52 VETERINARY BACTERIOLOGY 



to an external stimulus is called a tropism. Mold hyphae will 

 often grow toward a moist medium, while the conidiophores which 

 they bear rise at right angles to its surface and seek to produce 

 the spores as far as possible from a moist surface. These phenom- 

 ena are known respectively as positive and negative hydrotropism. 

 The influencing of the direction of the growth by the action of 

 chemicals is called chemotropism. The forms of mold and 

 bacterial colonies, when growing upon artificial media, are 

 largely determined by this factor. For example, many molds 

 radiate in practically straight lines from their point of origin in 

 a medium, and every branch quite exactly bisects the angle between 

 the two filaments on either side. This mutual repulsion of the 

 hyphse is doubtless due to certain of the products excreted by the 

 cells. Heliotropism, or the influence of light on the direction of 

 growth, is also observed in some forms. J 



Influence of Reaction of Medium on Growth. Many organisms 

 are quite exacting in their requirements as to the reaction of the 

 medium in which they are grown. Some forms grow best in a 

 medium slightly acid, others refuse to develop except in one which 

 is slightly alkaline. The majority of bacteria, however, grow well 

 in a medium that is neutral to litmus. 



ANTISEPTICS AND DISINFECTANTS 



An antiseptic is anything that will inhibit the growth of micro- 

 organisms without necessarily destroying them. In the broadest 

 sense, this term would include such physical agencies as the action 

 of cold and heat, but in practice it is generally confined to chemical 

 substances. A disinfectant is a substance that will destroy patho- 

 genic bacteria. Inasmuch as pathogenic and non-pathogenic 

 bacteria are both destroyed by the same substances, there is 

 little real difference in the meaning of disinfectant and germicide 

 (something that will destroy all bacteria). A deodorant is any 

 substance that masks disagreeable odors or eliminates them entirely 

 by removing their cause. A deodorant may or may not be a dis- 

 infectant or antiseptic. These latter terms are relative ones only, 

 for any disinfectant if sufficiently diluted becomes an antiseptic. 



Theories of Action of Antiseptics and Disinfectants. Germ- 

 icides may destroy microdrganisma by forming compounds with 



