METABOLISM OF MICROORGANISMS 361 



Young of some common higher animals: 



Cattle 4 



Swine 7 



Chicken 10 



Fish 10 



Yeast when grown under aerobic conditions is probably the most 

 efficient of the microbial cells ; molds come next and, from the meager data 

 available, bacteria are third. Animals, even the most efficient, are far 

 below aerobic microorganisms in their ability to convert food into living 

 cells. 



Slow-growing animals are less efficient than animals that attain ma- 

 turity in a short time. This is to be expected since cells that grow 

 slowly use up a larger proportion of the food for maintenance than rapidly 

 growing cells. 



Perhaps the most noteworthy figures are those comparing aerobic 

 and anaerobic growth. For example, the same species of yeast gives 

 about 10 times more weight of cells under aerobic than under anaerobic 

 conditions. Producers of baker's yeast understand this fact and blow 

 enormous quantities of air through the medium to obtain high yields 

 of yeast. Under anaerobic conditions the nutrients, e.g., glucose, are 

 converted mainly into ethyl alcohol and carbon dioxide instead of into 

 yeast cells. It is impossible to have high yields of cells and alcohol 

 in the same fermentation. The situation is analogous to that with 

 cattle; the farmer obtains a high production of either beef t)r milk, but 

 not both from the same animal. 



The effect of aerobic conditions in stimulating the growth of cells is 

 seen also by comparing the two bacteria, A. vinelandii and CI. aceto- 

 hutylicum. The first is an aerobe and gives high yields of cells. The 

 second, an anaerobe, gives a low yield of cells but large amounts of 

 products such as acetone, ethyl alcohol, and butyl alcohol. 



Molds are aerobic microorganisms. If allowed to grow under favor- 

 able conditions, e.g., penicillin fermentation, they are about as efficient 

 as yeast in converting nutrients into cells. In citric fermentation, growth 

 is deliberately restricted in order to promote the yield of citric acid. 



Metabolic rate 



Microorganisms transform matter at a much faster rate than do animals. 

 It takes about 100 days for an adult human being to consume his own 

 weight of food. Cattle accomplish this in 40 days and swine do it in 

 20 days. Yeast cells take about 30 minutes to metabolize their weight 

 of nutrients. The mold A. niger converts its own weight of glucose to 

 gluconic acid in about 2 minutes, and a urea-splitting bacterium trans- 

 forms its weight of urea to ammonia in a few seconds. One reason why 



