^II^■SI()I,()(;^■ 



1:^7 



of ultra\i()l(>( lij;hl (Kcliici-, lMtl('ii<:;('i- and 

 McCoy). 



Action of Stiniulant8 and Poi8on8 



Poisonous substances usually alTcct niicro- 

 or<!;anisnis in one of the follo\viii<>; ways: 1. 

 At certain low concentrations, which ai-e 

 definite for each substance, there is no aj)- 

 preciable action. 2. At somewhat higher con- 

 centrations, these substances may exert a 

 St inudatina; effect u])on growth. '.]. A further 

 increase in concentration of th(> substance 

 produces a bacteriostatic effect upon growth. 

 4. Still further increases ha\-e a microbicidal 

 effect. There is a sharp line of demarcation 

 between stages 2 and o, but hardly any be- 

 tween 3 and 4, because a concentration 

 which has a bacteriostatic effect may be- 

 come bactericidal on prolonged action. 



With regard to their antimicrobial effects, 

 chemical substances are looked upon as in- 

 dil'ferent agents, as stinudants, or as poisons, 

 depending entirely upon the activity of the 

 particular substance. 



Microbial stimulants have been classified 

 into three groups: (a) chemical stimulants, 

 (b) effective nutrients, including oxygen, 

 and (c) effective metabolic products. When 

 ])resent in very small doses, toxic substances 

 may act as stimulants; fluoride, for ex- 

 ample, increases the production of zymase 

 by yeasts. The chemical stimuli are often 

 differentiated between those that affect the 

 germination of spores and those that favor 

 growth and reproduction. 



Kruse proposed the following rules foi- th(^ 

 toxic action of poisons upon microlx^s: 1. 

 The bacteriostatic or l)actei'icidal action of 

 a poison increases with the concentration. 2. 

 \\\\\\ an increase in the amount of inoculum, 

 the toxic effect of the medium decreases or 

 disappears. 8. The weaker the nutritive 

 power of the medium, the greater is the 

 toxicity; it is strongest in water and weakest 

 in protein solutions. 4. The poison has a 

 greater bactericidal action at higher tem- 

 peratures. 



The action of a poison and the resistance 

 of the cell dejK'iui upon the rapidity with 

 which the sul)stance passes thioiigli the 

 l)lasnia membrane and penetrates into the 

 c(41. The fat -soluble substaiutes are, tlierc- 

 fore, moi-e poisonous. The strongest poisons 

 ai-e the metallic salts. AgNO.3 in a concentra- 

 tion of 1 :8()0,()()() will kill most bacteria, and 

 llgClo is still more toxic; they are most 

 soluble in ether, alcohol, and fats. Iodine 

 and otluM' substances, such as alcohols, 

 chloroform, ether, and CS2 behave in a 

 similar manner. Copper salts are not soluble 

 in lipoids, l)ut tend to foi'm complex organic 

 compounds with the cells. 



Of the various microbial poisons, the fol- 

 lowing groups are found to be particularly 

 effective: salts of certahi heavy metals, in- 

 cluding silver, gold, mercury, and copper. 

 The last two have found an extensive ap- 

 plication as fungistatic and fungicidal 

 agents. Iron has little antimicrobial activity, 

 and lead, nickel, and zinc are not active as 

 poisons. ]\Iineral acids, particularly those of 

 the halogen group, have a powerful action. 

 Of the soaps, only the salts of saturated 

 fatty acids are poisonous. Potassium per- 

 manganate and peroxides have a strong 

 effect. Formaldehyde is effective. Ethyl 

 alcohol is active only in certain dilutions 

 with water. Of the aromatic compounds, 

 phenol and its derivatives, especially the 

 chlorinated and brominated compounds, are 

 most important. 



A number of other organic compounds, as 

 substituted ammonium salts, salicylanilide, 

 dichlorodihydroxybenzomethane, and many 

 others, lun-e found extensive use as fungi- 

 cides. The dyes may be added to the list of 

 bactericidal agents. 



In recent years, a new type of antimi- 

 crobial agent has gained universal recogni- 

 tion. It comprises the antibiotics, or com- 

 pounds of microbial origin. Their effect upon 

 ictinomycetes are discussed in detail in 

 Chapters 14 and 15. 



