5O BACTERIOLOGICAL METHODS 



R esearch Laboratory of Parke, Davis and Company. Potassium 

 tellurite is said to be the most satisfactory reagent. In dilutions 

 of i : 50,000 this substance forms characteristic black compounds 

 with all of the more common micro-organisms when in the living 

 state. The reaction does not take place in the presence of dead 

 micro-organisms and the different organisms do not all react in 

 the same degree or manner. Some are much more susceptible 

 than others. The Bacillus coli appears to be the most sensitive 

 to the reagent. With most species of bacteria the time re- 

 quired to produce the characteristic color and precipitation reac- 

 tion ranges from 12 to 96 hr. at a temperature of 37 C., but with 

 the colon bacillus a distinct coloration or color ring becomes visible 

 several minutes after the reagent is added. King and Davis 

 summarize the experimental results as follows: 



1. Nearly all of the more common micro-organisms (bacteria and yeasts) react 

 with potassium tellurite, forming characteristic, black compounds. 



2. This capacity depends on an active stage of metabolism of the reacting 

 organism, and the action is, in all probability, a reduction of the tellurite. 



3. The "tellurite reaction" can be used as an indicator of microbial life, and is 

 especially suitable for revealing microbic contamination. 



4. A dilution of i : 50,000 of the salt seems to be most suitable for its action as a 

 general microbic indicator. In this concentration, it produces no irritative action 

 when introduced into test animals. 



5. The bacteria of the "colon- typhoid group" show differences in resistance 

 to the antiseptic action of potassium tellurite and in the appearance of their reaction 

 with this salt. These variations are sufficient to suggest the experimental use of 

 potassium tellurite for differential diagnosis in the group. 



6. The intensity of bacterial action on potassium tellurite depends upon the 

 individual resistance of the bacterium and the concentration of the salt present. 

 The velocity of reduction of the tellurite is apparently a specific function of an organ- 

 ism, apart from its resistance to antiseptic action. With the colon bacillus, the 

 "tellurite reaction" is almost instantaneous. 



7. Treatment with potassium tellurite has practically no influence on the bio- 

 logical characteristics of an organism. 



3. Numerical Limits of Micro-organisms in Foods and Drugs 



What should be the maximum limit of the number of bacteria 

 and other micro-organisms in food and drugs within the intent 



