370 METABOLISM OF MICROORGANISMS 



Bacillus poly my xa, and Bacillus brevis. The amino acid content of 

 bacitracin is given in Table 5^ and presents no unusual features. There 

 are several polymyxins, A, B, C, D, and E, and each one contains large 

 amounts (more than 50 per cent) of the unusual amino acid, L-a,y-di- 

 aminobutyric acid. A second distinctive feature is the presence in the 

 molecule of a nine-carbon fatty acid, probably 6-methyloctanoic acid. 



Tyrothricin is not a homogeneous substance but consists mainly of 

 gramicidin, a neutral cyclic polypeptide. On hydrolysis gramicidin gives 

 five amino acids and ethanolamine, NHoCH^CHoOH. 



Bacitracin resembles penicillin in being most active against gram-posi- 

 tive bacteria. It causes kidney damage (evidenced by albumin in the 

 urine) . Because of this toxic effect, its use is limited to combating local 

 infections. 



The polymyxins are very potent against gram-negative bacteria, in- 

 cluding the very resistant Proteus and Pseudomonas bacteria. Unfortu- 

 nately, the polymyxins cause more or less kidney damage, so their use 

 will probably be limited to refractory infections that do not respond to 

 other treatments. 



Tyrothricin is the oldest commercial antibiotic, but probably the least 

 used of the commercial products. It acts on gram-positive bacteria 

 but is not suitable for injection or oral administration. It is used only 

 for topical purposes, tluit is, where it can be brought into direct contact 

 with the infecting oi;ganism, e.g., surface abscesses. 



Because of the millions of gallons of media that must be used for the 

 production of antibiotics, the fermentation is done in deep tanks of 

 5-15,000 gallon capacity. Sterile air is forced through the medium at 

 the rate of about one-half volume of air per volume of medium per minute. 

 The medium is also stirred vigorously to increase aeration. Deep tank 

 fermentation was first developed for the production of penicillin and 

 later applied to the production of other antibiotics and vitamins. 



Vitamin B12 is produced simultaneously with streptomycin, aureomycin, 

 and terramycin. Hence producers of these antibiotics obtain a second 

 valuable product in the same fermentation. Vitamin B12 is also pro- 

 duced commercially by a mixed aerobacter-proteus type of fermentation. 

 The vitamin is formed by many different kinds of bacteria and molds. 

 Yeasts produce little, if any, of it. 



By yeast 



Baker's yeast can grow under both aerobic and anaerobic conditions. 

 If an abundance of air and a low concentration of sugar [e.g., 0.5 per 

 cent) are supplied, the end products of metabolism are mainly carbon 

 dioxide and yeast (50 per cent of the weight of sugar is obtained as dry 

 weight of yeast) ; there is practically no alcohol. See Fig. 14-2. 



