. (The true spore-bearing cells little or not at all 



fungi and moulds.) differentiated ; no sexual reproduction. 



The bacteria. The yeasts 



(blastomycetes). 



The bacteria may be subdivided into lower and higher forms. The 

 lower forms are the more numerous and consist of minute unicellular 

 masses of protoplasm. The largest of the forms met with in animals 

 are 9,, or micromillimetres (a [Jt = ^TTOT f an mcn )> long by less than 

 \t*. thick. The smallest known bacteria measure 0.5/^ long by 0.2// 

 thick, while others are invisible with any magnification which we now 

 possess. 



The higher forms (see streptothrix) show advance on the lower along 

 two lines: (1) On the one hand they consist of filaments made up of 

 simple elements, such as occur in the lower forms. These filaments 

 may be more or less septate, may be provided with a sheath, and may 

 show branching, either true or false. The minute structure of the 

 elements comprising these filaments is analogous to that of the lower 

 forms. Their size, however, is often somewhat greater. The lower 

 forms sometimes occur in filaments, but here every member of the 

 filament is independent, while in the higher forms there seems to be a 

 certain interdependence, among the individual elements. For instance, 



