Microbes Must Eat 69 



rapid changes upon additions of sjnall amounts of acids or alkalies. 

 To help visualize degrees of sourness as they correlate with pH, 

 the following list of common substances is presented: 



Table 3 



THE ;;H OF COMMON MATERIALS 



pa 



Hydrochloric acid 1.0 



Human gastric contents 2.0 



Ginger ale 3.0 



Wines 3.0 



Sour pickles 3.2 



Tomatoes 4.2 



Beans 5.5 



Human saliva 7.0 



Human blood plasma 7.4 



Sea water 8.2 



It has been mentioned that each organism has its own pH 

 range which limits growth of that species, and Table 4 lists a few 

 such organisms. 



Table 4 



THE pH LIMITS FOR MICROBIAL GROWTH 



L Molds 1.4-9.0 



2. Yeasts 2.5-9.0 



3. Most bacteria 5.0-9.0 



A. Escherichia coli, the common organism found in the intestines 



of all warm-blooded animals 4.8-10.0 



B. Clostridium tetani, the etiological agent in lockjaw 5.5-8.3 



C. Salmonella typhosa, the typhoid organism 5.6-8.5 



PREREQUISITE IV: STERILITY 

 Physical Methods 



Moist Heat Sterilization. Sterility means one thing to the 

 layman and something quite different to a microbiologist. So often 

 we hear people say that they have sterilized the baby's bottle by 

 boiling it for ten minutes. In some cases this treatment may very 

 well sterilize the bottle, but because of spores, those resistant 

 bodies so useful in helping some organisms withstand periods of 

 unfavorable environment, we cannot always be sure that mere 



