208 LABORATORY EXERCISES IN BACTERIOLOGY. 



may be employed before staining with the aniline-water-gentian-violet solution; or 

 hematoxylin may be used after aniline-water-fuchsin solution. 



The above description of staining methods and the necessary reagents is neces- 

 sarily brief, reference being made to but very few of the many valuable methods appli- 

 cable to general or special use. For more detail and a more complete list of the methods 

 proposed reference should be made to the standard text-books and systematic works 

 on bacteriology. The special methods of staining spores, flagella, etc., are given in 

 connection with the paragraphs devoted to such bacterial structures. 



PHYSICAL CHARACTERISTICS OF BACTERIA. 



i. Shape of Bacteria. Bacteria are single-celled organisms. In the unstained 

 preparations, or better after staining, they are seen to present certain shapes, which 

 are in the main characteristic. Uniform persistence of such shape or its variability 

 under altered conditions of growth constitutes an important feature in the oligo- 

 morphism and pleomorphism of bacteria. The oligomorphous varieties (eubacteria] 

 are by far the most numerous and include all the known types important in medicine ; 

 the pleomorphous, exhibiting a wide possibility of adaptation and growth, are essen- 

 tially saprophytic and numerically limited and of little except general interest. It 

 is not meant that the shape of a given oligomorphous bacterium is absolutely fixed 

 and invariable, however; certain variations in shape and size being essential in the 

 growth of these as of any germs, and some minor modifications in both of these features 

 and in the physiologic phenomena being manifest from the influence of special condi- 

 tions of development (evolutional and involutional forms) . However, such variations 

 are not of such degree as to change the essential type. Three morphologic major 

 types may be distinguished the globular or short oval forms, the long ovals or rods, and 

 the curved rods. These served as the basis for the older classifications, the first being 

 known as cocci (sing., coccus), the second as bacilli (sing., bacillus), and the third as 

 spirilla (sing., spirillum) ; and these terms are constantly used in general application 

 where better definition is not demanded. The modern classifications, while based 

 upon the general outline of bacterial shape, include other features and are much more 

 complicated than such a simple division. In the classification of Migula, adopted 

 with minor modifications in these pages, the first group is included under the family 

 name Coccacece, the second under the term Bacteriaceae, and the third under that of 

 Spirillacea. The presence or absence of organs of motility, of definite capsules, of 

 branch-divisions of the bacterial cell, the mode of grouping, and a number of other 

 features are the basis for further division. A rod-shaped organism showing true 

 branching is spoken of as a mycobacterium; the claustridium is a rod-shaped germ with 

 a thick, clubbed extremity or thick in the middle and tapering at the ends. 



These shapes can usually, when marked, be distinguished without special prepara- 

 tion ; but for their clear demonstration they are best prepared by one or other of the 

 methods of staining. 



Exercise 41. Prepare two films of cocci from a known culture of Micro- 

 coccus pyogenes aureus, taking care not to add too much of the growth to 

 the drop of water used in making the film lest the preparation be so crowded 

 that observation with the microscope will be difficult. Stain one film with 

 the ordinary watery solution of gentian violet, without heat, for four or 

 five minutes. Wash well with water. Stain the second film bv Gram's 



