32 BACTERIA 



morphologically, bacteria are relatively simple. Biologically and 

 chemically their structure is as complicated as that of the highest cells 

 of the animal body. In common with all proteins, bacterial cellular 

 substance contains a powerful poison. This is true alike of pathogenic 

 and non-pathogenic bacteria and true of all proteins bacterial, vege- 

 table and animal. 



In fact, bacteria in their essential parts are living molecules, of 

 definite chemical composition and structure. In different species the 

 molecular composition differs. As Benecke has shown, the pyocyaneus 

 requires for its growth, in addition to protein substances, two ele- 

 ments, potassium and magnesium. The former cannot be replaced by 

 sodium or ammonium, nor the latter by calcium. The tubercle bacillus 

 needs glycerin in order to assimilate amino-acids, carbohydrates and 

 organic acids. Cramer found that the cholera bacillus assimilates as 

 much as 95 per cent, of the nitrogen in alkaline bouillon, and at most 

 3 per cent, of that in Uschinsky's solution. The obligate parasites 

 feed only on the fluids and tissues of the animal body and some only 

 on some particular species of animal. The influenza bacillus grows 

 only on media containing hemoglobin or some chemically related 

 body. 



Metabolism. Every living thing must feed, assimilate and elim- 

 inate. It thrives and multiplies when food is abundant and conditions 

 of life are favorable. It hungers and dies when the food supply fails. 

 These things are as true of unicellular as of multicellular forms of 

 life. All bacteria, even the anaerobic, absorb oxygen and eliminate 

 carbonic acid. In this gaseous exchange more oxygen is absorbed than 

 is eliminated in the form of carbonic acid. This shows that some 

 oxygen is used in nitrogen metabolism. When the food supply is 

 suddenly withdrawn in the midst of rapid multiplication, the death- 

 rate is great, and greater among the young than among the older 

 individuals. Fisher has shown that when a growth of cholera bacilli 

 twenty-four hours old is suddenly transferred to salt solution, all the 

 organisms die within thirty-six hours. With older cultures some live 

 bacilli are found much later and with a seven-day culture, some will 

 be found to be alive after fifty days. When the food supply is gradu- 

 ally withdrawn the death-rate is not so high and the more hardy 



