14 LABORATORY EXERCISES IN BACTERIOLOGY. 



spherical shape; bacteriacece (including the non-motile bacteria and the motile bacilli 

 and pseudomonads) , unbranched and non-sheathed rods; mycobacteriacecz, rod-shaped 

 organisms approaching the hyphomycetes in forming mycelial-like threads and some- 

 times truly branching; spirillacea, curved rods; and chlamidobacteriacece, filamentous 

 bacteria covered with more or less definite envelopes (Fig. 1). 



For further division of these families reference may be made to the classification in 

 Lesson IX (vide Chester, Determinative Bacteriology). 



Bacteria occur widely in nature, both as free and parasitic organisms, scarcely any 

 substance being free from their presence. In their growth they approach the character 

 of the protozoa because of the absence of chlorophyll from their constitution, rendering 

 them unfit to obtain nutrition, like the chlorophyllous plants, from inorganic substances ; 

 they are therefore to be found in greatest profusion in situations where organized food is 

 available, and are least plentiful where such food is not obtainable for one or other rea- 

 son. The vegetable nature of bacteria is assumed mainly from the evident close relation- 

 ship to other forms of protophytes, often seen in gradation of structural and functional 

 characters ; and in the fact that at least in a number of the bacteria, if not all, cellulose 

 or modifications of cellulose have been demonstrated as a more or less important con- 

 stituent of the cell-wall. Their large proportion of proteid material, as well as many of 

 their structural and functional features, also ally them to the lower animal life ; but 

 from a preponderance of vegetable features they are generally accepted as members of 

 the vegetable kingdom. 



Their importance to the physician lies in their frequent parasitic occurrence. When 

 found growing free in dead organic matter they are spoken of as saprophytes. Of the 

 parasitic forms the greater number are productive of no important influences deleterious 

 to the well-being of the host, and may occasionally be of positive service; such are 

 spoken of as non-pathogenic parasitic bacteria. However, a number are known to possess 

 the power of inducing disease in the body of the individual invaded and are hence spoken 

 of as pathogenic bacteria. It is to be recognized, however, that saprophytism and para- 

 sitism may in turn be possible in numerous instances for the same bacterium, should 

 circumstances demand ; and while an organism may ordinarily be a saprophyte, it may 

 be capable of living as a parasite, and in the latter condition might possibly be actively 

 pathogenic. Hence, while for the most part the essentially pathogenic bacteria demand 

 the physician's attention, there are more reasons than that of general interest which re- 

 quire the study of other forms as well as of these latter. 



The study of these low vegetables is known as Bacteriology and includes not only 

 the general consideration of the morphology and physiology of these microorganisms, 

 but also the technique of observations for the identification of the organism in ques- 

 tion, and its relations to disease. 



