ANTIBIOTICS AND CHEMICAL DISINFECTANTS 221 



TABLE 26. BACTERICIDAL EFFECTS OF PENICILLIN, GRAMICIDIN, AND 

 TYROCIDINE UPON S. HEMOLYTICUS 



From Dawson, Hobby, Meyer, and Chaffee (171). 



* 10 |Xg of each preparation was added to i milliliter of culture. 



t In thousands per milliliter. 



lethal effects. The conclusion was reached that the mechanism of bac- 

 tericidal action must be considered as a separate problem for each type 

 of organism, and, one may add, for each type of compound. 



Although the major difference in the action of antibiotic substances 

 and chemical antiseptics is based upon the selective antibacterial nature 

 of the former, still an attempt may be made to correlate the two types 

 of compounds. Marshall and Hrenoff (621) constructed a disinfectant 

 spectrum for antibacterial substances with a flexible blending of differ- 

 entiated degrees of activity. The first, or ineffective, band covers a 

 range of dilutions of an agent between zero concentration and the high- 

 est dilution which still exerts no action on bacteria. The second, or stimu- 

 lative, band comprises a range of relatively high dilutions in which 

 there is a slight stimulation of bacterial multiplication; this range is 

 ordinarily narrow, but it may become broad. The third, or inhibiting, • 

 and the fourth, or bactericidal, bands merge indistinguishably. The 

 fifth, or impractical, band covers a range of concentrations of the dis- 

 infectant that are too great for practical purposes (Figure 21). 



By establishing the normal rate of multiplication of bacterial cells in 

 a given culture without the disinfectant, one can determine the retarda- 

 tion of that rate by the disinfectant. This rate approaches zero at com- 

 plete inhibition with no multiplication and no deaths. A further increase 

 in the concentration of disinfectant results in the death of some organ- 

 isms per unit of time, and eventually a concentration is reached at which 

 all organisms die rapidly. Any rate of multiplication greater than zero 

 but less than normal can be considered as the bacteriostatic zone, and 



