128 Microbes and You 



exist, and this is exactly what might occur if false conclusions are 

 drawn from improperly prepared gram stains. A microscopic ex- 

 amination, usually with the oil immersion objectiye, will reyeal the 

 form and the size of an organism, in addition to its gram reaction. 

 By underlining or circling the applicable terms that appear on the 

 descriptive chart, a quick glance will point out to the observer the 

 outstanding characteristics of the organism. 



Hanging Drop 



By examining a hanging drop of a young (less than twent)^- 

 four hours) broth culture under the high dry objective of the 

 microscope, motility and typical arrangement of the species can be 

 determined. 



Spores 



Spores fail to stain during ordinary gram staining, but by the 

 application of heat with dyes such as malachite green or carbol 

 fuchsin, spores can be stained and their size and location can be 

 ascertained after suitably counter-staining the preparation with a 

 contrast dye such as safranin or methylene blue which colors the 

 vegetative cells and the non-spore components of spore-bearing 

 bacteria. 



CULTURAL CHARACTERISTICS 



To a trained observer the cultural characteristics can supply 

 valuable clues as to the possible genus, and sometimes even as to 

 the species of the test organism. It is unwise, however, to allow 

 these criteria to be the only studies made, since closely related, yet 

 distinct, species may have similar cultural characteristics. 



Streak Plate 



Because a streak plate allows all colonies to develop on the 

 surface of the medium, such considerations as form, surface ap- 

 pearance, elevation, edge of the colony, and the speed of microbial 

 growth can be compared. Again, these characteristics in them- 

 selves are not conclusive evidence as to the genus or species name 



