THE CHEMICAL CHANGES PRODUCED BY BACTERIA 55 



glycolysis on the organism. Stephenson and Whetham (1924) compared the 

 aerobic and anaerobic breakdown of dextrose by Bad. coli in a free supply of 

 air. The breakdown in the early stages of growth was almost anaerobic ; oxygen 

 uptake was minimal, and lactic acid and similar incompletely oxidized substances 

 were produced. During the later stages of growth, when the release of free energy 

 from these incomplete oxidations was diminishing, atmospheric oxygen was freely 

 utilized, and COj given off. 



A large number of carbohydrate substances are known to be attacked by bacteria ; 

 they range from 3-carbon compounds, through pentoses, hexoses and disaccharides to 

 polysaccharides like cellulose, starch and dextrin. Many of the bacterial species that 

 attack hexose sugars also ferment the related hexahydric alcohols, mannitol, sorbitol 

 and dulcitol, which are apparently broken dowoi in an analogous manner. By analogy 

 with yeast metabolism the breakdown of the more complex disaccharides and polysac- 

 charides is presumably achieved by a preliminary hydrolysis to simple sugars. Saccharose 

 is first hydrolysed to a mixture of dextrose and fructose ; lactose to dextrose and galactose, 

 and maltose to dextrose ; the enzymes concerned are, respectively, invertase, lactase 

 and maltase. WTien hydrolysis has reached this stage, the utihzation of the hexose 

 sugars proceeds along the lines indicated above. Direct evidence that this is the normal 

 mode of cleavage of disaccharides in bacteria is scanty (see Fleming and Neill 1927). 

 Moreover, some yeasts ferment certain complex sugars at a greater rate than they ferment 

 dextrose, a fact difficult to reconcile with the hypothesis of a preliminary breakdown to 

 the simple sugar (see Sobotka and Holzman 1934, Nord and Engel 1938, O'Connor 1940, 

 Leibowitz and Hestrin 1942). Wright (1936, 1937), finding that certain milk streptococci 

 ferment lactose and saccharose more rapidly than they ferment the constituent mono- 

 saccharides, suggested that in bacteria also the disaccharides may be attacked directly, 

 without previous hydrolysis to monosaccharides. The production of acids from saccharose 

 by a number of bacterial species, including Bad. coli, was inhibited by low concentrations 

 of iodoacetic acid, and yet no reducing sugar could be found in the test cultures. The 

 concentration of iodoacetic acid was nevertheless insufficient to inhibit the hydrolysis 

 of saccharose by yeast invertase. 



Little is known of the processes of carbohydrate synthesis in bacteria. The 

 best-established synthesis is that of 4-carbon carboxylic acids from 3-carbon 

 compounds and COg (see Werkman and Wood 1942, van Niel et al. 1942, Evans 

 et al. 1943). Among the more complex compounds, we may note the formation 

 of the polyglucoside dextran, and the polyfructoside levan, from disaccharides, by 

 the action of cell-free extracts. These polysaccharides are contained in the capsular 

 material of several bacteria. Hehre and Sugg (1942) produced a serologically 

 specific (p. 279) dextran by the action on sucrose of a cell-free enzyme preparation 

 from Leuconostoc mesenteroides. Hestrin and Avineri-Shapiro (1943, 1944) studied 

 the formation of levan from sucrose and raffinose with enzymes extracted from a 

 strain of Bact. aerogenes. They suggested that the energy requisite for the synthesis 

 of the polysaccharide might be produced by the breakdown of part of the substrate 

 into an aldose. 



The Action of Bacteria on Proteins and other Nitrogenous Substances. — The 



breakdown of proteins that occurs under natural conditions has long been known 

 to depend on the action of bacteria. This proteolytic activity is, however, con- 

 fined to particular species, many of which are anaerobic ; it is at least doubtful 

 whether even the proteolytic species are able to utilize complex proteins for growth 

 in the absence of other sources of nitrogen. 



Bainbridge (1911) found that many bacteria, including Proteus vulgaris, Bact. 



