68 BACTERIOLOGY. 
Most species of bacteria produce acids in the presence 
of sugar, which explains the fact that neutral or slightly 
alkaline cultures become acid at first from the small 
amount of sugar contained in the meat used for making 
the media. When the sugar is used up the reaction 
often becomes alkaline, as the production of alkalies 
continues after the acid formation has ceased. The 
substances producing the alkalinity in cultures are 
chiefly ammonia, amine, and the ammonium bases. 
The conversion of urea into carbonate of ammonia 
affords special evidence of the production of alkaline 
substances by bacteria: 
CO(NH,), + 2H,0 .=  CO,(NH,), 
Urea. 2 Water. Ammonium carbonate. 
Leube has isolated several organisms from putrefy- 
ing urine which separate ammonia from urea. The 
power of decomposing urea, however, is not wide-spread 
among bacteria. Out of twenty-seven organisms studied 
by Warington, only two were found to decompose urea. 
Of sixty species investigated by Lehmann, three only 
developed the odor of ammonia from sterilized human 
urine. 
Ptomains—Toxins. But besideammonium carbonate, 
a large number of basic crystalline substances have been 
recognized, especially by Brieger, as products of bacterial 
growth. These are now commonly known as ptomains, 
or putrefactive alkaloids (from ar@px, putrefaction). 
Nencki, and then later Brieger, Vaughan and others, 
succeeded in preparing organic bases of a definite chem- 
ical composition out of decomposing fluids—meat, fish, 
old cheese, and milk undergoing bacterial decomposition 
—as well as from pure bacterial cultures. Some of these 
were found to exert a poisonous effect, and for a long time 
